What size of garbage disposer max I can install if the same circuit has dishwasher

[Curiousv]

This is 20A circuit and one outlet has 2 plug ins
Obviously they wont be continuous loads
Dishwasher I think is 10A
So what is the max horse power or AMP disposer I can install in this same circuit?
Wiring is 12-3 cable

 

[wwhitney]

Several comments:

A "12/3" NM cable has 3 circuit conductors plus an EGC. So it would typically be used for an MWBC, where you have (2) 120V 2-wire circuits that share a single neutral conductor and where the ungrounded (hot) conductors are on opposite legs of the 120/240V service. If you have such an arrangement, and one duplex receptacle is the only thing connected to the load end, then the center tab on the ungrounded side of the duplex receptacle should be removed, and you have one 20A circuit on the top, and one 20A circuit on the bottom. The two circuits should supplied in the panel by either a double pole breaker, or two single pole breakers with a handle tie. Not by a tandem breaker, although opposite sides of a quad would be OK if handle-tied.

It's also possible you mean a "12/2" NM cable and have miscounted the EGC. In which case your original question is applicable. And then the answer is that if the circuit supplies only the one outlet (which means it supplies only that duplex receptacle and no other loads; an outlet is any location with load connections, whether by cord and plug or hard-wired), the only restriction is that the dishwasher current rating plus [correction: 125% times] the disposal current rating must be at most 20A. Or, if there is an effective method of ensuring that only one appliance can run at once (e.g. the switch that activates the disposal kills power to the dishwasher), then each of them may be up to 20A. [correction: 125% times the garbage disposal current may be up to 20A.]

Cheers, Wayne

 

[Sponsor]

 

[Fitter30]

1/2 hp

 

[wwhitney]

1/2 hp

If we are going by NEC Table 430.122, which says to treat a 1/2HP 115V motor as having an FLA of 9.8A, and we are assuming the garbage disposal and 10A dishwasher are jointly on a 20A circuit, that is not correct. As NEC 430.24 tells us to take 125% of the motor load plus the other load to determine our minimum conductor ampacity, and 125% of 9.8 plus 10A = 22.25A. While NM cable is limited to the 60C ampacity, which is 20A for #12 Cu. [Note that the small conductor OCPD limits in 240.4(D) do not apply to motor loads.] Thus we would be limited to a 1/3 HP motor which has a Table 430.122 7.2A FLA.

However, 430.6(A) Exception 3 says that for a listed motor-operated appliance that is labeled with both HP and FLA, we are to use the nameplate FLA, rather than the Table 430.122 value based on HP. Thus in this case the answer is any garbage disposal labeled 8A or less, or a garbage disposal labeled only in HP that is labeled 1/3 HP or less.

[. . . in a particular case,] the only restriction is that the dishwasher current rating plus the disposal current rating must be at most 20A. Or, if there is an effective method of ensuring that only one appliance can run at once (e.g. the switch that activates the disposal kills power to the dishwasher), then each of them may be up to 20A.

So this is incorrect, as I forgot the 125% multiplier for motor loads. For the mixed case, dishwasher current plus 125% garbage disposal current must be at most 20A; for the case of an individual 20A branch circuit for the garbage disposal, the garbage disposal current must be at most 16A.

Cheers, Wayne

 

[Fitter30]

If we are going by NEC Table 430.122, which says to treat a 1/2HP 115V motor as having an FLA of 9.8A, and we are assuming the garbage disposal and 10A dishwasher are jointly on a 20A circuit, that is not correct. As NEC 430.24 tells us to take 125% of the motor load plus the other load to determine our minimum conductor ampacity, and 125% of 9.8 plus 10A = 22.25A. While NM cable is limited to the 60C ampacity, which is 20A for #12 Cu. [Note that the small conductor OCPD limits in 240.4(D) do not apply to motor loads.] Thus we would be limited to a 1/3 HP motor which has a Table 430.122 7.2A FLA.

However, 430.6(A) Exception 3 says that for a listed motor-operated appliance that is labeled with both HP and FLA, we are to use the nameplate FLA, rather than the Table 430.122 value based on HP. Thus in this case the answer is any garbage disposal labeled 8A or less, or a garbage disposal labeled only in HP that is labeled 1/3 HP or less.


So this is incorrect, as I forgot the 125% multiplier for motor loads. For the mixed case, dishwasher current plus 125% garbage disposal current must be at most 20A; for the case of an individual 20A branch circuit for the garbage disposal, the garbage disposal current must be at most 16A.

Cheers, Wayne

1/2 hp is 6.3 amps. Under specs. Why wouldn't you consider this non continuous load multiplier is 0 because who run disposal for more than minute or two at the extreme.
://www.lowes.com/pd/InSinkErator-Badger-5XL-Garbage-Disposal-1-2-HP/5000109263?cm_mmc=shp-_-c-_-prd-_-kit-_-ggl-_-CRP_SHP_LIA_KIT_Online_A-B-_-5000109263-_-local-_-0-_-0&gad_source=1&gclid=Cj0KCQjw8MG1BhCoARIsAHxSiQmM43fEYDkeQWSfCFoVwz070Lx2f2CeDlxk2mMfX8OPewxgyxvruoYaAi2iEALw_wcB&gclsrc=aw.ds

 

[wwhitney]

1/2 hp is 6.3 amps. Under specs

OK, so that one would be fine with a 10A dishwasher on a 20A circuit, as long as the nameplate itself says 6.3a and doesn't just say 1/2 HP. Other models may vary.

Cheers, Wayne