number of taps

[mshields]

I've encountered a system consisting of a 150A CB in Nema 1 enclosure on the secondary of a 45kVA step down XFRM. The 150A feeds 1/0's in a wireway which are presently tapped 3 times with #6's to 3, 50A panels each with 50/3 MCB.

Wisdom of a 4th 50A tap not withstanding, is their a prohibition against tapping this 150A feeder a 4th time.

Client seems to think that we would need to increase the size of the wire in the wireway to be 200A rated; i.e. change out the 1/0 to 3/0.

I don't believe that to be the case.

Who's right?

 

[Carultch]

I've encountered a system consisting of a 150A CB in Nema 1 enclosure on the secondary of a 45kVA step down XFRM. The 150A feeds 1/0's in a wireway which are presently tapped 3 times with #6's to 3, 50A panels each with 50/3 MCB.

Wisdom of a 4th 50A tap not withstanding, is their a prohibition against tapping this 150A feeder a 4th time.

Client seems to think that we would need to increase the size of the wire in the wireway to be 200A rated; i.e. change out the 1/0 to 3/0.

I don't believe that to be the case.

Who's right?

If there is an overload, the 150A main would trip and catch it before the 1/0 wire fails.

This is the same reason why you can have a sum total of 400A worth of breakers in a 200A panel. It will ultimately be protected by the 200A main if there is a problem, and with enough load diversity, it will be very unlikely that all branch circuits operate at full load anyway.

Intuitively, I would agree with you. And if there is a chance that all four taps operate at once, I would advise on upgrading to a 200A feeder & OCPD to provent nuisance tripping. But there is no part of the NEC that limits the total amps of all the taps, to the total amps of the feeder.

 

[kwired]

If your total load is only 90 amps there may be nothing wrong with a fifth or even sixth tap.

If you did go with 200 amp conductor and 200 amp overcurrent device you still only have 45 kVA transformer, assuming 208 volts, the transformer is the weak link in the chain if you actually have more then 125 amps of load.

 

[Daniel malack]

I've encountered a system consisting of a 150A CB in Nema 1 enclosure on the secondary of a 45kVA step down XFRM. The 150A feeds 1/0's in a wireway which are presently tapped 3 times with #6's to 3, 50A panels each with 50/3 MCB.

Wisdom of a 4th 50A tap not withstanding, is their a prohibition against tapping this 150A feeder a 4th time.

Client seems to think that we would need to increase the size of the wire in the wireway to be 200A rated; i.e. change out the 1/0 to 3/0.

I don't believe that to be the case.

Who's right?

Determine the existing load and go from their. Just because you have 3 50 amp breakers doesn't mean you add them together to make 150 amp.

Sent from my SM-T350 using Tapatalk

 

[petersonra]

I've encountered a system consisting of a 150A CB in Nema 1 enclosure on the secondary of a 45kVA step down XFRM. The 150A feeds 1/0's in a wireway which are presently tapped 3 times with #6's to 3, 50A panels each with 50/3 MCB.

Wisdom of a 4th 50A tap not withstanding, is their a prohibition against tapping this 150A feeder a 4th time.

Client seems to think that we would need to increase the size of the wire in the wireway to be 200A rated; i.e. change out the 1/0 to 3/0.

I don't believe that to be the case.

Who's right?

You can tap it 100 times if you want to. You can have 5000 Amps worth of CBs downstream too. No matter what you do the 150A feeder breaker will protect the 1/0 conductors.

Really it is all about what the load calculation tells you. That is the only "wisdom" involved in deciding if it is Ok to tap again.