I am looking at the calculations example in the 2008 and 2011 NEC handbook of NEC 430.110. First it says the minimum ampere rating of the disconnecting means is 61.3A (1.15 x 53.3A) which I agree with. Then looking up the LRC of 220.1A it says you need 15HP. Then it says that the 15HP fails to meet the minimum current requirement of 61.3A which I agree. It says to go to the next size up of 20HP which obviously meets the 220.1A LRC and able to handle the full load rating of 53.3A. But that seems to contradict the 61.3A requirement as the 20HP 230V motor is 54A. It seems to me based on the example you would be required to use a minimum 25HP disconnect in order to handle 61.3A. In the real world, I would have used a SQ D 100A disconnect rated at 40HP, but according to their example I could have used a 60A disconnect rated at 20HP based on 3 phase 240V.
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430.110 Handbook Question for Disconnect Size
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Originally posted by Bjenks View PostI am looking at the calculations example in the 2008 and 2011 NEC handbook of NEC 430.110. First it says the minimum ampere rating of the disconnecting means is 61.3A (1.15 x 53.3A) which I agree with. Then looking up the LRC of 220.1A it says you need 15HP. Then it says that the 15HP fails to meet the minimum current requirement of 61.3A which I agree. It says to go to the next size up of 20HP which obviously meets the 220.1A LRC and able to handle the full load rating of 53.3A. But that seems to contradict the 61.3A requirement as the 20HP 230V motor is 54A. It seems to me based on the example you would be required to use a minimum 25HP disconnect in order to handle 61.3A. In the real world, I would have used a SQ D 100A disconnect rated at 40HP, but according to their example I could have used a 60A disconnect rated at 20HP based on 3 phase 240V.

Ok, now you got me thinking that 430.110 (A) talks about a disconnect that has an amp rating only (such as generaluse switch, circuit breaker, molded case switch, or attachment plug and receptacle) which you just go 115% of the single motors fullload current rating. However if it an unfused motor rated disconnect you can forget about the 115% and do 100% of the current rating and HP.
430.110(C) tells you how to come up with an equivalent single motor HP and Amp rating value for multiple motors and resistance loads on a single disconnect of any type.
430.110(C)(1) focuses on the HP rating by taking the larger of the total amps without the 115% and the LRC using 430 tables. The exception is that if you are using a disconnect with an amp rating so large that it can continuously handle the LRC and the resistance equipments current then you can do a HP rating without adding the resistance loads.
430.110(C)(2) says the same thing as 430.110(A) for nonmotor rated switch methods you have to use 115% of the motors current. The exception then says if you have a listed unfused motor rated switch you can forget about the 115% and do 100% of the current rating and HP of the hypothetical single motor.
So I think under Solution it starts out just talking about a disconnecting means which would require the HP rating and 115% Amp rating. which is 61.3A and 15HP with the 61.3A making you go with a 20HP).
But with a HP rated nonfused disconnect you could go with the 20HP rated at 53.3A. But I think I am grabbing at straws to have this make sense. The key would be that for motor rated disconnects I just use 100% for the Amps and HP. For nonhp rated I would use 115% of the current, but I am not sure if this is flawed.Proverbs 16:3
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