Hi all,
I installed a 1200A Square D I-line breaker today and am getting some very odd amp readings on the load side of the breaker.

The facility is a large screen printing factory with 208v, 2500A, three phase service. Approximately 60,000 sq. ft., with 1/3 production and offices and 2/3 warehouse. Most of the machinery, Hvac, etc. is three phase. The lighting is all 120v.

Customer called and said they were having problems with an 800A breaker tripping. This breaker is the main for an I-line sub-panel which in turn feeds several subs with a wide variety of loads. We checked the load and the breaker was carrying 760A. Today we changed out the breaker to a 1200A.

When we were doing the load check we noticed that the two 500mcm parallel conductors were not sharing the load equally. We wrote that off as a poor installation with unequal length parallel conductors. Wrong! Our new installation shows the same symptoms.

Today we pulled out all the old conductors and installed a new 1200A breaker with three parallel runs of 500mcm CU on the load side. All the phase wires were cut to exactly 88" . The three neutral conductors were all cut to 110" .

The problem is that the parallel runs show an imbalance of 30% on any given phase. The lugs are torqued. What's up with that???

How much imbalance is normal and acceptable with parallel conductors??

GJ