No secondary protection for lighting circuit.

[RTW]

I have a lighting circuit without over current protection on the secondary side of the transformer. The feeder for the primary side of the transformer originates at a bus box 100 feet away from the transformer. Power to the transformer comes by way of (2) #12awg with a 15 amp circuit breaker 3 feet ahead of the transformer. The transformer is a 3KVA, 1 phase, 480/120. The secondary side or the transformer feeds six, 120 volt, 175 watt lights, by way of (4) #10awg which originate at the transformer and peel off as they go out to the lights. The lights are located 150 feet away from the transformer. Again the secondary side has no over current protection. I have read sections 450 and 240 in the NEC and become more confused after each reading. Can anyone direct me to a wiring diagram that shows how to apply these sections to the example I have described?

 

[david luchini]

From what you describe, I think you have two violations.

The first is the protection of the secondary conductors. 240.21(C)(1) allows the secondary conductors on a single phase, 2 wire transformer to be protected by the primary OCPD, IF the primary OCPD is in accordance with 450.3 and if the primary OCPD is not greater than the value determined by multiplying the secondary conductor ampacity by the secondaryrimary transformer ratio.

The #10 secondary has an ampacity of 35, multiplied by 1/4 = 8.75. The 15A primary OCPD exceeds this value, so the #10 is not properly protected by the primary OCPD.

The second violation is transformer protection per 450.3. Transformers don't require secondary OCPD IF they meet the requirements of "primary only protection" in T450.3(B). For primary only protection of a transformer with a primary current less than 9 Amps, (the 3kVA has a primary current of 6.25A) the max rating of the primary OCPD is 167% of rated current. This would give a max primary OCPD of 10.44A. The 15A c/b on the primary exceeds this, and therefore is in violation.

If you put a 30A c/b in the secondary, then you would be in compliance with both sections.

 

[RTW]

Thanks David, but I am trying to maintain the no secondary protection going out to the lights and therefore the confusion begins.

 

[david luchini]

Thanks David, but I am trying to maintain the no secondary protection going out to the lights and therefore the confusion begins.

Replace the 15A c/b on the primary with 5A fuses. This would give you compliance with the "primary only" transformer protection, and would properly protect the #10 secondary conductors.

 

[RTW]

David, Thanks so much for your help.