Sizing Long Tap Feeders

Status
Not open for further replies.
M

muhandas

Senior Member
A feeder tap protected by 125A Circuit Breaker in a subpanel connects to a 480:208/120 Transformer. I understand that the feeder tap provisions of 240.21(B)(3) concerning the sizing of the primary transformer conductors apply if the total length of the primary and secondary conductors is no greater than 25 ft. However, if the length does exceed 25 ft. is it correct to apply 210.20 for the sizing of both the primary conductor and the OCPD in the subpanel using the load connected to the transformer as a criterion (assuming, of course, that none of the other conditions such as manufacturing buildings, etc. come into play)?
Many thanks for your comments.
Heinz R.
 
The tap rules allow you to connect a conductor to a feeder that has an OC device rated higher that the ampacity of the tap conductors. If you can not apply the rules241.21.B 1 thru 5 then 241.21A says to use 210.19. Don't forget to check table 450.3(B) for the transformer protection.
How long is your circuit?
 
Tx. for quick reply, Bob. I'm doing a plan review and don't know exactly how long the runs will be, just wanted to provide a "heads up" to both the architect and the eventual inspectors. I had checked the transformer protection table 450.3(B) and things are ok there. In our case, the transformer is rated 75KVA, the demand transformer load is 61.4 KVA, the OCPD for the feeder that leads to the transformer primary is 125A and the feeder conductors are #3. That calculates to a demand primary current of about 74A. That would be OK in context of 241.21(B)(3) but if the length of the conductors exceeds 25 ft. it seems to me that 210.20 needs to be applied and, therefore, the OCPD will need to be reduced to protect the 100A ampacity of the feeders (which are sized correctly even for 210.20). Am I missing something?
 
Heinz,
That would be OK in context of 241.21(B)(3) but if the length of the conductors exceeds 25 ft. it seems to me that 210.20 needs to be applied and, therefore, the OCPD will need to be reduced to protect the 100A ampacity of the feeders (which are sized correctly even for 210.20). Am I missing something?
Click to expand...
You can't protect the secondary conductors of this transformer with the primary OCPD. See 240.21(C)(1). You must protect the secondary conductors per the tap rules in 240.21(C). If you can't use one of these rules based on the location of the panel, then you will have to provide an OCPD for the secondary conductors at the transformer.
Don
 
Status
Not open for further replies.
Top