What are the effects of trimming the diameter of a wire down to half its gauge?

Question

I have a three-phase, 4 AWG 259/28 stranded power transmission cable (3 conductors total) running ~200m into a slip ring. The only way to connect to the slip ring is by using its 8 AWG pig tails. These pig tails are rated 600V and 40A, which is sufficient for our needs.

I would like to trim down each of the 4 AWG conductors so they can fit into an 8 AWG butt connector or c compression clamp to splice to the pig tails. As far as I can tell, when trimming a conductor down you are simply changing the resistance/length of the cable in that area and its rated amps.

When trimming the conductor from 4 AWG to 8 AWG, would the mΩ/m of the trimmed section shift from 0.8152 to 2.061, as shown in wikipedia's table on wire gauges, or is there more going on here than I think?

Since the diameter of the 4 AWG conductors will suddenly change, will there be power transfer or heat issues that I am unaware of?

Answer 1

If you simply cut strands from the 4 AWG wire, you will have an unpredictable resistance between those strands and the remaining strands. If you don't use all of the strands for the majority remaining length of the full wire size, the voltage drop will be the voltage drop will be somewhere between the voltage drop of 8 AWG and 4 AWG for that length.

Depending on the method of supporting the wire, vibration or just the weight of the heavier wire could put excessive strain on the short length of reduced-size wire.

I would attach ring lugs for 8 AWG on the leads form the slip rings and lugs rated for 4 AWG on that wire. Then, in a junction box, connect each phase of 8 AWG to 4 AWG using short bolts through the ring lugs. Tape up the lugs with appropriate electrical tape.

Answer 2

If you are affected by the electrical codes, you will violate code doing that. You may also affect the cable's strength in unforeseen ways.

Any chance you can obtain different lugs for the machine?

Otherwise my approach would be to run 8 AWG from the slip rings to some convenient location for a junction box, and then splice the 8 to the 4 there. I would use a lug multi-tap connector, e.g.

One is insulated, the other naked. (the insulated one's wires enter from the same side, you can get any variation). Both under $10 at your local electrical supply. All these can handle a wide range of wire. For insulated ones, I select where its maximum size equals my largest wire; that way that wire's insulation fits snug into the rubber. The plastic plugs cover setscrews and unused wire holes.

Answer 3

You have no problem.

You say the amps will be less than 40.

8 gauge wire will handle 40-55 amps.

There will be no heat build up.

Yeah sure, the resistance will increase by a negligible amount, like 4µΩ for a couple of millimeters.

This is not necessary and may not be an option depending on your circumstances. If you want you can not cut the extra strands but peel them back about 3 inches. Then cut the strands going into the CTAP so the peel back strands are about 2" longer than the other strands. The once the connector is crimped you can take the peeled back strands and twist them around the outside of the crimp. And solder them if you want(if you use the copper CTAP).