3 Phase Bastard Leg

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# 1 RULE

# 1 RULE

The #1 RULE positively identify what you are working before you do anything .
 
Jraef said:
From those measurements, the ONLY possibility is that you have a 240/120V 3 phase 4 wire delta source with a center tapped leg between two of the phases. That is not “Wye” at all and if they were referring to the primary side of the transformer being Wye, that was irrelevant and superfluous information that only confuses things.

So the ONLY two issues you have with this system are that 1) you can only feed 120V loads from two of the phases, and 2) if you feed 240V single phase loads where either pole comes from the “stinger” leg, the breaker cannot have a “slash rating”, meaning it must be rated for straight 240V, not “120/240V” as the cheaper ones are. Other than those two issues, you can use this in any way you need. No need to change anything.
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If using a 277/480 panelboard with a system like this (because few if any 240V straight rated single pole breakers exist), can you connect 1-pole circuits to the high leg, and use them to feed 208V loads?
 
Carultch said:
If using a 277/480 panelboard with a system like this (because few if any 240V straight rated single pole breakers exist), can you connect 1-pole circuits to the high leg, and use them to feed 208V loads?
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If you have loads that need 208V phase to neutral, technically yes. The problem with doing that is that it is "unconventional", meaning that it is not what one would EXPECT to see, so the NEXT GUY to look at it may not fully understand the cobbled up arrangement and it can lead to errors that could be dangerous. I try to stay away from unconventional arrangements for that reason.
 
gar said:
189707-2430 EDT

markebenson:

Those measurements tell you that you have a wild leg delta. If there are two transformers, then it is an open delta. Three transformers and it is a closed delta. Between two legs of the supply you have a center tapped secondary with the center tap being nreutral. This neutral should be earthed some where.

.
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Thank you very much for the clarification I appreceate it sir!
 
Jraef said:
From those measurements, the ONLY possibility is that you have a 240/120V 3 phase 4 wire delta source with a center tapped leg between two of the phases. That is not “Wye” at all and if they were referring to the primary side of the transformer being Wye, that was irrelevant and superfluous information that only confuses things.

So the ONLY two issues you have with this system are that 1) you can only feed 120V loads from two of the phases, and 2) if you feed 240V single phase loads where either pole comes from the “stinger” leg, the breaker cannot have a “slash rating”, meaning it must be rated for straight 240V, not “120/240V” as the cheaper ones are. Other than those two issues, you can use this in any way you need. No need to change anything.
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Unfortunately the venue will use 208/240 only for 1 air conditioner and one water heater and all other devices required 110/120v and there is a new 3 phase panel for this load.
 
markebenson said:
Unfortunately the venue will use 208/240 only for 1 air conditioner and one water heater and all other devices required 110/120v and there is a new 3 phase panel for this load.
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What is 208/240? The service is 240V phase to phase and 208V B phase to neutral, I don't think either device will use both 240V and 208V.
 
what i mean is:

the new 200a panel will not power any 208v devices
the new 200a panel will not power any 240v devices


all devices connected to the new panel will be 1 phase to neutral approx 110-120v.

thx
 
Mark, this seems to be above your head, don't you have a more experienced electrician that can assist you on site?

Roger
 
AH Ha! Therein lies the problem. The new panelboard should be 120/240 single phase. Then both busses can supply 120 volts to loads. There is no way to power 120 volt loads on all three phases of a high leg delta service.

Have them return the panelboard and get the correct one. :D
 
Frank DuVal said:
There is no way to power 120 volt loads on all three phases of a high leg delta service.
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You could if you put a 208V to 120V transformer phase to neutral on the high leg, but then of course your B phase to A and C phase voltages will be 138.5V. instead of 240V. I am not recommending it, of course. :D
 
I've always heard that you should not attempt to power anything from the high leg to the neutral, period.
 
LarryFine said:
I've always heard that you should not attempt to power anything from the high leg to the neutral, period.
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But why not? Other than that the high leg may be already fully loaded, what harm could you do as long as you fed a 208V load with it?
 
ggunn said:
But why not? Other than that the high leg may be already fully loaded, what harm could you do as long as you fed a 208V load with it?
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The slash rating of the breaker would be violated. Unless you are using a fused disconnect or a 277/480V panelboard to connect it, you are limited to 120V phase to ground nominal.
 
Carultch said:


The slash rating of the breaker would be violated. Unless you are using a fused disconnect or a 277/480V panelboard to connect it, you are limited to 120V phase to ground nominal.
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Pre coffee remark redacted :D
 
ggunn said:
But why not? Other than that the high leg may be already fully loaded, what harm could you do as long as you fed a 208V load with it?
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electrofelon said:
I have heard the voltage regulation is poor. I Dont remember if that is only if its open delta, or its BS irregardless (not a word).
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The connected load doesn't care. The source can have issues depending on conditions.

A small load relative to the source probably never have any problems.

On a full delta the current splits two ways from the neutral point, on a completely balanced system half goes thru the "A" point and half goes through "C" point on the way to "B".

On an open delta there is only one path through the source from "N" to "B". Say B-C is the open side, then the A-N section of coils carries not only A-N current but also carries B-N current. Becomes easier to overload that section of coil if you put too much B-N load on it.

Then you can complicate it more with a bank made with different sized transformers.
 
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