# 3 phase question

1. Sep 24, 2008

### triden

My dad and my uncle were wiring in a disconnect & panel in my dad's new shop the other day. It is a 600v 400A 3-phase connection. When they were wiring the 3 phase connection from the main disconnect to the panel, my uncle (whose an electrician) says to make all the conductors the same size. He said that if you don't, the shorter conductor of the 3 will get really hot.

Is there any reason to why he would say this? I can't think of any reason unless maybe the cable was longer...instead of shorter. Then the increased resistance may cause the conductor to heat - not sure. Any ideas?

2. Sep 24, 2008

### stewartcs

Perhaps he meant the same diameter of wire (AWG) instead of length.

CS

3. Sep 25, 2008

### triden

He specifically said "Don't make one conductor shorter than the other's - they have to be the same length". That's what confuses me..I don't think he was implying diameter.

4. Sep 26, 2008

### stewartcs

Nothing comes to mind as to why that would happen (unless it would unbalance the load somehow due to the cable reactance). I think it would have to be a significant difference in length even for that.

I'd ask him to explain his reasoning.

CS

5. Oct 2, 2008

### subtech

This applies when there are multiple conductor runs for each phase.
It is not advisable to route cables differently and cause difference in lengths when large loads and high currents are likely.
Routing, transpositions in conductors, cable tray details, and even structures near service conductors must be considered for high current applications.
It gets a little involved...

6. Oct 3, 2008

### dlgoff

Then could you please explain why different lengths aren't good? Seems okay to me in the OPs situation.

7. Oct 4, 2008

### subtech

It is ok in the OPs situation if there was a single conductor run for each phase. In the OP, the uncle is mistaken.

As I said, the "equal length rule" does apply when there are multiple conductor runs for each phase.
In multiple conductor runs when amperages are in the hundreds or thousands, routing, transpositions in cable positions, and even cable spacings are important.
Look here: