# Three Phase Motor Power Cable Voltage Rating

Power Supply - 3 Phase 400V

As per BS 7671, is it acceptable to use 4 cores (Br Bl Gy & Yellow) 300/500V cable be used for 3-phase motors?

What is the recommended voltage rating of power cables for three-phase motors?

Thanking you in advance.

• Cable voltage ratings (a/b volts) are a) to Earth and b) between cores, and as 400V 3-phase is 400V between lines and 230V between any line an N or Earth, then  300/500V cable should be adequate.

- Andy.

• Being picky & pedantic.....

Are these (cable) ratings intended to be rms or peak?

One could argue that 230V rms (to earth) is really 230 * sqrt(2) = 325V since the cable insulation will be exposed to the peak of the sinewave.

Equally, 400V rms line-line is 565V peak.

Sooo... shouldn't we be talking about a 400/600V cable?

• Are these (cable) ratings intended to be rms or peak?

I'd hope they were rms (like most things a.c.) - if not a lot of people are in a lot of trouble...

- Andy.

• Though I might add that voltage rating isn't the only consideration - things like physical robustness may be needed for some environments, which usually implies a "chunkier" cables - which often naturally come with a higher voltage rating due to the thicker layers of insulation and sheath.

- Andy.

• Voltage ratings for cables are RMS voltages, not peak. 300/500 volt cable is intended for use on systems with up to 300 volts to earth or 500 volts between conductors.

• Nobody else has mentioned the use of the green & yellow core as a phase conductor.

I hadn't seen that suggestion - I thought the OP was suggesting Brown/Black/Grey for the lines, and (Green/)Yellow for PE - balanced loads like motors often don't need a N.

Re-using a G/Y core in a multicore cable as a live conductor isn't actually prohibited these days (likewise using a "line" coloured core as PE - which carries the same risks, if at the opposite end) - usually the problem is consequential the lack of a c.p.c. which is usually mandatory even when supplying double insulated equipment. For SWA however the armour could be used.

- Andy.

• Oops.  I misread the original post!  I read it as brown, blue and green+yellow.