A wire heats itself: the power it dissipates is I²R, and that heat has to leave through the insulation into the surrounding air. The wire is only allowed to reach its temperature rating (105, 135, 150, 200 or 260°C, set by the insulation), so the room it has to heat up in is the difference between its rating and the ambient air, the ΔT. A 200°C wire in a 60°C bay has a ΔT of 140°C.

1. Free air (figure 3). A single copper wire strung in still air, read off the curve for its size at your ΔT. Aluminum takes the copper value times 0.8 (3.8.8.1.1).
2. Bundle (figure 4). Wires in a bundle warm each other and the middle of the bundle cannot shed heat, so each wire is derated. The factor depends on how many wires are in the bundle and on how many of them are carrying current at the same time, as a percentage of the bundle. Below 20% the 20% curve applies.
3. Altitude (figure 5). Cooling by air depends on air density, so a wire that is fine at sea level is not fine at 60,000 ft.
4. Multiply. The result is the most one wire in that harness may carry continuously.

Loading matters as much as count. Twelve wires with only two on at a time barely warm each other; twelve wires all on at full current, the spec's example 2, run at 43% of the free-air rating. That is why the calculator asks for the bundle loading: the share of the bundle carrying current at the same time, worked out from your circuit analysis. Do not guess low. If you do not know, use 100%.

Ampacity keeps the wire from overheating. Voltage drop keeps the load working. For power circuits, MIL-W-5088L 3.8.8.4 requires the total impedance of supply and return paths to be such that the voltage at the load is within the limits of MIL-STD-704; the spec gives no table, so this calculator lets you choose the limit. As a starting point it offers the FAA AC 43.13-1B typical allowances for 14, 28, 115 and 200 V systems; those are typical values, so use the limit in your own system specification.

Copper resistivity is 1.724 × 10⁻⁸ Ω·m at 20°C, rising 0.393% per °C. A wire running near its rating is hot, so its resistance is well above the cold value found in tables; by default the calculator estimates the operating temperature from the same figure-3 curve and uses that hot resistance. Switch to "20°C (cold)" to compare with a table. A dedicated return wire doubles the length; using the airframe as the return does not, but then the structure's resistance is part of the drop, so add it under "contacts, splices".

A wire has to be protected by a device that opens before the wire is harmed. The rule of thumb the calculator applies: the device rating must not exceed the wire's allowable current, and the wire must carry the load. So there is a window: load ≤ device ≤ wire. If the load exceeds the device the breaker nuisance-trips; if the device exceeds the wire the wire is unprotected. When the checks fail together, the fix is a larger wire, not a larger breaker. This is a screening check only; the actual trip curve, the wire's short-circuit heating and your fault-current analysis decide the protection design.

• Aluminum (3.8.2): needs procuring-activity approval; size 8 and larger only; not on engine-mounted accessories or in areas of severe vibration; not where connections are made and broken often; not for runs under 3 feet or in corrosive fumes; only with approved terminations.
• Minimum size (3.8.8): size 22 for airplanes, helicopters and lighter-than-air vehicles; 24 and smaller are prohibited, with an exception for high-strength copper alloy size 24 in missile bundles (3.8.8.3). Direct attachment to engine-mounted accessories: nothing smaller than size 20, and size 20 must be high-strength alloy (3.10.19).
• Terminations (3.8.8.1.2): the ratings here are wire ratings only. Connector contacts, relays, breakers and switches have their own limits and can be the hottest part of the circuit.
• Plated conductors (3.8.1): tin- and silver-plated copper degrade at continuous high temperature, raising resistance and reducing flexibility.

MIL-W-5088L 6.7 example 1: a harness of 10 size 20 and 25 size 22 wires, all 200°C rated, in a 60°C ambient at 60,000 ft, with 7 of the 35 wires (20%) carrying current. Read from the figures: free-air ratings of 21.5 A (size 20) and 16.2 A (size 22) at ΔT = 140°C; a bundle factor of 0.52 for 35 wires at 20%; an altitude factor of 0.79. So a size 22 wire may carry 16.2 × 0.52 × 0.79 = 6.6 A and a size 20 wire 8.8 A. The harness total is (25 × 6.6 + 10 × 8.8) × 0.20 = 50.6 A. Press "Spec 6.7 example 1" above to reproduce the per-wire figure, and use the mixed-harness card for the total.

• It implements the MIL-W-5088L method as printed. AS50881 has been revised since; if your program is on AS50881, check its current figures.
• The curves are digitized from a scan of a 1991 document. The spec page shows the scans next to the re-plotted curves so you can check them.
• It treats the bundle as one uniform thermal environment. Conduit, insulation blankets, hot ducts, and wires that run through different zones need their own analysis (3.8.8.2 and 3.10.16).
• It does not cover shielded-cable heating, RF coax, fault-current withstand, or connector contact ratings.

What replaced MIL-W-5088?
SAE AS50881, "Wiring, Aerospace Vehicle". MIL-W-5088L was made inactive for new design on 30 September 1998, with future design directed to AS50881.

What is the smallest wire I can use in an aircraft?
Size 22 (3.8.8), with a missile-only exception for size 24 high-tensile copper alloy in bundles.

How much does bundling derate a wire?
It depends on both the number of wires and how heavily the bundle is loaded; figure 4 is the source. The spec's examples give 0.52 for 35 wires at 20% loading and 0.43 for 12 wires at 100%.

Does altitude matter that much?
Yes. Figure 5 gives 0.91 at 20,000 ft and 0.79 at 60,000 ft.

Can I use the Table I numbers directly?
Only if your case matches Table I's assumptions exactly (70°C, 60,000 ft, 20% loaded, 33 or 9 wires). Otherwise use the figures, which is what this page does.