Overcurrent Protection for 120/208V Panel

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Delcobob

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I have a typical set up for a transformer. I have a 277/480V panelboard with a 3P100A breaker feeding a 75 kva 480V - 120/208V transformer. The secondary side has 3/0 copper conductors (less than 10' long) feeding a 200A 120/208V 42 ckt panelboard. I have always installed a 200A main breaker in the low voltage panel for this installation. Now I have an engineer that tells me the main is not necessary, according to 450.3. I see his point, but I still insist the main is required as per 240.21 (C). Can anyone shed some light on this situation for me? Thanks.
 
... agree with you

... agree with you

Bob,
I think the panel main breaker is not necessary to protect the transformer since you can fall under the primary-only percentage in 450.3.
But in order for the cable to be protected you either need your 200A main, or the sum of the panel breakers must be less than or equal to the ampacity of the 3/0 feeder, per 240.21 (C) as you pointed out.
JM
 
I think 450.3 is misleading. Read 240.21(C)(1) carefully. "Multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device." So unless you have a delta-delta transformer you will need secondary overprotection which is usually most easily provided with a main breaker in the secondary panel.
 
The for this type of installation the secondary conductors require overcurrent protection. Look at 240.4(F).
 
And don't forget 408.36 overcurrent protection for panelboards.

450.3(B) actually deals with the optional sizing of the transformer PRIMARY protection if secondary protection is provided.
 
This is a common issue. The problem is that a lot of code users don't understand that the OCPD rules in Article 450 are only for the protection of the transformer. They have nothing to do with the protection of anything, including the conductors, that are connected to the secondary side of the transformer. Too many code users only look to Article 450 and forget about the conductor protection rules found in 240 and the panel protection rules in 408.
Don
 
If you comply with 240.C.2. the paragraph 240.21.C.2(1)b says that if conductor rating is "Not less that the rating of the device supplied by the conductors". Since the ampacity of conductors are not less than the ampacity 200 amp panel, it could be inferred that no main breaker is required.
I have seen this used to install a panel without a main breaker. However,408.36 says that a main is required.

Changed the paragraph number to 240.21(C)(2)(1)b. Error noted by Infinity below.

Thanks Trevor.
 
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bob said:
If you comply with 240.C.2. the paragraph 240.C.2(1)b says that if conductor rating is "Not less that the rating of the device supplied by the conductors". Since the ampacity of conductors are not less than the ampacity 200 amp panel, it could be inferred that no main breaker is required.
I have seen this used to install a panel without a main breaker. However,408.36 says that a main is required.
Click to expand...


There is no 240.C.2(1)b?:confused:
 
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