Three-Phase Electrical Power Supply

You need to verify what type of service your have. 120/240 volt delta or 120/208 wye. The reading you have might be normal for the delta service. If it is a wye service, the loads are not balanced.

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Agree with previous answer on 120/240 open delta, also know as "wild leg" service.

If you have a 120/208 wye system then I suggest you measure amperes on each 120v (1P) circuit and not how many 1P circuits you have on each phase. If you have spaces on 'C" phase you may be able to move a few 1P breakers to "C" phase

Also, I would question whether you have a real problem anyway (for 120/208 wye) if your service is 100 amp or more. Just leave it.

thank you for reply,i wil check with service provider.

Check if the transformer bank supplying the 3phase power is connected in open delta. If it is, then this is normal. One phase has always lower current compared to the other two.

In the utility where I work, we provide open delta txfr banks to our customers and it is always the case.

zorandodo; check & see if you have a 3 phase motor running drawing 8 amps, you may have a open delta for 3 phase the other load is a 17 amp general load. perry

hello sir......

wat is the basic concept and necessity of using STAR and DELTA connections .... for ex (in transfomers many combinations of star to delta ,star to star ,delta to delta used ).Give the concept in technically also practical applications . my mail id is ktva361_eee@yahoo.co.in

Expect a favour replay

tnx

Think from the angle of phasor diagrams, V_ph, I_ph, V_line, I_line on both sides, the concerned conductor sizes and insulation requirements.

In case of doubt ask your professor.

Any way this topic is covered in CR4, and available at leeast a million places on net.

PS : I don't understand why the SMS languages are being used around.

Hi Zorandodo,

It just sounds like there is some of the single phase loading (either Lx-N or Lx-Ly), and you mention 'main lugs for big panels' so I would assume a 25A load on two phases is not at your maximum capacity. Things to note would be what type of building is it, if nit is cold then there could be a couple of heaters running on Phase a and b, and it could be daytime and the area lighting is on phase c but turned off.

If you are really concerned, do an audit of the building, note the phase it is connected on or where it is an item that is connected to two phases like an arc welder. And again, I don't see that you really have a problem.

Regards,

Trevor.

Check followings

01. Nuetral of all three phase.

02. Current can be flow thru Earth if nutral is loos or not adiquate for C phase

thank you all for great tip's.will need to do some home work but now will be much more easy.

In reading, I'm still not sure of the description given, but if you are speaking of the currents in your feeders, etc. previous answers are probably correct.

If you are saying that your individual motors are drawing that unbalanced a current between phases, I think that you have a problem with supply.

Three phase machinery shouldn't "recognize" a delta wild leg.

In the last response i'm speaking of the usual industrial setups. I have no experience with exotic wiring systems for specific purposes

Howdy ZORANDODO,

By referring to "C phase" you have identified your system as Delta. Your amp values reveal that you have a 4 wire Delta system, 120/240 3 phase.

Your amperages are quite normal. You have identified and stated the reason many distribution systems are designed as or are converting to Wye.

"C" phase usually carries significantly lower amps due to all of the "plug load (120v)" being on the center grounded (earthed) phase between A and B phases, and only the 3-phase loads show up on the "C" phase. These systems, especially when system wide, are difficult to balance.

A 3 phase delta is naturally balanced when all loads are 3-phase, but when some single phase to grounded neutral is introduced, the imbalance is unavaoiable.

Regards, CJM

CJM is correct, most electricians when linking up a single phase circuit (120/220v) just grab the first two legs in a distribution board and voila, 120/220V from three phase. They neglect to check the loading across the circuits ending up in a mess like this. Here in South Africa, electrical council sometimes check and if your load is badly unbalanced, will fine you!! Better get a good sparky to check it for and correct for you.