Three-phase three-wire secondary

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rbb

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A 150KVA 480/240v three-phase delta-delta transformer is protected primary side by a 225-amp circuit breaker. The secondary is corner grounded and the secondary conductors are comprised of 2-4/0 AWG THWN-2 Cu conductors per phase and a No. 2 AWG bonding jumper in each of two 2-1/2 inch conduits. The secondary conductors run 150 ft. to an auxiliary gutter where they are tapped to supply five 60-amp fused switches and two 100-amp fused switches. The calculated load is 360-amps.

1) Does this installation comply with the code?

2) What is the required bonding jumper size between the auxiliary gutter and the 100-amp switches and the 60-amp switches assuming the tap conductors are No. 1 Cu. and No. 4 Cu respectively?
 
Re: Three-phase three-wire secondary

I may be wrong but I dont believe the secondary can run that far without overcurrent protection to begin with.
 
Re: Three-phase three-wire secondary

I would say #6 copper Or #4 alum. for the #1.
and #8 copper or #6 alum. for the #4.
250.66
 
Re: Three-phase three-wire secondary

You will be tapping a tap at the gutter for your disconnects. Thats a violation. You will have to use the same size conductors as the secondaries from the transformer to the gutter.
 
Re: Three-phase three-wire secondary

bwilson:

The secondary conductors are protected at their rated ampacity. How is this tapping a tap?
 
Re: Three-phase three-wire secondary

I see nothing in the first post about OCP at the gutter. First the tap conductors from the transformer to the first OCP cannot be over 10 ft. unless industrial or outside. 240.21(c)(2),(3),(4). Second unless these conductors have OCP at there terminations another tap (at the gutter) is a violation of 240.21. Unless I am compleatly out to lunch this is the way I see it. If not please let me know
 
Re: Three-phase three-wire secondary

Sorry about the first statement. I see it is a delta - delta transformer. But I believe that the sencond part is correct. You cannot tap a tap
 
Re: Three-phase three-wire secondary

In this situation,I would say the conductors from the xfmr to the gutter is a feeder and from the gutter to the disconnects is a tap.definition of feeder is all circuit conductors between the service equipment,the source of a seperately derived system,or other power supply source and the final branch circuit overcurrent device.
 
Re: Three-phase three-wire secondary

Tap conductor: as used in this article, a tap conductor is defined as a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4. In this case you would have #1 and #4 on the secondarie side of the transformer protected by a 225A OCP on the primary side of the transformer
 
Re: Three-phase three-wire secondary

Yes the conductors from the parallel 4/0's to the disconnects are tap conductors but the 4/0's themselves are not tap conductors,they are properly protected by the 225 amp primary overcurrent protection device.read the highlighted part in the handbook just after 240.4f
 
Re: Three-phase three-wire secondary

Read the definition of a tap conductor in Art. 240
By definition the conductors from the secondary lugs of the transformer are taps as are the conductors from the gutter to the disconnects.
 
Re: Three-phase three-wire secondary

not with a 3 phase delta delta transformer having a 3 wire single voltage secondary with it's primary protected as it is.If it were 240/120v system or other it would be a violation,but I do'nt feel it is in this instance.
 
Re: Three-phase three-wire secondary

Bwilson, I have read the definition and in this case I do not see that these are tap conductors. :)
 
Re: Three-phase three-wire secondary

In this case you would have #1 and #4 on the secondarie side of the transformer protected by a 225A OCP on the primary side of the transformer.

which should be fine according to 240.21b2 the ampacity of the #1 is 130 amps and the ampacity of the #4 is 85 amps. 1/3 of 225 = 75 amps.
 
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