120/240V 3P 3W calcs

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jself

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Forgive my ignorance but I have never run into 3phase 240v power before, however I have run into a situation with a very old building and it has an Open Delta transformer providing power to it in just that configuration. My question is probably simple to the seasoned pro's here: with 208 3-phase power 120 is basically achieved (in load calculations) by taking the 208v and dividing it by the square root of 3, that relationship makes sense to me, and in this I can achieve 120v 1phase, 208v 1phase, and 208v 3ph. However the relationship in 3phase power with 240 to 120 is not the square root of 3 its half. Am I able to get 120v 1phase, 240v 1phase, and 240 3phase in the same manner and if so what are the relations? I would be happy with being pointed to a document that might address this information if it is convenient.

Thanks all,
Jason
 
Re: 120/240V 3P 3W calcs

A 3?, 208/120 system is a WYE connected transformer. The relationship is as described by you, line volts = phase volts x 1.732.

A 3?, 240/120 system is a Delta connected system. The mid-point of the "C" winding is grounded providing 120V to neutral between L1 and L3. Line voltage = phase voltage or 240V. L2 will have a higher voltage to neutral. This is called ther "high-leg".

The system you are describing is a simply a Delta connection with one winding missing. All connections and voltages will read the same as a "closed-delta". The olny difference is reduced power, effectively 57.7%. (1.732 / 3 = .577)
 
Re: 120/240V 3P 3W calcs

Thanks,
That makes sense....The important thing I found out while researching this subject was that for 3? load you simply divide the load across the 3 phases, for 1? load you would just add that to the total 3? load. That knowledge plus what you have said and what I have found have made me feel alot more comfortable with this type of setup.

Thanks again,
Jason Self
 
Re: 120/240V 3P 3W calcs

Originally posted by jself:
...for 1? load you would just add that to the total 3? load.
Click to expand...
Well... not exactly.

The 3? loads are carried equally on all 3 240V conductors.

1? 240V loads can be distributed between any of the 3 240V conductors. How they are connected affects the load applied to each transformer differently.

The 120V load is carried entirely on the A? (L1) and C? (L3) and N conductors. The 120V load is carried entirely on one transformer. The other 1 or 2 transformers in a delta arrangement do not contribute any to supporting the 120V load.

B? (L2) isn't used for 120V loads since it's 208V from B? to N.

It's common for the second transformer in an open delta system to be much smaller than the transformer that is supplying the 120V loads. It's KVA rating will limit your 3? power and will also limit how much 1? 240V load can be applied to A?-B? (L1-L2) and to B?-C? (L2-L3).

So be sure and look at the KVA ratings of both transformers, and look at how any 240V 1? loads are distributed.

Edited to add:

I've seen a number of installations where all 240V 1? loads, as well as all 120V loads, were supplied off of one transformer, and the use of the second transformer was limited to 3? loads only. This was done by keeping all 1? loads in one 1? panel and all 3? loads in a separate smaller 3? panel. The 3? panels were sometimes labeled "use for 3? loads only".

240V 1? loads applied to A-B or to A-C affect only one of the two transformers. Apply that same load to B-C and you affect both transformers.

My point is, you have to really look at how the existing system is configured and installed -- don't just assume there's only one way something can be done.

[ August 03, 2005, 02:10 PM: Message edited by: tx2step ]
 
Re: 120/240V 3P 3W calcs

Tex,

I beg to differ with you on one point.

You said:

The 120V load is carried entirely on the A? (L1) and C? (L3) and N conductors. The 120V load is carried entirely on one transformer. The other 1 or 2 transformers in a delta arrangement do not contribute any to supporting the 120V load.
Click to expand...
This is true for an open delta, but for a full delta, the other two windings contribute. Consider the case of a balanced load across the 120/240V winding. For identical xfrmrs, this winding will provide 2/3 of the load current; the other two xfrms provide the other 1/3.
 
Re: 120/240V 3P 3W calcs

The open delta was so commonly used, it seems to be what everyone thinks of when they talk about the high leg delta.

Our services are open delta with equal sized transformers, so we do load the B phase with 240V single phase loads. The traditional way is with the large transformer supplying the original single phase service and then upgrading by adding the small transformer and a 3 phase panel for 3 phase loads only.

If we did not load the B phase, we would have an even worse imbalance on the backup generator than we do now. If backup is part of the picture, I would consider asking for a service which will allow loading B as needed for single phase 240 V loads.
 
Re: 120/240V 3P 3W calcs

ahh such sweet memories...... the open delta...
how many of us oletimers have had the opportunity to see (even possibly be responsible for) that beautiful burst of light as the 120v lighting circuit is accidently connected to B phase :)
 
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