# Difference between maximum amps for chassis wiring and power transmission

I am putting together a test system that'll be carrying over 50Amps.

I need to rate the wires that will be powering the device under test and also querying voltages and signals on the device.

Looking at the AWG table hosted by powerstream there are two current guidelines, one gives the maximum amps for power transmission and another gives the maximum power for chassis wiring. There is a huge difference between the two e.g. for AWG24 3.5A versus 0.577A

I would hazard a guess that chassis wiring is less than 1 metre and power transmission is greater than 100m. Or maybe chassis wiring is something that carries a signal or is only on for a limited amount of time?

What exactly is meant by chassis wiring and power transmission, what is the difference between the chassis and power transmission wiring?

• From the article you linked to, "The Maximum Amps for Chassis Wiring is also a conservative rating, but is meant for wiring in air, and not in a bundle." Nov 9, 2017 at 14:31

Chassis wiring assumes each wire is routed separately, and power wiring assumes they are wired in a bundle. In chassis wiring the cooling of the conductors is better, because they are all exposed directly to air. In a bundle, some of the wires are not in direct contact withe the air.This is why maximum current for chassis wiring will be more compared to an equivalent power transmission.

• the term bundle is vague, i.e. is three wires a bundle or is there a set threshold of wires that needs to be crossed to be classified as a bundle? Jun 28, 2018 at 21:54
• Two wires (eg. live and return) carrying current are a bundle. If a wire is PE, it does not count. Jun 28, 2018 at 22:00

There's a statement above the table:

The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very very conservative

To me, it does look extremely conservative. Maybe it assumes cables tightly bundled inside a cabinet full of hot equipment.

For cables in free air, in a cooler environment, you could increase the current considerably, provided the volt drop is acceptable.

• Thanks, any idea what the 700 mil rule is? Jun 28, 2018 at 22:23
• I hadn't heard it before. But 1 mil = 1/1000 inch. A circular mil is the area of a circle with diameter 1 mil. The area of a circle is A = π(D/2)². If we set A = 700, and re-shuffle the equation, then D comes out at about 30 mils, or 0.03 inch. Jun 28, 2018 at 22:46

Use the 60C column of the table below. This is generally closest to what most people use without getting into the nitty gritty of what type of wire is being used and what type of load is being fed. https://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes

Beyond that, you haven't specified your voltage. If you're using 50V and above, you really need to pay attention to the voltage rating of the wires insulation.