Minimum Feeder Size

[steve66]

I have an existing 480V feeder I plan to tap to feed a new toilet building (via a new transformer. The load will be very small (a few vending machines, 3 lights, an Exhaust fan, a water fountain, and a Hot water heater.

225.39(D) (2002 NEC)seems to require a minimum of a 60A disconnect on the primary of the transformer. Does anything NEC code require a minimum size for the feeder or the transformer? I was thinking of using a 15KVA xformer with #10 or #12 feeder.

Steve

 

[charlie b]

Re: Minimum Feeder Size

I believe the minimum 60 amp disconnect is talking about the disconnect of the 120/240 coming into the building. It is not telling you that a transformer's primary protection must be at least 60 amps. You can't use a 60 amp breaker as the feeder to at 15KVA transformer.

You can, however, use a 30 amp breaker and #10 conductors for the primary feeder, and use a 60 amp overcurrent device as the building's disconnecting means.

But I'm confused about something. Is the transformer going to be inside the building? If not, then see my discussion above. If the transformer is inside the building, then I think you can do this:

(1) Provide a overcurrent protection for the primary feeder (i.e., from the upstream panel) with a rating no higher than 45 amps (i.e., 250% of 15KVA at 480 volts). Pick a feeder conductor that can be protected by that same breaker.

(2) Place a 60 amp rated, non-fused, disconnect switch at the entrance to the building. This will satisfy 225.39(D).

 

[steve66]

Re: Minimum Feeder Size

Strange, service conductors would have to be sized for the full 60 amps per 230.42(B). Outside feeder requirements often parallel service requirements, but I guess this is one case where they are different.

The transformer will be inside the building, so it's primary disconnect will be the building disconnect. It just seems really odd to require a 60 amp disconnect for the primary of a 30KVA transformer.

Steve

 

[steve66]

Re: Minimum Feeder Size

Now I'm even more confused. What exactly does a 60A disconnect rating mean? If I use a circuit breaker enclosure and install a 100A frame breaker that is only rated at 45 amps, is that a 60A disconnect??

Steve

 

[charlie b]

Re: Minimum Feeder Size

I believe that a 100 amp frame, 45 amp trip breaker would satisfy the "minimum 60 amp disconnect" rule. Yes, it is strange.

 

[john m. caloggero]

Re: Minimum Feeder Size

Section 225.39(D) requires a disconnecting means rated not less than 60 amperes. By definition in Article 100, a disconnecting means is a device by which the conductors of a circuit can be disconnected from their source of supply. Since the disconnecting means is a circuit breaker, a 45 ampere rated breaker in a 100 ampere frame does NOT meet the requirement for a 60 ampere disconnecting means.

 

[don_resqcapt19]

Re: Minimum Feeder Size

John,
But a 60 amp fusible disconnect with 45 amp fuses would?
Don

 

[steve66]

Re: Minimum Feeder Size

Posted by Mr. Caloggero:

Since the disconnecting means is a circuit breaker, a 45 ampere rated breaker in a 100 ampere frame does NOT meet the requirement for a 60 ampere disconnecting means.

But I can't protect my 15 KVA xformer with a 60A breaker, so now I need two disconnects in series. One for the 60A service disconnect, and one for the transfomer protection. I just don't see any way that makes the installation safer?

I'm still wondering if the intent of the code was to require a minimum of a 60A feeder? That would seem reasonable for a 208V service, but it seems like overkill when feeding a outhouse from 480 volts.

Steve

 

[charlie b]

Re: Minimum Feeder Size

It seems like the code is saying that there is no building that can get by with less than 60 amps. But I really think, as I said earlier, that the intent was related to 60 amps worth of 120 volt loads. That is not, as I must admit, what the words say. But I think that is what is needed.

I see two solutions, other than your "two disconnects in series" solution. One is to place the transformer outside the building, and use that panel's main breaker (inside) as the disconnecting means. The other is to request a variance from the AHJ.

 

[steve66]

Re: Minimum Feeder Size

I guess this isn't really a problem. I'll just make the breaker at the first building a 45 amp to protect the transformer. Then the breaker at the transformer can be a 60A breaker as a main disconnect

Steve