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#66667 - 06/13/06 04:53 PM 200 amp Service 3 Phase  
rhpope  Offline
Member
Joined: Feb 2006
Posts: 18
North Carolina
I need some clarification on 120/208Y 3Ø 4W vs. 120/240 3Ø 4W Delta.

The scenario is that I will have a 200 amp 3Ø service coming into the main distribution panel.

1. For 120/208Y 3Ø 4W, I would be able to get 200 amps/leg at 120V and since this is a Wye, I should be able to get 200 amps at 208V too because of how a Wye is configured.

2. For 120/240 3Ø 4W Delta, amperage wise this should be the same as 120/240V 1Ø where you get 200 amps at 120V and 100 amps at 240V due to how the Delta is configured.

Am I right or wrong in my understanding of the difference between these two types of 3Ø power as far as the amperage is concerned?


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#66668 - 06/13/06 06:55 PM Re: 200 amp Service 3 Phase  
Roger  Offline
Member
Joined: May 2002
Posts: 1,716
N.C.
Both systems will only supply 200 amps (in theory) total on any leg or phase regardless of the combination of load(s).

The Wye will not supply 200 amps of 120v loads plus 200 amps of 208v loads each.

The Wye will allow you to use more of the panel though (120v breaker spaces)

Roger


#66669 - 06/14/06 06:34 AM Re: 200 amp Service 3 Phase  
rhpope  Offline
Member
Joined: Feb 2006
Posts: 18
North Carolina
Roger,

To clarify what I meant about your response, at 120V you can draw 200A or at 208V you can draw 200A not both at the same time and the same goes for the 120/240 1Ø or 3Ø, 200A at 120V or 100A at 240V. As you stated, of course all this is in theory since it all depends on the size of the transformers on the utility pole.

I am basing my questions on the idea that if you take (3) 1Ø transformer all of the same KVA rating that produce say 20A on the load side that you will get the following with the different configurations:

(Qty. 1) 1Ø transformer will give 20A @ 120V or 10A @ 240V 1Ø

(Qty. 2) 1Ø transformers wired into an Open Delta will give 20A @ 120V or 10A @ 240V 3Ø

(Qty. 3) 1Ø transformers wired into a Wye will give 20A @ 120V or 20A @ 208V 3Ø


All this as stated previousy that it is at one voltage or the other, not both at the same time.


#66670 - 06/14/06 07:10 AM Re: 200 amp Service 3 Phase  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
Not quite.

A 200A service is one that can nominally supply 200A on each of the incoming ungrounded conductors. I say 'nominally' because the ratings of the transformer and the trip curve of the main OCPD come into play, but in general this means that you have service conductors with an ampacity of 200A and OCPD with a trip rating of 200A.

In the case of 120/208 3 phase 4 wire wye, a 200A service means that on any leg you can have 200A of current flow. You could have a total of 72KVA delivered as 200A on each of 3 120V line to neutral loads. You could have a total of 72 KVA delivered as 115.4A on each of 3 208V line to line loads. If you have a single load connected from phase A to phase B (or any other pair of phases), then you could supply 200A to it, but only deliver 41.6KVA. If you have a single load connected from phase A to phase B carrying 200A, and a single load connected from phase C to neutral, carrying 200A, then you will deliver 65.6KVA to the load. (As well as have 4 current carrying conductors....hmm, wonder what this says about 'full boats' [Linked Image] )

For 120/240 3 phase 4 wire delta, again, on any leg you can have 200A of current flow. You could have a total of 48KVA delivered as 200A on each of 2 120V line to neutral loads, with the 'stinger' leg unused. You could also deliver 48KVA as 200A through any 240V load connected phase to phase. Finally, with a balanced three phase load, you could deliver a total of 83.1KVA as 115.4A on each of 3 240V line to line loads.

-Jon


#66671 - 06/14/06 10:02 AM Re: 200 amp Service 3 Phase  
renosteinke  Offline
Cat Servant
Member
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
It's really a PoCo call, and not a design issue.
A large number of the "delta" systems out there are so-called "open Delta," that is, there are only two transformers used.

Other Delta arrangements have one transformer much larger than the other two, to allow for the additional single-phase loads.

Given a choice, I'd take the "Wye" every time!


#66672 - 06/14/06 10:14 AM Re: 200 amp Service 3 Phase  
rhpope  Offline
Member
Joined: Feb 2006
Posts: 18
North Carolina
renosteinke,

Is there a particular reason that you prefer Wye over Delta?

What are the pros and cons besides the fact that the Delta has a 208V High Leg on the B phase and with the Wye you can get 120V circuits on all phases?


#66673 - 06/14/06 10:22 AM Re: 200 amp Service 3 Phase  
Radar  Offline
Member
Joined: Apr 2004
Posts: 349
Los Angeles, CA
Ditto Reno - take the Wye if you happen to have a choice, but in my experience, the poco is going to tell you what you'll be getting, and you design accordingly.
Quote
(Qty. 2) 1Ø transformers wired into an Open Delta will give 20A @ 120V or 10A @ 240V 3Ø

Small point of clarification here - open deltas are pretty much the same as a closed delta except they need to be derated to 67% of nameplate value because of the missing transformer.

Radar


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#66674 - 06/14/06 11:07 AM Re: 200 amp Service 3 Phase  
Roger  Offline
Member
Joined: May 2002
Posts: 1,716
N.C.
Rhpope, the transformer configuration will not change the fact that 200 amps per phase is 200 amps per phase regardless of whether the loads are phase to ground, phase to phase, or any combination of them.

Radar, actualy it is more like 58% of a three winding Delta.

Roger



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