Cable Derating Guidelines

De-Rate Cables by Temperature.
Refer to this page for a manufacturers listing of Wire and Cable.
Cable manufacturers will provide different numbers based on the insulation used for the wire. The [TFE] table below lists copper wire with a Teflon [TFE] insulation. Teflon insulation has a higher operation temperature range then other insulators, for example PVC. The [TFE] table below is based on data derived from MIL-STD-975, using 70oC as the operating temperature. To derate based on number of wires in a bundle:
IBW = ISW x (29 - #wire) / 28 @ [1 to 15 Bundled wires]
IBW = ISW x (0.5) @ [more then 15 Bundled wires]
ISW = Single wire
IBW = Bundled wires
To derate by temperature use;
Derate by 80% at 1500C, 70% at 1350C, or 50% at 1050C {Per MIL-STD-975}
To derate for Commercial wire use the table below;

Copper Wire Correction
Temperature Derate Ampacity
40oC 0.88
45oC 0.82
50oC 0.75
55oC 0.67
60oC 0.58
70oC 0.33

Ampacity Tables for many conditions (Commercial Wiring):
IEEE Standard 835, IEEE Standard Power Cable Ampacity Tables
IEEE Standard 848, Procedure for the Determination of the Ampacity De-rating of Fire Protected Cables
ICEA P-54-440, NEMA Pub. No. WC 51 - Ampacities of Cables in Open-Top Trays
Use the National Electrical Code [NEC] system for premises wiring, and not the Military data on this page.

Voltage Drop per Foot:
V = I x L x (R/1000) x 1.004 x (T-20)
V = Voltage Drop, I = Current, R = Resistance per 1000 foot
L = Length of wire in feet, T = Estimated wire Temperature under load oC
Refer to the AWG Table for Resistance per length

The table below provides derating guidelines for 2000C rated Teflon (type PTFE) wire. The data shows both single wire and multi-conductor wire at 700C in a vacuum (no air flow). If the wire is rated to 1500C derate the values in the table by 80%, 70% for 135C wire, and 115% for 2600C.

Copper Wire TFE Insulated at 700C
Current Carrying
Single Wire
Current Carrying
Bundled Wire
Multi-Conductor Cable
00 169.0 amps 87.5 amps
0 147 75.0
2 108 50.0
4 81 40.0
6 60 30.0
8 44 23.0
10 33 16.5
12 25 11.5
14 19 8.5
16 13 6.5
18 9.2 5.0
20 6.5 3.7
22 4.5 2.5
24 3.3 2.0
26 2.5 1.4
28 1.8 1.0
30 1.3 0.7

The data on this page should be considered engineering guidelines, as among other factors, air flow in the final application is not provided. However; it's common practice when derating a component to use free-air at 250C, in a system with no forced air.

This page provides derating data based on temperature and current through the wire. Other Engineers may also derate based on voltage drop over the length of the wire. Or using a larger gauge wire to reduce the current and voltage drop of the interconnection.

Related Cable Data:
Unit Test Acronyms
DUT Definition

Wire Selection, Equipment Chassis
Cable Selection, Equipment Chassis
Electronic Chassis Design

How to Derate Components;
Guideline for Derating Electronic Components

What is the application:
Size the wire for each application.
Control signals; derate by voltage drop.
Current carrying; derate by wire temperature.
Low weight; derate by current.

PC motherboard

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Modified 6/13/15
Copyright © 1998 - 2016 All rights reserved Larry Davis