delta high leg and receptacles

[dannyrzk]

Good afternoon, If you add a Delta Hight leg transformer and a 120/240 delta panel to provide power for some specific Dehumidifiers in a building. Can somebody help me understand, why I'm being told not to add 120v circuits, to this panel?

thank you!

 

[Beaches EE]

Is there a neutral in this panel? If yes, then the high leg to neutral cannot be used for 120 volt single phase. The other 2 phases should be fine unless there is a load balancing issue.

 

[synchro]

Many transformers for high leg delta have a limit on the amount of unbalanced load current between the two 120V windings of 5% of the total transformer kVA:

Is there a 5% limit for the single phase loading on Delta-Delta connected Transformers with 240/120 Center Tapped secondaries? | Schneider Electric USA

 

[dannyrzk]

Thank you both Beaches EE and Synchro, the 5% unbalanced information makes a lot of sense.

Thanks!

 

[GoldDigger]

So two reasons:
1. Do not add 120V loads on the 120-240 legs because you might exceed the 5% limit (which is only valid in the first place if you have a three phase single core transformer instead of a single phase main pot and one or two stinger pots for the high leg. ) Your description indicates that you are using a single transformer.
2. Do not add single phase breakers to the "empty" 1/3 of the panel slots that connect to the B phase conductor since that will give you the wrong voltage.

 

[electrofelon]

I just want to clarify that this is very common and not prohibited. . I am not disagreeing with the other responses, just keep in mind in practice you can likely connect much more than 5% of 120v loads if they are relatively balanced across the center tap.

 

[jap]

I just want to clarify that this is very common and not prohibited. . I am not disagreeing with the other responses, just keep in mind in practice you can likely connect much more than 5% of 120v loads if they are relatively balanced across the center tap.

This is interesting.

For example: How much neutral load could you put on a 3ph 120/240v standard dry type transformer with a lighting tap?

How can you balance 120v loads across the center tap if you only have 1 wild leg and not using 2 opposing phases?

Jap>

 

[jim dungar]

I just want to clarify that this is very common and not prohibited. . I am not disagreeing with the other responses, just keep in mind in practice you can likely connect much more than 5% of 120v loads if they are relatively balanced across the center tap.

The vast majority of wild-leg systems are derived using transformers/windings with individual cores and in an open delta arrangement.
The 5% loading comes into play with a single core transformer which are not real common in large sizes,but are not rare in the smaller sizes like 30kVA.

The problem with exceeding the 5% limit is partly due to the issue of simply turning loads off, thereby creating an imbalance, which is fairly likely to occur when used to supply general lighting and receptacle loads.

 

[electrofelon]

This is interesting.

For example: How much neutral load could you put on a 3ph 120/240v standard dry type transformer with a lighting tap?

How can you balance 120v loads across the center tap if you only have 1 wild leg and not using 2 opposing phases?

Jap>

Look at it this way:. Take your typical 75 kva 480 - 120/240 three phase transformer. 5% of 75kva is about 31 amps at 120. Many different situations and variables of course, but for most situations I come across, I'm not too worried about the center tap seeing more than that considering diversity and "cancelling" of neutral load from opposite phases.

 

[synchro]

The 5% loading comes into play with a single core transformer which are not real common in large sizes,but are not rare in the smaller sizes like 30kVA.

The problem with exceeding the 5% limit is partly due to the issue of simply turning loads off, thereby creating an imbalance, which is fairly likely to occur when used to supply general lighting and receptacle loads.

So having 10% of a single core transformer kVA on 120V loads will have absolutely no risk of exceeding the 5% unbalance limit. Diversity may be applied to exceed that as electrofelon says, but the specific loads should be considered more carefully in this case.

 

[jim dungar]

So having 10% of a single core transformer kVA on 120V loads will have absolutely no risk of exceeding the 5% unbalance limit. Diversity may be applied to exceed that as electrofelon says, but the specific loads should be considered more carefully in this case.

I said nothing about a 5% unbalance.
I said the 5% loading limit is due to a problem with unbalance, often caused simply by turning off loads.

In electrofelon's example, he talks about 'cancelling of neutral loads', my comment is about issues when no cancelling can possibly occur.

 

[electrofelon]

In electrofelon's example, he talks about 'cancelling of neutral loads', my comment is about issues when no cancelling can possibly occur.

Admittedly, there is no allowance in the NEC for consideration of diversity, so i think going "by the book" we could connect 5% A-N and 5% C-N. In practice, I wouldn't mind going a bit higher......ask an EE how often MCC's are fully NEC compliant.

 

[synchro]

I said nothing about a 5% unbalance.
I said the 5% loading limit is due to a problem with unbalance, often caused simply by turning off loads.

In electrofelon's example, he talks about 'cancelling of neutral loads', my comment is about issues when no cancelling can possibly occur.

Sorry. I did not mean the 5% as being a relative amount of unbalance, but I can see in retrospect it might've been interpreted that way. I was referring the limit of 5% of the transformer kVA rating on loading applied to the center tap, which results from unbalanced 120V loads. But I should have stated that more explicitly and clearly.