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??
Time to look at load downstream. You may have loads that are not balanced from the sub panels. Look for harmonics and surge type loads. eg a shear or press that cycles its motor(s) on and off perhaps quickly.
Originally posted by Redsy: nesparky, I don't see how that would cause unequal division of current on a parallel feeder.
That is exactly my question!
Parallel feeders are resistors in parallel. Ohms law says that if the currents are unbalanced, we know the voltage is the same, the resistance has to be out of balance by the same amount as the current.
What kind of amperage numbers are we talking? How many conduits?
The feeders are run through the switchgear, no raceways, except a nipple between the two cabinets. Downstream the load is very diverse. Sub-panels for lighting and HVAC. Three phase loads for equipment, every other type of load you would expect in the typical commercial/industrial setting.
We were measuring with two different cheap Amprobe clamp on meters. Both meters showed the same results, I don't know if these read true RMS or not.
The load was very light. We did this on Saturday, with the factory shut down. No equipment was operating.....the only loads were lighting and Hvac. Normal load with the plant operating is 760A on this feeder.....on Saturday we were seeing about 200A.....the three parallel feeders were showing numbers like 55A/63A/78A....all three phases were similar.
FWIW there is considerable phase imbalance. At full load of 760A the neutral is carrying about 150A of current. This seems to be a separate issue. Some of the load needs to be shifted from A & B to C phase to balance everything.
[This message has been edited by golf junkie (edited 09-23-2001).]
GJ, I had a similar situation a while back. While I don't have an answer for you, I can tell you this. My situation was a temp generators. We ran 12 4/0 type W per phase from each generator to a termination cabinet to parallel the two generators. All cables were exactly 40ft in length. While load testing the generators with a load bank (a purely resistive load) I took some amp readings on the cables out of curiosity. To my surprise there was a variation of about 50A in while pulling a total of 1000A. Some cables were very close to each other some were quite different. I never could explain why especially in a controlled situation.
Re: Imbalanced Parrallel Conductors#127656 09/24/0106:55 AM09/24/0106:55 AM
A couple of thoughts- 1) Is the unequal division of current consistent as a percentage or as an absolute value? If the imbalance remains 20-25 amp at higher load, I wouldn't be as concerned. If the imbalance increases proportionally and it looks like the conductors will exceed their individual ampacities, then I would be more worried. 2) Are the cables bundled in any manner? Do they pass through the nipple ABCN? Making sure that they are grouped together this way for the whole length might help.
How about swapping the highest reading pair in the breaker, and then re-assessing. There must be some sort of R value that is inconsistent, possibly in the new breaker and/or termination ( maybe you could check the heat on all terminations??)