Garbage Disposal Ampacity

[Geraldselectric]

I have a question about this test question:

What size 120v branch circuit is required for NM cable supplying a 1200va disposal? I came up with with 12.5a requires #14, which is the answer.

My question is about hard wired versus Cord N Plug. Shouldn't a test question state how the disposal is to be wired? Isnt a Cord N Plug disposal required to be 80% of the circuit and #14 would be limited to 12a and a #12 would be required?

 

[haskindm]

Look at 210.23(A)(1) and (2). The 80% restriction is is for equipment NOT fastened in place. Is your garbage disposal portable? No, so no restriction.
There is a 50% restriction on cord and plug connected equipment if other equipment is served. Since I assume this is a dedicated circuit, this restriction should not apply. You have already added the 25% for this being a motor load, so I believe you have it covered and your answer is correct.

 

[Geraldselectric]

Thanks. I see that 50% is for equipment not fastened in place.

I have seen quite a few times where a disposal was not dedicated, but both a disposal and dishwasher was cord n plug into the same duplex receptacle.

Is this acceptable as long as a 15a receptacle is kept to a total of 12a and a 20a receptacle kept to 16a max between the 2 devices?

 

[Dennis Alwon]

A 1200 va disposal is a very huge disposal. I think the ones I have seen draw about 6-7 amps.

Anyway, I too have seen the dishwasher and disposal together on the same circuit. I don't believe it is legal in most cases (esp. with today's appliances)however, it probably will never create a problem since the disposal is on for such a short period of time-- usually 10 secs or so. I don't believe that is long enough time to trip the breaker.

I find that most dishwashers with the heaters in them draw about 12 amps. This is more than 50% of the circuit so I would say that these appliances need a separate circuit.

The disposal you are talking about will draw 10 amps so a 15 amp circuit is fine but if other things are on the circuit it would need to be 20 amp. art. 210.23(A)(2) as haskindn has pointed out.

 

[Geraldselectric]

Thanks. What seems to be even more confusing is that 210.23(A)(2) address Cord N Plug NOT Fastened in place to be 50%. Both the disposal and dishwasher are fastened in place. So, I am now understanding there is no derating.

Also, I am understanding for both of them to be on a duplex receptacle, I size the disposal to 125% and add the dishwasher nameplate and keep it under 15a for #14 or 20a for a #12 respectively?

6A disposal x 125% = 7.5a plus 12a Dishwasher = 19.5a? So, the receptacle would have to be rated at 20a (not 15a) with a #12?

 

[Dennis Alwon]

Thanks. What seems to be even more confusing is that 210.23(A)(2) address Cord N Plug NOT Fastened in place to be 50%. Both the disposal and dishwasher are fastened in place.

Article 210.23(A)(2) is talking about Equipment (hence DW, Disposal and other equipment) that is fastened in place.

Now the disposal that draws 6 amps can be connected to other appliance (cord & plug connected) not fastened in place, lighting, or both. So I can wire the disposal with a 15 amp circuit and also have my kitchen undercabinet lights on the same circuit.

6A disposal x 125% = 7.5a plus 12a Dishwasher = 19.5a? So, the receptacle would have to be rated at 20a (not 15a) with a #12

No--- I don't believe you can put the disposal and dishwasher on the same circuit given the amp loads I stated regardless of you using a 20 amp recep.

First if you did a calculation you would have to use 125% of the largest motor not the smaller motor.

DW---12amps*1.25= 15
Disposal 6 amps

That totals 21amps. No good.

 

[haskindm]

Dennis,
I don't believe you can treat the entire dishwasher as a motor load. Usually the motor in a dishwasher is a small part of the load, the majority of the load will be the heating element.

 

[Geraldselectric]

ok, thanks for the clarificaition. I was wondering how others are getting away with a duplex receptacle for both.

 

[Dennis Alwon]

Dennis,
I don't believe you can treat the entire dishwasher as a motor load. Usually the motor in a dishwasher is a small part of the load, the majority of the load will be the heating element.

You may be right. What do you do then since the heating loads should be 125%. Take the largest of the heating load, dw motor, or disposal and multiply that by 125%???

Seems odd to separate it out but it sounds right.

If that were the case then you should be able to put both on a 20 amp circuit. I don't think you would need a 20 amp duplex receptacle since neither unit draws more than 15 amps.

 

[infinity]

A 15 amp duplex is permitted on a 20 amp circuit. Did you open up the box and look at the receptacle? It's possible that there is a 3 wire MWBC feeding the duplex. One circuit for each receptacle.

 

[infinity]

Dennis,
I don't believe you can treat the entire dishwasher as a motor load. Usually the motor in a dishwasher is a small part of the load, the majority of the load will be the heating element.

A dishwasher is an appliance. IMO, unless it's a continuous load (which it isn't) you do not add 25% to the circuit calculation.

 

[Geraldselectric]

Kind of getting back to the original question, does table 210.21b2 put the 80% limitation on any Cord N Plug when using a duplex? I think the only way to use a duplex is by MWBC as Infinity suggested.

 

[infinity]

A duplex on one circuit is two receptacles so either one or both could have a maximum load as it appears in that table.

 

[Geraldselectric]

I have a question about this test question:

What size 120v branch circuit is required for NM cable supplying a 1200va disposal? I came up with with 12.5a requires #14, which is the answer.

My question is about hard wired versus Cord N Plug. Shouldn't a test question state how the disposal is to be wired? Isnt a Cord N Plug disposal required to be 80% of the circuit and #14 would be limited to 12a and a #12 would be required?

So, if this disposal is hard wired, #14 is OK. But, if Cord N Plug, I must use a #12 with a 20A receptacle due to 12.5a?

 

[chris kennedy]

Just a tip. Answer test questions as they are stated. Don't read things in that aren't there. Thats how they get you.:wink:

 

[Dennis Alwon]

So, if this disposal is hard wired, #14 is OK. But, if Cord N Plug, I must use a #12 with a 20A receptacle due to 12.5a?

Gerald-- it is okay to use 14 whether it is hard wired or cord and plug. The issue is wether or not you extend the circuit not if it is cord and plug connected. Read 210.23(A)(2) very slowly.

 

[Geraldselectric]

Yes, I need to read it slowly. 210.23(A)(2) is confusing, because what If I extend the circuit with another fastened in place load such as the dishwasher? IMO, the 50% does not apply in this situation? - Thanks