A phase heat

[BOB poole]

I have encountered an issue i would like sopme input on.
480v to 208/120 45kva dry type step down transformer, feeding a 150a contactor, which in turn feeds a 150a load center with a main.

It's using 1/0 cu
a phase between the contactor and load center main is getting so hot you can not touch it... Slowly burning the insulation off.
Current measures at 45-50 amps on all three legs. Nuetral current is 40a +-.... Panel is ballanced fairly evenly. Loads are mostly electronic gaming equipment.

The conductor on a phase is very hot to the touch. No apparent problems on b&c.

All terminations were pulled loosed and checked. Nothing obviously wrong here either.

Transformer does not appear to have any hot spots. Primary current seems normal. All voltages are normal. Bonding jumpers in good condition, and bonded to xo, a ground rod, and building steel, as well as having a properly sized equipment grounding conductor, which is bonded as well.

What do you think could be causing the heating?

 

[augie47]

Addressing your question, I would suspect an internal contactor problem which could probably be confirmed by a Fall of Potential check or an IR scan.

On a side note, feeding the contactor directly ahead of the OCP device is most likely a violation of 240.21(C)

 

[dmagyar]

Harmonic's may be it

Harmonic's may be it

Bob, with what you've said where everything else looks good but with the load being mostly electronics' you have to consider harmonics being responsible for the heating. Check that out, or get a testing company involved as you may need some backup for additional work that may be necessary to mitigate the problem. Hope this wasn't a design-build.

P.S. What is controlling the contactor?

 

[dbuckley]

With a neutral current about the same as phase currents, with a roughly balanced load, that confirms there are sgnificant harmonic currents present. Being a bunch of gaming machines, a/k/a computers, that is not a big surprise. My maths gives the load at about 18KW (50A x 120V x 3 phases) so a 45KVA transformer should survive that harmonic assault, even though it'll be buzzing a bit (at 180Hz - a buzz, not a deep hum).

The oddment is why just one phase condutor is getting hot.

First question: Is it hot for the whole length, thus ruling out termination issues? If it is getting hot for its whole length, barring physical conduction issues, then something doesn't add up. The heat produced is given by I^2 x R, and thus either the current is higher than you think, or the resistance is higher.

Then the second question would be what is the voltage across each of the phase and neutral cables, measured from termination to termination.