Transformer Question

Status
Not open for further replies.
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I got this pm and I am putting it out to the group.

husmball said:
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?
Click to expand...
 
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.
 
Last edited:
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.
 
iwire said:
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.
Click to expand...

embarrassingly, I ask "why"
 
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.
 
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.

(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,
Click to expand...
 
iwire said:
I think this clears it up, at least the requirement if not the reason.
Click to expand...

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.
 
augie47 said:
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.
Click to expand...

iwire said:
I think this clears it up, at least the requirement if not the reason.
Click to expand...

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.
Click to expand...
 
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."
 
Last edited:
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).
 
steve66 said:
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."
Click to expand...

See post 3.
 
Status
Not open for further replies.
Top