Primary XFMR Conductors

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gary b

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I am installing a 15kVA, 480-V 3-phase XFMR. Primary FLA is 18A. Using maximum primary protection only (250%) I am installing 40-amp CB. Using 240.4 am I required to install 8-AWG (60 column, 310.16) or can I use 12 AWG and be compliant?
 
Sorry, miss information. I am using both primary and seconday protection. 250% on primary and 125% on secondary. Can I use 12 AWG for the primary?
 
First use the 75C ratings unless you have checked the equipment and found 60C ratings. The answer is maybe. Look at the tap rules beginning with 240.21. See if your installation qualifies for A, B or C. Look at B3.
 
I thought you used the 60 degree column (110.14(C)(1)(a) for 100 amps or less unless you check the listings for higher temps then you can use the 75 degree column.
That said, my question is if you raise the OCPD using the percentages listed in Table 450.3(B) is it required (or even a good idea) to use a larger corresponding conductor ampacity rating as per 240.4? I thought the tap rules in 240.21 (C) applied the secondary not the primary conductors.
 
That said, my question is if you raise the OCPD using the percentages listed in Table 450.3(B) is it required (or even a good idea) to use a larger corresponding conductor ampacity rating as per 240.4?
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The rules of 450.3B are for the protection of the transformer only. In general, the primary protection is set at 125% of the primary current. Due to the transformer inrush current or some other load on the transformer it may be necessary to increase this rating and the max setting is 250%.

The primay conductors must be protected at or below their ampacity.
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Don
I agree with the statement but according to the tap rules, the primary OC device may be set at a higher rating that the ampacity of the primary conductors. The conductors are apparently considered protected when these provisions are met.
 
Bob,
Yes you can use the tap rule in 240.21(B)(3), but it is difficult to comply with in the real world. I believe that is the only tap rule that would apply to the primary conductors.
Don
 
gary b said:
I thought you used the 60 degree column (110.14(C)(1)(a) for 100 amps or less unless you check the listings for higher temps then you can use the 75 degree column.
That said, my question is if you raise the OCPD using the percentages listed in Table 450.3(B) is it required (or even a good idea) to use a larger corresponding conductor ampacity rating as per 240.4? I thought the tap rules in 240.21 (C) applied the secondary not the primary conductors.
Click to expand...
Just remember you can go by the 75degree column if all terminals in that circuit are rated at 75 degree or more even if it's under 100 amps. But if there is a 60 terminal in that circuit then you do have to us the 60 degree column.
 
Interestingly Exhibit 240.9 in the NEC Handbook shows an 8 AWG THHW CU primary feeder conductor protected by a 110 Amp fuse on the transformer illustration. This is way above the ampere rating of the wire. I believe the illustrator is showing an example of applying the maximum rating of the 250% rule to the conductor installation. This OCPD provides short-circuit protection, and I think the secondary OCPD would provide O.L. protection. I am not sure about this though. I understand this to be ?maximum? rating and if the XFMR will work on a 50 amp CB that is what should be installed.
 
Maybe there is an easy way to explain this. Both the conductor and the primary winding of the XFMR must be protected from overcurrent. The feeder wire to the primary is sized based on the FLA of the primary winding adjusted for the Table 450.3(B) percentages (if used). The OCPD is sized based on the wire selected to meet the calculated load of the primary. As long as the OCPD protecting the conductor is within 250% of the XFMR primary FLA the OCPD can do double duty and protect the primary winding without the need to add an addition OCPD at the XFMR. I believe there are many electricians out there that miss this and are not matching the primary conductor to the OCPD!
 
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