Specifying
Reading an M27500 cable part number
An M27500 designator is not a catalog code. Every position selects a requirement the specification defines, and three of them behave in ways that surprise buyers: one letter carries both a marking method and a minimum shield coverage, the shield letter changes meaning depending on how many shields there are, and the jacket letter changes meaning depending on the shield letter. Misread any of those and you order a cable that is not the one on the drawing.
What the specification covers
MIL-DTL-27500 is a detail specification for “cable, power, electrical and cable special purpose, electrical, shielded and unshielded”. It does not define the wire — it defines a cable built from wire that another specification defines, which is why an M27500 number always contains a reference to a basic wire specification such as one of the MIL-W-22759 or MIL-W-81044 slash sheets.
Its own classification is four constructions:
- Unjacketed — 2 to 15 color-coded wires spirally laid, no outer jacket.
- Jacketed — the same, with an overall outer jacket.
- Shielded — a single wire, or 2 to 15 spirally laid, with one or two overall shields.
- Shielded and jacketed — with one or two shields and one or two jackets.
Note where a single conductor is allowed: only in the shielded constructions. A one-conductor unshielded “cable” is just wire, and belongs under the wire specification instead.
The positions, and what each one fixes
| Position | Selects | Worth knowing |
|---|---|---|
| Basic number | The specification itself | The finished cable is identified by this specification's number, not the wire's |
| Identification method | How the wires are identified and the minimum shield coverage | The trap below |
| Conductor size | The basic wire size | “All wires used in the cable will be of the same size” — mixed gauges are a different construction entirely |
| Basic wire specification | A letter symbol for a specification, type and class | This is where MIL-W-22759/13, /14 and the rest are selected |
| Number of wires | 1–15 shielded, 2–15 unshielded | Cables of 10 to 15 conductors are limited to conductor size 12 and smaller |
| Shield style and material | A single letter | Separate symbol sets for single and double shields |
| Jacket | Material and temperature rating | Its meaning depends on the shield symbol |
Three things that are easy to get wrong
1. The identification letter is also a shielding requirement. The specification gives one set of letters for “an unshielded cable or wire, or a cable with a minimum shield coverage of 85 percent” and a different set “when a minimum shield coverage of 90 percent is required”. The same marking method appears in both sets under different letters. So a character that looks like a printing preference is carrying a coverage requirement, and substituting the “equivalent” letter from the other set quietly changes the shield.
2. The shield letter depends on how many shields there are. Shield material is designated by a single letter per the specification's table — but that table has a symbol column for single shield style and a separate column for double shield style. The same material has two different letters. Reading a double-shield letter against the single-shield column gives you the wrong material, not an error message.
3. The jacket letter is interpreted through the shield letter. The specification is explicit: a single jacket symbol is used for cables with an outer jacket only; a double jacket symbol used with a double shield symbol describes a jacket between two shields plus another over the outer shield; and a single jacket symbol used with a double shield symbol describes two overlaid shields under one outer jacket. Three physically different cables, distinguished by which pair of letters is present.
These are the constructions that matter for EMC and for how the assembly is terminated, and they are not visible from the outside of a finished cable.
Cite the revision. The specification says so itself
Copies of this specification circulate freely, and the one most easily found is Revision H, dated 10 December 1997, superseding MIL-C-27500G. The document carries its own instruction on every page: “Check the source to verify that this is the current version before use.”
Take it literally. Tables of symbols are revised, and the work is also published in a NEMA form. A drawing that cites a designator without naming the revision it was written against is ambiguous, and it will be resolved by whoever is reading it that day. This is the same discipline as specifying a circular connector and the counterfeit standards, where a bare citation points at different obligations depending on when it is read.
Practical checks before you release the drawing
- Name the specification and the revision the callout was written against.
- Confirm the basic wire specification separately — M27500 selects it, it does not define it.
- Check every conductor is the same size, and that the count is allowed for that size.
- State the shield coverage you need, and verify it matches the identification letter you chose.
- Say how many shields and how many jackets, in words, beside the designator.
- Specify the shield termination method separately — the cable number does not cover it.
The last point is where cable numbers and assembly requirements meet. See harness drawing requirements for the rest of the package and the RFQ checklist for what a supplier needs to quote without assumptions.
This is a buyer's orientation, not a substitute for the specification. Confirm the current revision against the published document before relying on any symbol.
Last reviewed 2026-09-20