A drawing calls for a “200 A bolt-down fuse holder,” and two suppliers offer visually similar covered bases. One proposal fits a MIDI fuse; the other expects a MEGA fuse. Matching the ampere number does not establish that the fuse holes, stud size, lug stack or cover fit the intended assembly. Nor does it establish that the fuse and holder are both suitable for the system voltage.
This guide keeps the mechanical assembly decision separate from protection design. The responsible engineer still selects the fuse current, time-current behavior, DC voltage and interrupt capacity using the actual circuit. For that separate decision, see the DC fuse fault-current guide.
Freeze the fuse identity before comparing holders
Put the fuse manufacturer's complete part number and revision on the RFQ. Record its family, current and DC voltage ratings, hole center-to-center pitch, hole size, body envelope and installation instructions. Then ask for a holder part number that lists that exact fuse as compatible. “MIDI-style,” “MEGA-style” and a common current rating are not enough to release a substitution.
The 2026 Littelfuse MXM MEGA–MIDI holder datasheet illustrates why. Its compatibility table lists examples of MIDI fuses with 30 mm or 36 mm pitch and M6 holes; listed MEGA variants use 51 mm or 56 mm pitch and may use M6 or M8 holes. The MXM family has sliding-tab arrangements and M6/M8 holder variants designed for selected combinations. Those figures describe the listed Littelfuse parts, not a universal dimension rule for every brand or an approval to mix unspecified products.
For a useful supplier comparison, attach the proposed fuse drawing next to the holder drawing and mark the same hole centers, stud diameter and contact faces on both. If a supplier proposes a “compatible equivalent,” request its own controlled drawing and compatibility statement. Do not use a photograph or a familiar outline as the entire fit check.
Separate the holder limit from the fuse limit
Check the holder's maximum operating voltage and current conditions independently of the fuse. Blue Sea Systems AMI/MIDI Safety Fuse Block PN 7720 is a concrete counterexample to a shortcut: its published maximum is 32 V DC, with a 200 A maximum conditional on suitable cable and fuse. Victron's MIDI fuse datasheet lists separate 32 V and 58 V MIDI fuse variants. A 58 V marking on a selected fuse does not raise PN 7720's 32 V holder limit.
The MXM datasheet describes a holder family with variants and compatible fuses reaching different voltage classes. It explicitly says to consult the fuse datasheet for input/output stud torque. Do not copy its family maximum or a torque figure to an arbitrary fuse-and-holder combination. The project reviewer should compare the particular fuse, holder, terminals and installation against the maximum circuit voltage and expected service conditions.
An ampere rating is also not an interrupt rating. If prospective fault current or protection coordination has not been resolved, leave that engineering row open even when the two pieces physically fit. The fuse coordination guide covers the time-current evidence to request.
Draw the actual lug stack and cover envelope
A holder quotation should identify what arrives in the box: base, studs or screws, nuts, washers, cover, busbar or linking pieces, and fuse. Some product records state that the fuse is sold separately. Blue Sea PN 7720 explicitly does so; the Littelfuse MXM datasheet also identifies its fuse as sold separately. A price for the base alone must not be compared with a complete protected assembly without adjusting the bill of materials.
Mark the supply and load lugs on an exploded drawing, including palm thickness, hole size, orientation, cable exit and required bend space. Show where each washer and fastener contacts the fuse and lug. The cover must close in that real configuration without forcing the cable or exposing an unintended live contact. Blue Sea PN 7720, for example, publishes an M8 x 1.25 terminal stud, an M5 x 0.8 fuse screw and cover breakouts for cable access. Its published screw and stud torques are different; use them only for PN 7720 in the manufacturer's current instructions, not as generic MIDI values.
For a multi-fuse block, also draw each link or busbar and its exposed-contact boundary. The MXM datasheet offers interlocking holders and separate busbar accessories, but the specific configuration and included parts still need to appear on the proposed BOM. If the assembly contains mixed MIDI and MEGA positions, mark each position's fuse family and part number so a replacement fuse cannot be chosen solely by amperage or color.
The battery cable lug geometry guide helps define the ring-terminal drawing. The fuse-holder thermal guide addresses the separate current and temperature evidence; neither check is replaced by a closed cover.
Use a worked drawing comparison, not a generic “fits” claim
Suppose a purchasing worksheet names a 30 mm-pitch, M6-hole MIDI fuse from the Littelfuse MXM compatibility table. A supplier proposes a base meant for one of the table's 51 mm-pitch MEGA examples, describing both as “bolt-down fuses.” The 21 mm pitch difference is visible before electrical ratings are considered: 51 minus 30 equals 21 mm. The proposed base is not released on the strength of a matching 200 A label. Ask for a holder that documents the exact MIDI fuse, or for a different engineer-approved fuse and holder pair with a new drawing and protection review.
This is a comparison of named manufacturer examples, not a claim that every MIDI is 30 mm or every MEGA is 51 mm. The MXM table itself lists other pitches and M6/M8 choices. A controlled two-part drawing, not the acronym, determines the actual fit.
Use the following matrix for each offered assembly:
On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.
| RFQ decision | Evidence to request | Hold point before order release |
|---|---|---|
| Fuse and holder identity | Exact manufacturer, part numbers, revisions and stated compatibility | “MIDI/MEGA compatible” without a named mating fuse |
| Mechanical interface | Hole pitch, hole size, stud or screw thread, fuse and lug drawings | Holes or contact faces do not match the proposed stack |
| Electrical limits | Fuse and holder voltage/current documents; fuse interrupt and time-current data | Holder limit is inferred from the fuse, or fault evidence is missing |
| Included parts | BOM for fuse, base, nuts, washers, cover, links and labels | Quote omits a needed fuse, cover or accessory |
| Installed envelope | Cable route, ring-terminal orientation, cover closure and access for the specified tool | Cover cannot close or connection cannot be inspected or tightened as instructed |
| Change and receipt | Approved drawing revision, sample photos, part/lot identification and substitution notice | Delivered parts cannot be matched to the accepted configuration |
Request a marked-up exception list for any row the supplier cannot meet. Do not treat a blank field as confirmation. Incoming inspection can check part markings, count, drawing fit and cover presence, while current, voltage and interrupt claims require the applicable controlled product evidence and project engineering review.
Turn the fit check into a controlled drawing review
A useful fit review is more specific than laying a loose fuse across a holder. Begin with a drawing package for the actual fuse part number. Record the revision and the direction from which the drawing is viewed. Identify the two electrical contact faces, the centers of the mounting holes, the body length and the space occupied by any window or visible indicator. Ask the holder bidder to mark those same features on its own drawing. If a value is missing, the buyer cannot resolve it by scaling a photograph. A photographed ruler can help raise a question, but it is not a controlled dimensional specification.
Next place the proposed ring terminals into the same assembly view. The cable lug may have a hole that passes over the stud and still have a palm that crowds the fuse body, another lug, a molded barrier or the cover. Note the number of lugs at each end, their orientations and any permitted stacking order. Ask whether the supplier's fuse-to-terminal contact sequence and hardware stack match the manufacturer's instructions. The purchaser should not improvise an extra washer, omit a supplied part or reverse the stack simply to make a cover close. Such a change can alter contact pressure or the height of the energized assembly; have the responsible engineer and manufacturer review it.
Show the mounting panel as well. A holder may fit the fuse but collide with an enclosure wall, neighboring holder or cable bend. Mark the fastener access direction, the space a tool needs, and the direction in which the cover opens or lifts. If a cover has breakouts, request a drawing or sample showing which breakout is removed for each cable route. A new opening may change the accessible-contact boundary, so inspect the finished configuration rather than relying on an unopened catalog photo. A dimensioned section through the tallest lug stack is often more informative than a top view alone.
Finally, make the evidence traceable: a single line on the review sheet should name the fuse, holder, cover, hardware and lug revisions that were checked together. If one item changes, that line becomes a trigger for a new fit check. This is particularly useful when purchasing receives a substitute during a shortage. It avoids the mistaken conclusion that all holders with the same broad fuse-family label have the same mounting geometry. The Littelfuse MXM table itself is a reminder that several hole pitches and holder variants can exist within a manufacturer's range.
Compare bids on the same assembly, not the same label
Imagine three quotations for a buyer's approved MIDI fuse. Bid A names a holder, cover and separate fuse line but omits the terminal hardware. Bid B prices only a base and labels it “universal MIDI/MEGA.” Bid C includes a holder, cover, fasteners and a named fuse but proposes a different fuse revision. These are illustrative bid descriptions, not real supplier offers or measured performance. The first comparison is not price per holder; it is whether each offer describes the same complete, reviewable assembly. Ask A for the missing hardware and instructions, ask B for exact compatible part numbers and drawings, and send C's proposed fuse change back to engineering before commercial ranking.
Write a comparison sheet with one row per part, its manufacturer number and revision, quantity per assembly, included or separately priced status, and the drawing or document used to validate it. Add a column for unresolved exceptions. For example, a “cover included” statement still needs the specific cover part and confirmation that it closes over the specified lugs. A “fuse included” statement still needs the selected fuse number and its applicable voltage and interrupt information. This approach prevents a low base price from hiding a separate fuse, a cover accessory, or the labor and approval work created by a substitute.
Keep electrical and mechanical acceptance in separate columns. A holder with the correct hole pitch is not automatically suitable for the maximum DC voltage; a fuse of the right current is not automatically able to interrupt the available fault current. Conversely, a strong electrical rating on a fuse datasheet does not establish that a bidder's ring terminals fit under a particular cover. The Blue Sea PN 7720 record gives a named 32 V DC holder example, while the Victron fuse catalog shows why a fuse variant's voltage marking needs its own reading. Those sources are examples of different products; they are not an approved paired assembly.
Do not award an exception merely because it seems minor on the quote. If the proposed fuse changes, the protection study may need review. If the holder or cover changes, the physical assembly and touch-access review may need review. If cable lugs change, both fit and termination instructions may need review. Commercial evaluation can resume when those affected evidence rows are closed. This is a practical hold-point sequence, not a claim that every project uses the same approval authority or test method.
Plan a first article that answers the remaining questions
Before approving a volume order, ask for a first-article assembly made from the quoted BOM. The request should identify the fuse and holder part numbers, accepted drawings, cable-lug samples, intended cover and mounting panel or representative fixture. Photograph the assembly before the cover is fitted, after all connections are installed and after the cover is closed. A photo can document configuration and workmanship, but it does not replace controlled rating evidence or an engineer's electrical assessment.
Have the assembly reviewer check the markings on the delivered fuse and holder against the approved list. Confirm that the stud or screw passes through the intended fuse holes without forcing the component sideways. Inspect that the fuse sits on the intended contact surfaces and that the terminal palm does not touch a nearby feature it should clear. Record whether the specified hardware is present and placed as directed. The acceptance record should name the actual instruction revision used for any tightening operation; do not substitute a torque copied from another manufacturer or another screw on the same base.
Close and reopen the cover with the final cable route in place. Check whether the cover latches or fastens as designed, whether a cable pushes it open, whether a wire or ring terminal can be pinched, and whether service access is possible without disturbing adjacent circuits. If a cutout or breakout is needed, inspect the final edge and exposed-contact boundary. These are sample inspection questions, not an assertion that any named supplier has passed the checks. If the project requires a defined accessibility probe or insulation distance, the responsible engineer should set that criterion from the applicable equipment standard and assess the completed assembly, not infer compliance from a plastic lid.
Record any mismatch as a corrective action tied to the part and drawing revision. A changed washer, cover or cable route should produce a revised approved assembly record; a quietly adjusted sample should not become the production reference. For larger orders, define what incoming inspection can repeat quickly—identity, count, markings, visible damage and cover presence—and what requires separate engineering evidence, such as an electrical rating or a temperature-rise claim. The fuse-holder thermal guide outlines the distinct evidence needed when heating is part of the decision.
Make service replacement part of the original RFQ
The person replacing a fuse later may see only an amperage marking and a covered base. A purchasing specification should therefore make the permitted replacement unambiguous. Request a label or service list that names the approved fuse family and part number, plus any alternative explicitly approved by engineering. Record where the fuse number can be read with the cover open and where the assembly revision can be found. If several holders sit side by side, identify each circuit and its separate approved fuse; identical-looking bases need not protect identical loads.
Ask how a technician will remove the cover and reach the fuse without using the cable as a handle. The service drawing should show the orientation of hardware and lugs before removal, and the current instructions that govern reassembly. If the supplier proposes a cover tether, interlocking holder or busbar link, include that part in the service BOM. The Littelfuse MXM material shows examples of accessories and interlocking arrangements, but the proposal must say which ones are supplied for this order.
Plan for controlled substitution during the product's life. A changed fuse supplier can bring a different pitch, body height or stud requirement even when its current rating is unchanged. A changed holder can bring a different cover, cable exit or voltage limit. Require advance notice and a documented comparison against the approved fuse, holder, lugs, cover and installation drawing. If the change cannot be proven equivalent on the relevant points, treat it as a new assembly review rather than accepting it at receiving inspection. This protects the buyer from a service manual that lists one part while later lots contain another.
Service planning also clarifies what should be supplied with the first order. Spare fuses, spare covers and extra hardware may be separate line items. Ask for their exact numbers and quotation terms; do not assume they are included with the holder. The goal is a replaceable, documented assembly, not simply a holder that looked correct on the day the purchase order was signed.
Keep protection approval distinct from the fit release
Mechanical fit can be reviewed from drawings and samples, but the fuse selection still belongs to the circuit's protection design. Give the engineer the maximum system voltage, prospective fault current at the installation point, protected cable and equipment, and any upstream or downstream protective devices. Request the exact fuse datasheet, including its DC voltage, interrupt rating and time-current information. A generic catalog maximum for a fuse family is not a substitute for the selected variant and installation conditions. If that information is missing, mark the electrical gate open even when the physical assembly looks complete.
The holder has its own limits and installation conditions. Ask whether the holder rating is stated for a bare base, a covered base, a particular cable size or an ambient condition. Read the applicable manufacturer's instructions for the offered revision. If the product record does not address the intended configuration, request written clarification; do not calculate a new rating from visual resemblance or extrapolate from another company's fuse. The PN 7720 example is particularly useful because a 58 V fuse elsewhere does not turn a 32 V holder into a 58 V holder.
Likewise, a fit check does not prove acceptable temperature rise under load. Contact surfaces, cable terminals, hardware stack, enclosure air movement and duty cycle affect the installed assembly. The RFQ can ask the supplier for relevant model-specific test evidence and for the assumptions under which it applies. The engineer decides what additional project test or analysis is needed. We do not infer a temperature margin, interrupt result or safe torque from the holder's nominal ampere label.
These separate gates make an approval decision clearer. The drawing reviewer can sign off a named fuse/holder/cover fit while the protection engineer keeps the electrical release on hold. Purchasing can compare complete BOMs while neither sign-off is presumed. A quote should state exactly which evidence is provided, which item is an exception, and which decision remains with the buyer. That is more useful than a broad statement that a holder is “suitable for 200 A systems.”
Send a complete holder-assembly RFQ
Provide the circuit's maximum DC voltage, engineer-selected fuse requirement, proposed fuse part number, cable and lug drawings, available mounting space, cable-exit direction and intended environment. Ask each bidder to identify the exact holder and all supplied hardware, share fuse/holder compatibility documents, return an exploded assembly drawing and list any change from the requested configuration. Request its own price, MOQ and lead time for that defined BOM; none is assumed here. If an online MIDI holder catalog entry is used as a starting point, obtain the actual manufacturer's controlled data before treating its listing as a rating or certification.
In the inquiry form, select Fuse Holders & Fuse Boxes and attach the fuse and lug drawings. Send a MIDI or MEGA holder RFQ.
Buyer FAQ
Will a 200 A MIDI fuse fit any 200 A MEGA holder?
No. Current rating does not define pitch, hole size, contact geometry or compatibility. Compare the exact fuse and holder drawings and the manufacturer's compatibility statement.
If the fuse says 58 V, can I use it in a 32 V holder?
Not on that information alone. The fuse and holder each have a voltage limit. Blue Sea PN 7720 is marked 32 V DC even though 58 V MIDI fuse variants exist elsewhere; obtain a holder suitable for the actual system and matched fuse.
Are the fuse and cover included with a quoted holder?
They may not be. Named Blue Sea and Littelfuse examples state that fuses are sold separately. Ask for a line-item BOM showing the base, fuse, hardware, cover and accessories actually included.
Which torque should the assembler use?
Use the current instructions for the exact fuse, holder and connection. A mounting fastener, fuse screw and cable stud can have different instructions; do not transfer one product's figure to a different assembly.