wire size

[j rae]

I have a garbage disposal that draws FLA 24 amps. Nema 2 size starter with approite overloads. Can I use #12 on a 30 amp ckt breaker? I'm thinking of a motor ckt protection but need code number. THANKS

 

[augie47]

I think that's an excellent question.
I don;t want to make too much out of it, but I highlighted many possibilities in my mind,.
First...422 references us back to 430 Part II
Secondly, sizing of conductors is normally based on the Table 430.248 values, but if you have no HP.
422 references us back to 430 Part II
Third, if the motor is so marked, then 430.22(E) might be in place.

If we assume the rating should be 125% of nameplate then a 30 amp conductor would be sufficient, keeping in mind that NM is a 60? cable and only has an ampacity of 25 amps.

 

[j rae]

I think that's an excellent question.
I don;t want to make too much out of it, but I highlighted many possibilities in my mind,.
First...422 references us back to 430 Part II
Secondly, sizing of conductors is normally based on the Table 430.248 values, but if you have no HP.
422 references us back to 430 Part II
Third, if the motor is so marked, then 430.22(E) might be in place.

If we assume the rating should be 125% of nameplate then a 30 amp conductor would be sufficient, keeping in mind that NM is a 60? cable and only has an ampacity of 25 amps.

It is a 7.5 HP at 208 volt!!

 

[Dennis Alwon]

I don't see how a #12 would work. 24 amps at 125% is 30 amps. 12 is only good for 25 amps at 75C

 

[jumper]

I don't see how a #12 would work. 24 amps at 125% is 30 amps. 12 is only good for 25 amps at 75C

I assume that this is directed at OP and not Gus.

Gus just said:

If we assume the rating should be 125% of nameplate then a 30 amp conductor would be sufficient, keeping in mind that NM is a 60? cable and only has an ampacity of 25 amps.

 

[texie]

I don't see how a #12 would work. 24 amps at 125% is 30 amps. 12 is only good for 25 amps at 75C

I agree. If this a true Art 430 application (as opposed to an appliance) the table value for this motor is 24.2. 24.2X1.25 = 30.25 required ampacity. #12 is OK at 90 degree but not at 75 degree so you would need to go to a #10.
But this begs another question similar to the discussion a few weeks ago about rounding. Just for sake of discussion, is it 30 amps or 30.2 amps? Back to the discussion of weather Art 220 rounding applies to all calculations.

 

[Smart $]

...
But this begs another question similar to the discussion a few weeks ago about rounding. Just for sake of discussion, is it 30 amps or 30.2 amps? Back to the discussion of weather Art 220 rounding applies to all calculations.

I would say 30A, not 30.2A. Reasoning is 220.5(B) rounding in combination with 220.3 and its table, which opens the door to all calculations under those articles... unless specifically stated otherwise in those articles.

 

[texie]

I would say 30A, not 30.2A. Reasoning is 220.5(B) rounding in combination with 220.3 and its table, which opens the door to all calculations under those articles... unless specifically stated otherwise in those articles.

I'm inclined to agree and I think that is the way Mike Holt and many others teach it, but as you may recall this was hotly debated. In any event, in the OP's case, 30 or 30.2 amps won't cut it for 75 degree with a #12 THHN conductor.

 

[Dennis Alwon]

I assume that this is directed at OP and not Gus.

Gus just said:

you assumed right............

 

[hurk27]

Am I missing something?

430.6(A)(1) requires us to use the tables even if the HP is not listed on the name plate?

If this is a single phase motor then table 430.248 will require conductors for 44 amps.

If this is a three phase motor then table 430.250 will require conductors for 24.2 amps.

The above is for a 7.5hp motor at 208 volts circuit voltage.

I'm not sure I would not consider a garbage disposal motor to be considered a continuous load or the motor rated as continuous duty as 430.22 would require the 125%, but if the motor is marked with a duty rating then you could apply table 430.22 I think?

The funny thing I find with tables 430.248 and .250 are the over kill in the ratings, as using the old fashion way of 746 watts per HP a 7.5 single phase motor @208 volts would only be 26.9 amps and a three phase would be 15.5 amps.

Also if the motor has its own over current protection then we can size the OCPD for the SCGF from table 430.52

I don't see where the OP'er mention NM and I would assume this is a commercial installation from the size of it and the 208 volt rating which is a 3-phase WYE voltage, the FLA current rating on the motor seems that it is a 3-phase motor but I could be wrong? so I would think THHN/THWN conductors @75?C

Don't mind me as Im not very sharp on motors and the NEC.

 

[jim dungar]

The funny thing I find with tables 430.248 and .250 are the over kill in the ratings, as using the old fashion way of 746 watts per HP a 7.5 single phase motor @208 volts would only be 26.9 amps and a three phase would be 15.5 amps.

HP is the output of the motor, yor conversion provides the output KW of the motor. The wire needs to be sized based on the input KW to the motor. The differences between output and input involves the PF and Efficiency of the motor. We know that PF and Eff vary with the loading of the motor, so you need to find the worst case value, unless you know the actual loading of the motor.

This is too much work, especially when the actual motor performance curves are unlikely to be available, instead we use the NEC tables, which we have been told by NEMA are based on industry accepted worst case design values.

 

[MarineTech]

To give a more complete answer could you please provide some additional information.

What code cycle are you on?

Where is the Disposal Installed?

Conductor insulation type?

Branch circuit length one way?

Any required derates?

Starter specs?

Thanks

 

[Smart $]

HP is the output of the motor, yor conversion provides the output KW of the motor. The wire needs to be sized based on the (>) input KW (<) to the motor. ...

Umm... worst case would be "input" kVA, as wire needs to be sized to worst case amperes, which input kW does not account for.

 

[jim dungar]

Umm... worst case would be "input" kVA, as wire needs to be sized to worst case amperes, which input kW does not account for.

So are you trying to say that input KVA and input KW are not related?

It would probably help if you read my entire post where I said the difference between input and output involved both eff and PF.

 

[Smart $]

So are you trying to say that input KVA and input KW are not related?

I am not implying anything.

It would probably help if you read my entire post where I said the difference between input and output involved both eff and PF.

You assume too much. I did read your entire post and understood exactly what you wrote, prior to responding. Excluding PF implicitly in one statement and including explicitly in another leads to confusion among the unknowing... IMO... which justified the response.