Transformer Question

[Dennis Alwon]

I got this pm and I am putting it out to the group.

I have a 1500kva transformer and on the secondary side it is feeding a 800A breaker and a 400A breaker . My question is do they have to be cabled the same.
Like on the 800A breaker i was going to use parrallel 500 MCM , but on the 400A breaker i was going to use parrallel 4/0 ,

Is that OK?

 

[Dennis Alwon]

As I told the OP- I am not the best at this but I think 240.21 would apply-- sounds like taps.

 

[texie]

Assuming 75 degree terminations, the 400 amp breaker is OK and could even use 3/0. The 800 amp breaker will require 600 kcmil to be compliant with 240.21(C). I'm assuming this is 480, 3 phase and wonder why the transformer is so large for 1200 amp worth of breakers.

 

[iwire]

I will add that the 800 amp tap and the 400 amp taps both have to go all the way back to the transformer terminals.

They cannot run 1200 amps of conductors and then tap off those to each OCPD.

 

[augie47]

I will add that the 800 amp tap and the 400 amp taps both have to go all the way back to the transformer terminals.

They cannot run 1200 amps of conductors and then tap off those to each OCPD.

embarrassingly, I ask "why"

 

[iwire]

embarrassingly, I ask "why"

The secondary conductors are taps already and you cannot tap a tap.

 

[texie]

I suppose that I should have mentioned that to be sure this complies with 240.21(C)(2)(4) we would need to know the primary and secondary voltages and the primary OCPD size. I guessing it will but I think the computation is neccessary to be sure.

 

[augie47]

I thought that, but 240.21(C) does not identify transformer secondary conductors as "taps", nor do they fit the definition in 240.2.
They seemingly have feathers and quack, but I'm not sure they are ducks.

 

[iwire]

I think this clears it up, at least the requirement if not the reason.

(C) Transformer Secondary Conductors. A set of conductors
feeding a single load, or each set of conductors
feeding separate loads, shall be permitted to be connected
to a transformer secondary, without overcurrent protection
at the secondary,

 

[texie]

I think this clears it up, at least the requirement if not the reason.

Agreed, but we still need to meet 240.21(C)(2)(4) to be sure we meet the minimum conductor size requirement. This is normaly not an issue when you have a single OCPD on the secondary of a size appropriate for the transformer.

 

[david luchini]

I thought that, but 240.21(C) does not identify transformer secondary conductors as "taps", nor do they fit the definition in 240.2.
They seemingly have feathers and quack, but I'm not sure they are ducks.

I think this clears it up, at least the requirement if not the reason.

See also the second sentence of 240.21 Location in Circuit.

Conductors supplied under the provisions of 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4.

 

[steve66]

I'm pretty sure 2 parallel sets of 500's don't work to feed an 800 A breaker from a transformer.

I think they have to be parallel 600's.

Edit: 240.21(C): "The provisions of 240.4(B) shall not be permitted for transformer secondary conductors."

 

[augie47]

See also the second sentence of 240.21 Location in Circuit.

I'll buy that.
Thanks
(never did it or saw it done, just could not put my finger on "why"... brain farts today)

 

[texie]

Hopefully we'll here from the OP. Is this a trick question? In follow up to my post #7, which everyone is choosing to ignore, I don't think we can answer this without more detail. For example, if this is a 480 X 208 transformer it is not compliant, the 400 amp circuit will be to small. Do the math based on 240.21(C)(2)(4).

 

[texie]

I'm pretty sure 2 parallel sets of 500's don't work to feed an 800 A breaker from a transformer.

I think they have to be parallel 600's.

Edit: 240.21(C): "The provisions of 240.4(B) shall not be permitted for transformer secondary conductors."

See post 3.

 

[iwire]

Is this a trick question? In follow up to my post #7, which everyone is choosing to ignore,

I can't speak for the others but I did not feel the need to say 'ditto'.

 

[weressl]

The secondary conductors are taps already and you cannot tap a tap.

Unless it is an assembly, like an MCC, but they play by diff'rent rules.

(Surprisingly so few understand this.)

 

[iwire]

Unless it is an assembly, like an MCC, but they play by diff'rent rules.

(Surprisingly so few understand this.)

The elves at the factory can do any magic they want.

 

[steve66]

See post 3.

Duh on my part. I missed that.