Loads on high leg of open delta

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wirenut1980

wirenut1980

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Plainfield, IN
Is it allowed to connect loads across the high leg of a delta service? I always thought that the voltage would collapse if loads were connected to it? Is there anything in the NEC that prohibits or allows connecting loads across the high leg to neutral?

Thanks in advance!
 
You may use the high leg on any two-pole circuit (the 2-P breakers need to be fully rated for 240V).

The NEC does not prevent you from using the high leg to neutral connection, however finding a 1-pole 208V breaker may be impossible for your panel.

FWIW
Not all delta systems have a center tapped leg. Always describe your 240/120 3PH 4W system as a 'high leg delta' or simply a 'high leg', just never as 'delta' only.
 
Ok, so the next questions are is it practical to put loads on the high leg and has anyone ever seen this done? How is the voltage regulated on the high leg? I wonder if it is a function of the loading on the two transformers, but not sure exactly how. Or does the voltage vary as a result of the loads connected across the 208 V?

By the way, the reason for my questioning is that a customer (I work for a utility) has loads hooked up to the high leg and they are complaining about low voltage. They have measured about 190 V at times. I still need to find out more information and have not been over there yet to investigate, so I don't have a lot of details (I got the information second hand). So, in addition to the questions already asked, what would cause the high leg voltage to drop like that?

Thanks,:grin:
 
If you are running an open delta, I would expect fairly poor voltage regulation for any high-leg to neutral load, but it depends on how large the load is in relation to the size and loading of the main transformer.
 
wirenut1980 said:
Ok, so the next questions are is it practical to put loads on the high leg and has anyone ever seen this done? How is the voltage regulated on the high leg? I wonder if it is a function of the loading on the two transformers, but not sure exactly how. Or does the voltage vary as a result of the loads connected across the 208 V?

By the way, the reason for my questioning is that a customer (I work for a utility) has loads hooked up to the high leg and they are complaining about low voltage. They have measured about 190 V at times.
So, in addition to the questions already asked, what would cause the high leg voltage to drop like that?
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In days gone by, when there was a 100'-between-pullpoints limit (;)), the high leg was described as being 190v, and not the 208v which theory says it should be. However, I don't recall anyone ever attempting to connect any loads from the high leg to the neutral.

That being said, it's not good practice to connect any loads that way, and your drooping-voltage complaints may be part of why. The proper way to connect 240v loads with a high-leg system is as with any other: line to line.

You can connect any 2-wire 240v loads between any two phases, even across the open side of an open Delta. A 3-wire 120/240v load must not connect to the high leg, but only to the two 120v phases and the neutral, as with a 120/240v 1ph system.
 
wirenut1980 said:
. . . a customer . . . has loads hooked up to the high leg and they are complaining about low voltage. . .
Click to expand...
Tell the customer that he has a contract for 120/240 volts, 3 phase, 4 wire delta system and loads to the higher phase to ground is not included in this system. If he attempts to use that configuration, there is no guarantee of the voltage.

By the way, if this an open delta, the open part of the delta will tend to collapse a little under load but that is not the serving electric utility's problem. :smile:
 
charlie said:
By the way, if this an open delta, the open part of the delta will tend to collapse a little under load but that is not the serving electric utility's problem. :smile:
Click to expand...
Gee, that's funny. If a customer complained to me about that, I'd claim that it is the POCO's problem. :cool:
 
charlie said:
Tell the customer that he has a contract for 120/240 volts, 3 phase, 4 wire delta system and loads to the higher phase to ground is not included in this system. If he attempts to use that configuration, there is no guarantee of the voltage.

By the way, if this an open delta, the open part of the delta will tend to collapse a little under load but that is not the serving electric utility's problem. :smile:
Click to expand...

My thoughts exactly, neighbor. I have never seen anyone actually hook up any load from high leg to neutral.


I don't know about you, but I am ready for some warmer weather. Today should be nice!
 
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wirenut1980:

Do some measurements.

Under no load of any kind on the wild leg measure voltages from wild leg, call it C, to neutral, and to A and B. Also measure A to B, A to N, and B to N.

Apply their load from C to N, and measure that load current, and the voltages C to N, C to A, and C to B. Also A to B, A to N, and B to N.

Hopefully the loads on A and B related to N are not changing much during this test.

Find the transformer secondary impedances. You would also like to know the impedance of the wild leg load (the phase angle of the load current).

Draw the vector diagrams after these measurements and see if anything makes sense.

.
 
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