Transformer secondaries

[nickelec]

I have a question I think o know the answer to but just need verification. I have a transformer 277/480 primary and 110/208 secondary feeding a set of the panels I planned to come off the transformer into a troug/wire way. And feed 3 panels via taps. These Feeders would be less then 10' but I think that I need to hit a disconnect first then the wireway if I'm Correct. 240.21 (c) (4)? Or is it ok to to hit the trough and tap the feeder with out a disconnect before

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[ron]

A lot will have to do with how the transformer primary protection is sized? 125% or higher than the FLA or are you good with "primary only protection" per Table 450.3(B)?

 

[nickelec]

A lot will have to do with how the transformer primary protection is sized? 125% or higher than the FLA or are you good with "primary only protection" per Table 450.3(B)?

I'm not that great at remembering formulas but how do I find the Max fla on the secondary side of a 75 kva transformer being fed via a 277/480 100 amp breaker?

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[tw1156]

75,000VA / (480V x 1.73) = ~91A
75,000VA / (208V x 1.73) = ~209A

KVA remains the same on both sides of the equation.

 

[nickelec]

75,000VA / (480V x 1.73) = ~91A
75,000VA / (208V x 1.73) = ~209A

KVA remains the same on both sides of the equation.

I get different numbers when I do that equation is it me lol?

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[nickelec]

75,000VA / (480V x 1.73) = ~91A
75,000VA / (208V x 1.73) = ~209A

KVA remains the same on both sides of the equation.

Got it thanks
.

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[Smart $]

You have to run conductors all the way from transformer terminals to each panel's lugs. A transformer secondary conductor is considered a tap itself, so you cannot tap a tap.

 

[nickelec]

Ty very much I was not aware , again I don't do many transformers

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[nickelec]

You have to run conductors all the way from transformer terminals to each panel's lugs. A transformer secondary conductor is considered a tap itself, so you cannot tap a tap.

So it would be more practical to hit a disconnect then the trough so the load side of the disconnect aren't considerd taps anymore?

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[nickelec]

You have to run conductors all the way from transformer terminals to each panel's lugs. A transformer secondary conductor is considered a tap itself, so you cannot tap a tap.

How did you come up with the fact that secondaries are a tap is it somewhere in nec or just experience in doing g this before

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[nickelec]

secondary conductors must be terminated in a single circuit breaker or set of fuses.* Does this go for any length of conductors, basically saying it's never ok to install the setup I'm looking to without hitting a disconnect first not necessarily an OCPD?

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[Smart $]

So it would be more practical to hit a disconnect then the trough so the load side of the disconnect aren't considerd taps anymore?

Do you plan on your panels having a main circuit breaker (MCB) or just main lugs only (MLO)?
You should only need a [fused] disconnect switch if your panels are MLO.

How did you come up with the fact that secondaries are a tap is it somewhere in nec or just experience in doing g this before

The fact the section is in 240.21 is a clue... and I do have a fair amount experience. :blink:

secondary conductors must be terminated in a single circuit breaker or set of fuses.* Does this go for any length of conductors, basically saying it's never ok to install the setup I'm looking to without hitting a disconnect first not necessarily an OCPD?

There are a only a few, relatively rare cases where your secondary conductors can supply a load with no OCPD, or connect to multiple OCPD's. Note the single OCPD per secondary conductor is per set and you can run more than one set.... which is why I'm saying run one set to each MCB panel.

 

[nickelec]

Do you plan on your panels having a main circuit breaker (MCB) or just main lugs only (MLO)?
You should only need a [fused] disconnect switch if your panels are MLO.


The fact the section is in 240.21 is a clue... and I do have a fair amount experience. :blink:


There are a only a few, relatively rare cases where your secondary conductors can supply a load with no OCPD, or connect to multiple OCPD's. Note the single OCPD per secondary conductor is per set and you can run more than one set.... which is why I'm saying run one set to each MCB panel.

I planned on using MCB panels but like you said earlier even if I use MCB panels I would need to run 3 sets from the transformer. Or run one set to a fused disconnect then MLO panels Wich I think is the best way to go.

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[nickelec]

I planned on using MCB panels but like you said earlier even if I use MCB panels I would need to run 3 sets from the transformer. Or run one set to a fused disconnect then MLO panels Wich I think is the best way to go.

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So transformer, 200a fused Disconnect​, trough, 3 MLO panels . Is it permitted to tap after that fused disconnect? Or , are those conductors considered a tap as well?

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[ActionDave]

So transformer, 200a fused Disconnect​, trough, 3 MLO panels . Is it permitted to tap after that fused disconnect? Or , are those conductors considered a tap as well?

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What size wire are you running to the panels?

Unless you are protecting wires at more than their allowed ampacity at the start of a circuit you do not have a tap.

 

[nickelec]

What size wire are you running to the panels?

Unless you are protecting wires at more than their allowed ampacity at the start of a circuit you do not have a tap.

My partners orgianal thought was to come off the transformer with a set of 3/0 to a trough then tap 3 times to feed 3 200amp main breaker panels. I want to come off the transformer to a 200 amp disconnect then feed 3 MLO panels. Keep in mind the existing panels are old feeding lighting and general receptacles. Currently fed via a 200amp feed.

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[nickelec]

What size wire are you running to the panels?

Unless you are protecting wires at more than their allowed ampacity at the start of a circuit you do not have a tap.

Where does the circuit start at the primary ocpd or at the lugs on the secondary

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[ActionDave]

My partners orgianal thought was to come off the transformer with a set of 3/0 to a trough then tap 3 times to feed 3 200amp main breaker panels. I want to come off the transformer to a 200 amp disconnect then feed 3 MLO panels. Keep in mind the existing panels are old feeding lighting and general receptacles. Currently fed via a 200amp feed.

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If you have 200A wire coming off of your fused disconnect going to a trough and 200A wire going to each panel you don't have a tap, you have a splice.

 

[ActionDave]

Where does the circuit start at the primary ocpd or at the lugs on the secondary

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It starts at the load side of your first disconnect on the secondary, until then you don't have any protection for the secondary conductors which is why you have to use the tap rules. A tap has a limited definition in the NEC and is not simply another word for splicing big wires.

 

[Smart $]

I planned on using MCB panels but like you said earlier even if I use MCB panels I would need to run 3 sets from the transformer. Or run one set to a fused disconnect then MLO panels Wich I think is the best way to go.

So you run three sets of 200A conductors... what is the big deal? You would have to use 200A conductors from trough to panel main in your "tap a tap" scenario anyway.

And if you used one fused disconnect, are you going to run 400-600A conductor set on both line and load side? What did you gain? You still plan on tapping this feeder with 200A conductors to each panel.

What are you gaining? You said it is all less than 10'. I don't know if I'd even bother with a trough. Just run three conduits from transformer, one to each panel.