A DC circuit breaker can have the correct marked current yet be unsuitable in the proposed enclosure. The air immediately around it may be hotter than the outdoor or room ambient. Adjacent breakers, fuses, busbars, power supplies and terminals add heat. Tight spacing and partitions can restrict convection. Sunlight can heat an outdoor box. Cable size and terminal preparation affect both electrical and thermal behavior. A ventilation change can reduce temperature while weakening dust, water or condensation control.
The procurement question is therefore not “is this a 125 A breaker?” It is: what continuous current, protection behavior and connection temperature are supported for the exact breaker, trip technology, DC pole arrangement, enclosure, local ambient, conductor, mounting and concurrent load? A defensible quotation returns a product-specific derating source, a heat-loss schedule, a dimensioned layout, the ventilation basis and a representative closed-enclosure temperature-rise plan.
This guide covers that thermal evidence chain. It does not select a protective device or replace the manufacturer's instructions, governing assembly standard, protection study or qualified enclosure design. Use the DC circuit-breaker trip-curve and interrupting-rating guide for the separate protection and fault-clearing boundary, and the DC breaker pole-series connection guide for pole topology. No statement here claims an unverified SINAWATTS breaker rating, enclosure, certification, thermal design service, test capability, inventory, price, MOQ, lead time or project result.
Direct answer: what should a thermal RFQ require?
For every breaker location, require the bidder to return:
- breaker manufacturer, family, exact part number, frame, poles, trip unit and rating or setting;
- declared DC voltage, pole-connection diagram, grounding arrangement and application limits;
- reference temperature and exact derating table or curve for the offered execution;
- maximum specified local air temperature immediately around the breaker, including uncertainty;
- outside ambient, solar exposure, altitude, enclosure material, dimensions, finish, ingress requirement and mounting location;
- all concurrent internal heat sources with load-state and power-loss evidence;
- breaker mounting orientation, clearances, spacing, barriers and adjacent-device arrangement from applicable instructions;
- ventilation type, inlet and outlet locations, free area, filter, fan duty, controls and failure response where used;
- terminal, lug, busbar, conductor and tightening details for each pole;
- cable or busbar heat-dissipation assumptions and permitted insulation temperature;
- thermal model, manufacturer assessment or representative temperature-rise test plan;
- measurement points, sensor method, stabilization rule, load profile and acceptance criteria;
- first-article and production records; and
- change-control triggers for breaker, conductor, enclosure, ventilation, layout or load changes.
Ask for a thermal schedule, not “suitable up to 70 °C.” A broad operating-temperature range can mean that the device operates with reduced current or a changed thermal pickup at the upper end. It does not establish that the marked ampere value is continuously available in a crowded closed cabinet.
Name each temperature before using it
At least four temperatures may appear in one quotation:
- External ambient: air around the enclosure or switchboard.
- Internal enclosure air: air at a stated location inside the box.
- Breaker local ambient: air immediately surrounding the breaker, at the point and distance used by the applicable manufacturer method.
- Component or connection temperature: breaker body, handle, terminal, lug, conductor or busbar surface temperature.
These are not interchangeable. A weather report cannot substitute for breaker local ambient. A thermographic terminal reading cannot be inserted into an air-temperature derating curve. An average cabinet sensor may miss a hot layer near the top or a local plume from an adjacent device.
Schneider Electric's current ComPacT NSXm User Guide, document DOCA0185EN-01, defines ambient temperature as the air immediately surrounding the breaker and says devices must be derated if temperature inside the enclosure is above 40 °C. Its product-specific tables then show different ampere availability by nominal rating and temperature for named NSXm trip executions. That statement and those tables apply to the documented ComPacT NSXm configurations; they are not a generic 40 °C rule or derating curve for every DC breaker. Schneider Electric ComPacT NSXm environmental conditions.
Put the measurement definition in the RFQ. State sensor location relative to the breaker, whether the door and covers are fitted, which loads are energized, how long the system has operated, and the external ambient during the result. If the enclosure has vertical temperature stratification, use enough measurement points to capture the relevant breaker location rather than averaging away the worst zone.
Freeze the exact breaker and trip technology
Thermal behavior can differ by frame, poles, trip technology, current setting, accessory and connection. A thermal-magnetic trip element responds to local temperature differently from an electronic trip unit, while both still have current-carrying limits arising from conductors, contacts and terminals. A family-level temperature statement cannot replace the exact rating row, trip execution and installation conditions.
Freeze these fields:
On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.
| Breaker field | Required return | Thermal reason |
|---|---|---|
| Product identity | Manufacturer, family, full catalog number and revision | Selects the applicable data |
| Frame and current | Frame size, marked current and adjustable settings | Defines conductor path and overload configuration |
| Trip technology | Thermal-magnetic, electronic or other declared execution | Determines temperature effect on protection and settings |
| DC topology | Voltage, polarity, grounded/ungrounded system and pole diagram | Changes the energized poles and losses |
| Accessories | Auxiliaries, shunt trip, undervoltage release, motor operator and monitoring | Adds heat and space or has its own temperature limit |
| Mounting | Orientation, DIN rail/panel, terminals and barriers | Changes convection and documented installation |
| Connections | Lug/busbar type, material, size, plating and hardware | Controls resistance and heat transfer |
| Conductors | Material, cross-section, insulation, quantity, length and preparation | Carries current and removes heat from terminals |
Do not transfer an AC thermal table to a DC configuration without product documentation. The same molded case may have a specific DC pole-series arrangement, different voltage boundary or separate catalog execution. Thermal approval also cannot compensate for inadequate DC interrupting capacity or an unsupported polarity.
Accessories need their own review. A continuously energized undervoltage release, communication module or power supply can add heat and impose a lower local temperature limit than the main breaker. The breaker auxiliary, shunt-trip and undervoltage-release guide provides the control-circuit evidence table.
Read derating tables by exact row and condition
A derating table is a controlled product document. Record its document number or URL, revision, table title, breaker execution, reference temperature, mounting condition and any notes. Confirm whether the values represent maximum continuous current, trip-setting limits, thermal pickup shift or another quantity. Do not convert all of them into one unlabeled “derating factor.”
The Schneider ComPacT NSXm thermal-magnetic table provides a useful product-scoped example. For the documented TM-D 125 A row, it lists 125 A at 40 °C, 117 A at 50 °C, 109 A at 60 °C and 100 A at 70 °C. Other rows differ, and the same guide contains a separate table for its earth-leakage execution. These numbers belong to the named product and table only.
Consider an original procurement example. A project proposes a documented NSXm TM-D 125 A configuration with 105 A continuous load and predicts 60 °C local breaker ambient. The cited row's 109 A value is above the proposed 105 A load, leaving 4 A between the assumed continuous load and that table value. This arithmetic does not approve the design. The reviewer still has to confirm the exact device execution, DC application and pole diagram, protective settings, load definition, conductor, terminal, altitude, enclosure test and temperature-prediction uncertainty. A two-degree or load-estimate error may matter when the margin is small.
Do not interpolate or extrapolate unless the manufacturer method permits it. If the required temperature lies beyond the published table, obtain an applicable manufacturer decision or select a supported configuration. Do not claim a larger frame solves the problem until its protection range, DC performance, terminals, dimensions, cost and thermal evidence are reviewed.
Build a complete heat-loss schedule
List every credible heat source under the worst concurrent operating state. Include the breaker poles, conductor and busbar resistance, fuses and holders, disconnects, contactors, relays, power supplies, control transformers, SPDs with continuous electronics, meters, shunts, communication modules, heaters and fan losses. State whether each value is measured, manufacturer-published, calculated or assumed.
For resistive loss, the basic relationship is:
Power loss = current² × resistance.
Use resistance at an applicable condition and include all current-carrying paths. Schneider Electric's March 2025 Circuit Breaker Operating Temperatures data bulletin, 0600DB1603 R02/25, describes breaker heat sources and presents the same P = I²R relationship for pole loss. It also cautions that breaker families have different thermal behavior. The bulletin is written around its stated UL 489 context and Square D examples; its numerical terminal limits are not universal acceptance limits for an IEC or unnamed DC assembly. Schneider Electric data bulletin 0600DB1603 R02/25.
An illustrative calculation shows why current assumptions matter. Suppose a hypothetical path has 1.5 milliohms resistance per loaded pole. At 80 A, loss is 80² × 0.0015 = 9.6 W per pole. At 100 A, it becomes 100² × 0.0015 = 15 W per pole. The 25-percent current increase produces a 56.25-percent loss increase. These figures are invented to show the squared relationship. They are not a breaker resistance, enclosure result or SINAWATTS measurement. Actual resistance changes with temperature, construction and connection condition.
ABB's official S800PV technical catalogue gives a product-specific PV example. It states that ambient for the S800PV means the air at that breaker rather than air outside the combiner or inverter, and that internal contact resistance, cabling and nearby products heat the enclosure. Its worked example assumes 0.01 ohm total internal resistance at 100 A and calculates 100 W, then discusses the significant heating that a sealed enclosure can experience. It also warns about direct solar exposure. Use that example only to understand the calculation structure for the named ABB application. Do not assign its assumed resistance or solar temperature rise to another box. ABB S800PV Photovoltaic technical catalogue.
Treat conductor and terminal design as part of cooling
The connection does more than carry current. Cable and busbar also conduct heat away from the breaker terminal. Manufacturer tests and ratings may assume declared conductor sizes, lengths, material and insulation. A short thin pigtail, compact flexible conductor, multiple conductors in one lug or a different busbar can change both loss and heat removal.
For each terminal return:
- approved terminal or lug part number and kit;
- conductor material, size range, class and quantity;
- strip length, ferrule or lug preparation and insertion inspection;
- busbar material, plating, width, thickness and hole pattern where applicable;
- tightening torque, tool and hardware stack from the applicable instructions;
- terminal shield, spreader, extension or phase barrier;
- cable routing and bend force at the lug;
- insulation temperature class and allowable condition;
- expected current and local ambient; and
- temperature-rise acceptance source and measurement point.
A loose, contaminated, misaligned or under-engaged connection can create concentrated resistance that a cabinet-average thermal model does not predict. Conversely, a warm breaker body is not proof of a bad connection. Compare measured temperatures with product instructions, load, ambient, phase or pole balance, baseline data and applicable limits.
The cable lug crimp, tooling and pull-test guide covers termination process evidence. The DC busbar sizing and joint-temperature guide addresses busbar joints. Their evidence must use the same conductor and terminal configuration frozen for the enclosure review.
Specify spacing by function, not one generic gap
Spacing around a breaker can serve several different purposes:
- electrical clearance and creepage;
- arc-gas or exhaust space;
- room for terminal shields or phase barriers;
- heat convection and separation from neighboring losses;
- conductor bend and termination access;
- operating-handle and accessory clearance;
- safe inspection and maintenance access; and
- compliance with the tested enclosure or assembly arrangement.
Return the exact manufacturer drawing and identify which dimension closes each purpose. Do not copy a gap from another breaker frame or interpret “DIN modules” as a thermal-spacing instruction. Some devices are intended for side-by-side installation under stated conditions; others require spacing, a derating factor or an accessory. A blank filler module may improve one thermal arrangement but cannot be assumed to satisfy electrical or arc-clearance requirements.
Show vertical relationships. A lower device can heat the inlet air for an upper device. A horizontal shelf can trap a hot layer. A dense wireway may block natural circulation. Adjacent fuse holders or power supplies can dominate the breaker local ambient even when each component is individually within its catalog load.
The layout drawing should identify panel depth, door clearance, wire duct, partitions, barriers, busbars and cable bundles. Ask the thermal reviewer to use the actual arrangement rather than a component count alone. If the installation instruction specifies orientation, preserve it; rotating a breaker to solve wiring space may change the documented convection or operating arrangement.
Engineer ventilation with the enclosure function
Natural vents, fans, heat exchangers and air conditioning can lower internal temperature, but each creates interfaces that the RFQ must control. A vent can reduce ingress protection. A filter can clog. A fan can fail, draw contaminated air or create a hot recirculation path. A thermostat may sense a cooler location than the breaker. A powered cooling system may be unavailable during a high-load or fault recovery condition.
For natural ventilation, define inlet and outlet location, free area after guards and filters, internal airflow path, mounting orientation and the external environment. Keep openings away from rain, washdown, dust or insects according to the enclosure system. Verify that cable duct, partitions and installed conductors do not block the assumed path.
For forced ventilation, define fan part number, flow and pressure basis, supply, control thresholds, alarm contact, redundancy if required, filter, service interval and failure state. Ask whether the equipment must remain at full current after fan failure or enter a documented reduced-load state. Test that state. Do not base protective safety on an unmonitored fan assumption.
For outdoor boxes, provide solar exposure, color/finish, shade, orientation and mounting surface. “Ambient 40 °C” should not silently mean both outdoor air and breaker local air. If a sunshield, spacing from the mounting surface or thermal barrier is part of the calculation, include it in the BOM and drawing.
Ventilation must be assessed with condensation control. Night cooling and daily temperature swings can create moisture even when peak temperature falls. Any heater adds loss to the thermal schedule. Define heater operation, thermostat and conditions in both hot and cold cases.
Plan a representative closed-enclosure temperature-rise test
Use calculation or simulation to guide design, then validate where required by the governing assembly standard, manufacturer instructions and project plan. A credible test should represent the configuration sold, including enclosure, door, covers, internal barriers, mounted devices, conductor sizes and routing, terminals, ventilation and external mounting orientation.
Freeze the test record:
On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.
| Test element | Required definition |
|---|---|
| Configuration | Enclosure and complete internal BOM with revisions |
| Loading | Current in every circuit, coincidence, duty and control loads |
| External conditions | Ambient, airflow, solar simulation if applicable and mounting surface |
| Local measurements | Air around each breaker plus terminal, conductor, busbar and other critical points |
| Sensors | Type, attachment, location, accuracy, calibration and scan settings |
| Stabilization | Approved steady-state rule and total duration |
| Ventilation | Openings, filters, fan state, controls and failure test if required |
| Acceptance | Exact product, assembly, conductor and project limits with source |
| Evidence | Raw data, time plots, photos, load readings, deviations and sign-off |
Run the intended worst concurrent load, not one breaker in isolation unless the design basis permits that state. Capture external ambient and local breaker air at the same time as connection temperatures. If multiple orientations or seasonal cases can govern, test or justify them.
Thermography can support inspection but must be controlled. Schneider's 2025 bulletin says infrared techniques may help evaluate breakers and terminations, that emitted heat depends on load and ambient, and that interpretation requires trained personnel. It advises allowing initially energized breakers at least three hours to reach operating temperature and comparing images with stored images of the same breaker. These are Schneider bulletin instructions for its stated context, not a universal stabilization time for every test. Follow the applicable product and test plan. Do not open energized equipment or bypass covers solely to get a convenient image; qualified personnel and approved electrical-safety procedures are essential.
Record emissivity treatment, reflected temperature, camera distance and measurement target where thermography is used. Compare like loading and ambient. A visual color palette does not establish compliance. Contact sensors or manufacturer-defined methods may be needed at specific points.
Keep terminal rise and absolute temperature distinct
Temperature rise is the measured component temperature minus its defined ambient reference. Absolute temperature is the component reading itself. A permitted rise does not mean the same absolute temperature at every ambient. Likewise, an absolute temperature limit cannot be assessed without knowing sensor location and measurement method.
Different standards and product programs may use different reference ambients, conductors and allowable rises. The Schneider data bulletin summarizes UL 489 values for the products and context it names. Do not apply its 50 K standard-rated termination rise or 60 K 100-percent-rated termination rise to an IEC assembly, a different terminal material or an unnamed breaker. Obtain the governing limits and exact manufacturer's instructions for the project.
Use both local ambient and terminal readings in the report. If one pole is hotter, compare current, conductor, torque, contact condition and airflow. A delta between similar loaded poles can be diagnostic, but it does not replace the absolute acceptance limit. If all poles are hot because local air is hot, the enclosure design still needs attention.
Compare three hypothetical bids
The following examples are invented for procurement training. They are not customer projects, supplier replies or SINAWATTS test data.
Bid A offers a 125 A DC breaker and says its operating range reaches 70 °C. It gives no derating row, trip execution, pole diagram, power loss or local ambient definition. The enclosure is described only as “IP65.” Hold approval. Request the exact breaker documents and a closed-box thermal basis.
Bid B returns a product-specific derating table at 50 °C but uses the site's 40 °C outdoor air as the breaker ambient. It adds a fan without filter, alarm or failure behavior. This is a traceable start, but the temperature input and cooling system remain unresolved. Ask for local enclosure prediction or test, complete fan design and ingress impact.
Bid C provides the exact DC part and pole diagram, current-setting rationale, loss schedule, conductor/terminal BOM, manufacturer spacing, local-air model and a representative temperature-rise plan. Its quote identifies which values remain assumptions until first-article testing. Bid C offers the most reviewable path even though final acceptance is still conditional.
Do not rank bids by enclosure size or fan flow alone. Compare the scope and evidence: supported continuous current, protective function, thermal margin, ingress, maintenance burden, fan-failure consequence, terminal construction and validation status.
Create a thermal compliance matrix
Use one row per breaker location:
On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.
| Decision field | Buyer input | Supplier return | Release condition |
|---|---|---|---|
| Protection | DC voltage, fault study, load and grounding | Exact breaker, poles, trip data and interrupting evidence | Protection review accepted |
| Local ambient | External range, solar and concurrent loads | Predicted/measured breaker-air temperature and uncertainty | Inside applicable derating range |
| Continuous current | Load profile and growth allowance | Exact table/curve and calculation | Supported with documented margin |
| Heat loss | Coincident operating state | Device and connection loss schedule | All material sources included |
| Layout | Enclosure and access constraints | Dimensioned orientation, spacing and barriers | Manufacturer and assembly rules met |
| Connections | Conductor and busbar requirements | Approved lugs, sizes, preparation and torque | Configuration matches evidence |
| Cooling | Ingress and maintenance needs | Vent/fan/filter design and failure state | Thermal and enclosure functions closed |
| Validation | Required standards and release stage | Model and/or representative test procedure | Acceptance criteria passed |
Use status values such as supported, conditional, open and not applicable. “Complies” without a document reference, input and configuration is not a useful return.
First article, production and maintenance evidence
Inspect the first enclosure against the approved thermal BOM and layout. Verify breaker catalog numbers and settings, pole links, accessories, spacing, barriers, conductor and lug identities, strip lengths, torque records, fan direction, filter and sensor positions. Confirm covers and door seals are installed for the temperature-rise test.
Keep the actual test dataset with enclosure serial or build identification. Record instrument calibration, external ambient, local air, currents, voltages, time traces and final stable values. Mark unresolved deviations and repeat affected measurements after correction. A passing average cannot hide one failed terminal or an untested fan-failure case.
Production controls should preserve ventilation openings, component location and wiring space. A wire duct filled more densely than the first article can alter airflow. A substituted breaker or lug can change losses. A different conductor insulation or cross-section can change heat transfer. Treat these as controlled characteristics, not cosmetic choices.
Maintenance instructions should define safe inspection by qualified personnel, filter service, fan-alarm response, cleaning and review of stored temperature data if monitoring is used. Do not direct personnel to retorque energized connections. A hot-spot investigation should preserve load and ambient data, isolate safely under the approved procedure and examine connection and product conditions before simply increasing breaker size.
Reopen the thermal review after changes to current, duty, breaker, setting, pole topology, conductor, terminal, torque hardware, adjacent devices, enclosure, color, solar exposure, mounting surface, partitions, spacing, ventilation, filter or altitude. Document whether the existing test remains representative.
RFQ submission package
Send the bidder:
- DC one-line and grounding arrangement;
- load profile, simultaneous loads and protection-study reference;
- candidate breaker locations and required pole diagrams;
- external ambient, altitude, solar and contamination conditions;
- enclosure dimensions, material, ingress target and mounting;
- internal layout with every heat source;
- conductor and busbar schedule;
- thermal compliance matrix;
- required model or test cases and measurement plan;
- first-article and production-control requirements; and
- separate commercial fields for price, quantity, lead time, warranty and document delivery.
Commercial and capability responses must come from the actual supplier for this defined scope. This article makes no such promise for SINAWATTS.
Source boundaries checked on 2026-09-23
The Schneider ComPacT NSXm user guide, Schneider data bulletin 0600DB1603 R02/25 and ABB S800PV technical catalogue linked above were checked on 2026-09-23. NSXm ampere values apply only to the stated Schneider product and trip tables. The 2025 Schneider operating-temperature bulletin summarizes its stated UL and Square D context; it does not establish IEC acceptance for every breaker. ABB's heat example and local-ambient discussion belong to the named S800PV catalogue. Each source supports a method and an evidence boundary, not an unnamed product approval.
Send your DC breaker enclosure layout and thermal schedule for an RFQ. Include the one-line, exact load cases, external environment, enclosure drawing, conductor schedule, adjacent devices and required test records so the requested component and documentation scope are explicit. Final protection, enclosure thermal design and approval remain with the responsible manufacturers and project parties.
Buyer FAQ
Is the outdoor or room temperature the breaker ambient?
Not automatically. Product documentation may define ambient as air immediately around the breaker. Estimate or measure the relevant local air in the closed, loaded enclosure and preserve the location and operating state.
Does a 70 °C operating range mean full marked current at 70 °C?
No. Check the exact product's derating table, trip technology and notes. A device may operate at that temperature only with reduced continuous current or changed protection behavior.
Can I apply one percentage derating to every breaker size?
Do not assume so. Product tables can vary by nominal rating, frame and trip execution. Use the exact row or manufacturer method and avoid unsupported interpolation or extrapolation.
Will a larger enclosure always solve the problem?
Not by itself. Size affects heat dissipation, but layout, surface area, material, mounting, solar exposure, internal losses, spacing and airflow also matter. Validate the actual arrangement.
Does adding a fan preserve the enclosure IP rating?
Only if the complete vent, filter, fan and enclosure system supports the required ingress performance. Define filter maintenance, fan monitoring and failure behavior as well as airflow.
Is a hot breaker evidence of a loose terminal?
Not by touch or one image alone. Breakers generate normal heat. Measure with an approved method, record load and ambient, compare poles and baseline data, and apply product-specific limits. Qualified personnel must follow electrical-safety procedures.
Can thermography replace a temperature-rise test?
Thermography is valuable for comparison and inspection when controlled, but a formal release may require defined local-air and contact measurements, stable loading and an assembly-standard method. Use the governing plan and manufacturer instructions.
Why does conductor size matter if the breaker current is unchanged?
The conductor has resistive loss and also carries heat away from the terminal. Manufacturer ratings or tests can rely on specified conductor constructions and sizes. Use an approved terminal/conductor combination.
Should adjacent breakers be assumed to carry current at the same time?
Use the real operating scenario. If circuits can be simultaneous, include their coincident losses. If a demand factor is allowed by the governing design, document its basis rather than assuming diversity.
What changes require renewed thermal review?
Review changes to breaker or settings, DC pole arrangement, load, conductor, terminal, enclosure, spacing, partitions, adjacent devices, ventilation, filter, outdoor exposure, mounting and altitude. Retest when the existing evidence no longer represents the supplied assembly.