Transformers

[Clarence Elkin]

Installing a 30kva trans @ 480 on PRI. When the secondary feed from a transformer feeds a panel board, is a main breaker required, or can you use MLO ? If MLO is ok to be used, what over current protection will the feeders have? Does the primary disconnect serve as the over current protection for the transformer as well as the feeder wires?

 

[augie47]

Re: Transformers

Lots of variables...mostly covered in 240.20...among the unanswered questions:
what is thbe secondary voltage? three phase or single phase? primary wire size and over current device? industrial location?

 

[don_resqcapt19]

Re: Transformers

In general, if the secondary has a neutral, then the secondary condcutors require overcurrent protection. See 240.21(C).
Don

 

[bdarnell]

Re: Transformers

Originally posted by Clarence Elkin:
Installing a 30kva trans @ 480 on PRI.

Since this is no doubt 3 phase ( I don't know of a 30 kVA single phase) if it's delta-wye, see 408.36(D). The exception takes you back to 240.21(C)(1) which applies to single phase and delta-delta only.

As Don said, if it has a neutral, you need a main.

 

[cripple]

Re: Transformers

All panels are required to have an overcurrent device protection (main breaker) per section 408.36. The overcurrent protection device is not always required to be located on the panelboard, but permits the overcurrent protection device to be located at any point on the supply side of the panelboard.
The conductors on the secondary of the transformer are considered feeder tap conductors and are required to be protected to their ampacity with overcurrent protection. In addition, section 240.21(C) covers the requirements for sizing and protecting the conductors on the secondary of transformers. The only way the transformer secondary conductors can be terminated on main lug only panelboards is that they comply with 240(C)(1).

[ February 01, 2006, 11:03 AM: Message edited by: cripple ]

 

[don_resqcapt19]

Re: Transformers

The conductors on the secondary of the transformer are considered feeder tap conductors and are required to be protected to their ampacity with overcurrent protection.

No, they are "transformer secondary conductors" and covered by 240.21(C). Feeder tap conductors are covered by 240.21(B).
Don

 

[cripple]

Re: Transformers

I stand corrected; transformer secondary conductors are tap conductors are they not?
It looks like a tap no overcurrent protection on the terminal on the transformer secondary, the provisions of 240.4(B) cannot be applied and they are not service conductors.

 

[don_resqcapt19]

Re: Transformers

cripple,

I stand corrected; transformer secondary conductors are tap conductors are they not?

They meet the definition of tap conductors, but the overcurrent protection rules for "feeder tap conductors" and "transformer secondary conductors" are not the same. You can't apply the correct code section if you are calling them feeder tap conductors.
Don