FEEDER PROTECTION

[WA_Sparky]

Are there any possible issues with the hypothetical situation below regarding not having protection between the utility transformer and the bus feeding two multi family building services. The service disconnects and their respective feeders are sized for their calculated loads. The gear containing the two service disconnects and its feeder are sized for the combined loads of both services. Are we required to provide an additional means of disconnect to protect transformers secondary feeder?

The issue that is coming to mind is in a worst case scenario (even though it should never happen) what if the two services at a later date creep up above 1200A combined, 1300A? Their disconnects may not trip if each individual service is still below their designed limits but the 1200A feeder and bus would burn up. I cant see anything in the code book that says its needed. Is it assumed that the transformer is equipped with a fuse that would protect this feeder?

 

[david luchini]

Are there any possible issues with the hypothetical situation below regarding not having protection between the utility transformer and the bus feeding two multi family building services. The service disconnects and their respective feeders are sized for their calculated loads. The gear containing the two service disconnects and its feeder are sized for the combined loads of both services. Are we required to provide an additional means of disconnect to protect transformers secondary feeder?

The issue that is coming to mind is in a worst case scenario (even though it should never happen) what if the two services at a later date creep up above 1200A combined, 1300A? Their disconnects may not trip if each individual service is still below their designed limits but the 1200A feeder and bus would burn up. I cant see anything in the code book that says its needed. Is it assumed that the transformer is equipped with a fuse that would protect this feeder?

That seems as if it would comply with 230.90(A) ex. no. 3.

(As an aside, you're using the terms "service" and "feeder" backwards where highlighted above....there is one service and two feeders (800A and 1000A.))

 

[augie47]

Doubtful you can depend on the transformer to protect the feeder.
As drawn, do you not have a calculated load of 1425 on a 1200 amp buss and feeder ??

 

[david luchini]

As drawn, do you not have a calculated load of 1425 on a 1200 amp buss and feeder ??

Assuming Optional Method Multifamily Dwelling demand factors...There could be a demand factors, for instance, for 20 units on the first feeder and for 30 units on the second feeder. The service would then see the demand factor for 50 units, so it's not a straight addition of the two feeder loads.

 

[WA_Sparky]

Assuming Optional Method Multifamily Dwelling demand factors...There could be a demand factors, for instance, for 20 units on the first feeder and for 30 units on the second feeder. The service would then see the demand factor for 50 units, so it's not a straight addition of the two feeder loads.

Correct. The closer to the utility gear the less demand seen at the equipment due to 220.84 Dwelling unit demand factors. 800A feeds like 40% of the units the 1000A feeds 60% and service gear 1200A gets the demand for all 100% connected units.