Ampacity for a single unit

[rhamblin]

Is there a spot in the code that allows you to run higher ampacities than normal if you are only feeding one piece of equipment? Someone told me there was but I can’t find anything.

 

[infinity]

Can you provide some more information, what do you mean by "higher ampacities" are you referring to an OCPD size greater than than the conductor size?

 

[rhamblin]

Can you provide some more information, what do you mean by "higher ampacities" are you referring to an OCPD size greater than than the conductor size?

Yes, you are correct. Running a larger OCPD to smaller conductors.

 

[sameguy]

?hvac application?
What do you want to do?

 

[rhamblin]

?hvac application?
What do you want to do?

I don’t remember the particulars, but it was for a condenser for An AC unit. I opted to size everything the normal way. Just wondering what exceptions I can use next time.

 

[Dennis Alwon]

You can often do this with a/c units or heat pumps and in fact for most motors. This can be done because those units have overload protection built into them which will protect the conductor from overload.

 

[Dennis Alwon]

An example-- Heat pump with a minimum circuit ampacity of 12 amps but a max cir. ampacity of 25 amps. 14 gauge wire is all you need with a 25 amp breaker

 

[rhamblin]

An example-- Heat pump with a minimum circuit ampacity of 12 amps but a max cir. ampacity of 25 amps. 14 gauge wire is all you need with a 25 amp breaker

What code are allows this? 430 motors, 440 A/C units or somewhere else?

 

[Vince1971]

data plate on the equipment - MCA & MOP.

 

[Dennis Alwon]

What code are allows this? 430 motors, 440 A/C units or somewhere else?

Look at Table 430.52-- for an inverse time breaker you can use an overcurrent protective device at 250% of the FLC

Section 430.22 states we must use 125% of the FLC for the conductor size. So you see you can have a smaller conductor then the overcurrent protective device. The overload can be built into the motor but if not then it must be sized by 430.32. For most motors that will also be 125%

 

[jumper]

What code are allows this? 430 motors, 440 A/C units or somewhere else?

See 240.4(G) which exempts motors and such from 240.4(D).

 

[rhamblin]

Another situation, please check calcs.

Another situation, please check calcs.

I am installing a compressor from another plant. The Air Compressor nameplate lists Input power at 120 kW and 161 HP. It is 3 phase 480V, I calculate this to be 145 amps (rounded up). 125% of that is 182 amps, so 3/0 conductor. According to T430.52, since we are using a Inverse Time breaker, we can run 250% on the OCPD. Because 360 Amps is not a standard size, we should be able to go up to a 400 Amp breaker. Does this sound right? 400 amp breaker with 3/0 THHN conductors? I plan to add a lock to the breaker so there won't be a separate disconnect.