Can multiple single phase transfomres be connected to a single feeder tap? I was on site today and saw two 480-120/240 single phase transfomrers tapped from a common 480V three phase feeder? Thinking about this connection you would have one phase unbalanced with the other two, however the 480V would just see this as another three phase load not knowing what is connected downstream, I thought this was unusual maybe not? just curious on how you would determine the unbalanced load and size the feeder properly to provide adequate protection?
Feeder Tapped Transformer
- Thread starter Mike01
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The transformers having a 1ph3w secondary would require secondary protection wouldn't it?
But, without the 1ph transformer being provided with primary protection what would protect the upstream 3ph transformer should the 1ph transformer itself fail such as a winding failure?
Were the secondaries connected to one device or separate from each other ?
In any event, the primary over-current device would need to meed the requirements of 450.3(b).
With proper secondary protection, the primary device could be up to 250% of the transformer current.
In any event, the primary over-current device would need to meed the requirements of 450.3(b).
With proper secondary protection, the primary device could be up to 250% of the transformer current.
mistermudd
Senior Member
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- Washington State
It has been awhile but I believe you are describing an open delta. Three phase output but only 67% ??? output of a complete delta. Just picture a delta with one winding missing, still 240 volt phase to phase even across the missing winding.
two different services
two different services
There were two transfomrers each serving different 120/240V-1Ph, 3W panels [I believe one was around 50kVA the other 37.5kVA] the primaries were tapped from a three phase 480V feeder one xfmr A,B & the other B,C so the B phase would carry an unbalanced load from each xfmr. I never came across something like this before so when I saw it, it just got me thhinking thats all.
two different services
There were two transfomrers each serving different 120/240V-1Ph, 3W panels [I believe one was around 50kVA the other 37.5kVA] the primaries were tapped from a three phase 480V feeder one xfmr A,B & the other B,C so the B phase would carry an unbalanced load from each xfmr. I never came across something like this before so when I saw it, it just got me thhinking thats all.
Assuming the secondaries are protected at 125%, then the primary over=current protection can be 250%. In the scenario you listed, that would probably work. If the two transformers are considerably different in size, protecting the smaller often requires over-current too small to handle the larger especially taking in-rush into account. For that reason among others, it is not commonplace
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