Art. 450

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paul renshaw

paul renshaw

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Cordova, Maryland
Please correct me if I am wrong because I think I am ready to pull my hair out. Table 450.3(B) contains the sizing of OCPD for xfmrs, when you size from this, don't the conductors have to be protected per Art. 240? I have some installations that I will go into here that do not seem right to me. The local inspector is telling me that the conductors can be fused to these percentages also. I know motor ground fault and short circuit protection will many times be rated at higher than the conductors because of the overload protective devices, but I did not think that was the case with xfmrs. But, as my wife can attest to, I have been wrong and confused many times before.
 
You are correct, Table 450.3(B) is for sizing overcurrent protection for the transformer itself.

240.21(C) gives you the requirements for overcurrent protection of transformer secondary conductors.

Only 240.21(C)(1) permits you to use the transformer primary overcurrent device to protect the secondary conductors and only under the conditions specified in 240.21(C)(1).

Chris
 
Without getting too long winded, the secondary amps of one xfmr is 125 amps, they have #1 cu leaving the xfmr and going to a 150 amp breaker. The other xfmr has secondary at 156 amps with 2/0 cu going into a 200 amp breaker. It seems to me that the conductors are fused too high in both instances. What am I missing here?
 
Looks to me like they either used the 90 C column of 310.16 or thought they could apply 240.4(B).

Either way I agree they are in violation.
 
My problem with this is that the inspector is the one telling the GC to install this size wire for the secondarys, the original conductors were #2. How do I tactfully tell this guy( who I do not really want to pi$$ off) that he really is wrong and may not fully understand ART 450 ?
 
You might want to politely ask him where in 240.21(C) it permits what he is telling the EC. It might also be benificial to point out the last sentence of 240.21(C) where it says the the provisions of 240.4(B) shall not be permitted for transformer secondary conductors. This sentence was added to the 2005 NEC so he might not be aware of it.

Chris
 
I found a Square D bulletin that came with a xfmr and it backs what I told him, I also found some excerpts from articles on the web that say the same thing, hopefully he will learn from it and not take it personally.
 
So no matter whether primary or secondary, the wires have to be protected according to Art. 240? He was telling me that #2 could be fused at 200 amps because the primary OCPD can be up to 250%.
 
paul renshaw said:
So no matter whether primary or secondary, the wires have to be protected according to Art. 240?
Click to expand...

Correct, the secondary conductors from a transformer must be protected in accordance with 240.21(C)

paul renshaw said:
He was telling me that #2 could be fused at 200 amps because the primary OCPD can be up to 250%.
Click to expand...

Again, Table 450.3(B) is for overcurrent protection of the transformer primary and secondary windings only not the secondary conductors.

Chris
 
Not having the fear of looking dumb, I'm going to ask the following question:

Are there two sets of overcurrent protection installed (one for the transformer and one for the conductor)? Unless the transformer has built-in protection, I am having a hard time understanding a design where there is primary/secondary OCP for the transformer preceeded/followed by conductor OCP.

Please help me fill in the missing link.
 
Montanaboy said:
Not having the fear of looking dumb, I'm going to ask the following question:

Are there two sets of overcurrent protection installed (one for the transformer and one for the conductor)?
Click to expand...

When using primary protection only, Yes. Primary set at no more than 125%. Conductors protected per 240.21(C)
Montanaboy said:
Unless the transformer has built-in protection, I am having a hard time understanding a design where there is primary/secondary OCP for the transformer preceeded/followed by conductor OCP.
Click to expand...

when using primary and seconary protection, there will be ocp on both sides. The secondary ocpd can also be used to protect the secondary conductors if properly sized.
Rick
 
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