112kva XFMR to twin 200a panels

[Pipeandwire]

Hey guys,

trying to setup a the following:

(1) 175a feed to 112kva transformer primary 480volt delta to 120/208y secondary's.

I would like to double barrel the secondary taps and feed 2 separate 200a panels with each having their own 200a mains.

no protection from xfmr to panels -

the secondary's and panels would be within 10' of the transformer. I've read some conflicting responses throughout the innerweb and was hoping to get some clarification here.
following NEC.

thanks looking forward to any help

 

[augie47]

It is permissible.
450.3(b) allows the install as far as transformer protection as your primary is protected at no more than 125% FLA (next size up allowed), your secondary conductors meet 240.21(C) as far as overcurrent and your panels meet 408.36

 

[ron]

I believe Secondary overcurrent protection is required because it is a Delta-Wye Transformer. 240.21(C)(1) says Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

Note 2 to Table 450,3(B)
Where secondary overcurrent protection is required, the secondary overcurrent device shall he permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device.

There is a 350A breaker listed in 240.6 Standard Ampere Ratings, so (2) 200's would be too much.

 

[ActionDave]

I believe Secondary overcurrent protection is required because it is a Delta-Wye Transformer. 240.21(C)(1) says Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device....

He has his secondary overcurrent protection in accordance with 240.21(C)(2) though as long as his conductors are ten feet or less, individually, for both panels.

 

[augie47]

I believe Secondary overcurrent protection is required because it is a Delta-Wye Transformer. 240.21(C)(1) says Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

Note 2 to Table 450,3(B)
Where secondary overcurrent protection is required, the secondary overcurrent device shall he permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device.

There is a 350A breaker listed in 240.6 Standard Ampere Ratings, so (2) 200's would be too much.

You have to look at it as a two step process: Art 450 from Transformer protection and Art 240 for conductor protection. You will note in Table 450.3(b) in the 1st row that for transformer protection, "Primary only Protection" is allowed if the primary protection is 125% (Note 1 =next size up). His install meets that criteria so as far as the transformer protection he is fine.

For step 2, the conductors, you go to 240.21(C), "Transformer Secondary Conductors" where, as you note, secondary protection in required to protect the conductors. That protection would need to meet the criteria in 240.21(C)(2)-(6) with which he complies

 

[david luchini]

There is a 350A breaker listed in 240.6 Standard Ampere Ratings, so (2) 200's would be too much.

As Gus mentioned, if the transformer is being protected as primary only, then the secondary protection could be anything.

That being said, IF the transformer was primary and secondary protected per 450.3(B), the allowable rating of the secondary protection would be 400A, so (2) 200's would be acceptable.