Separate circuit for garbage disposal?


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Old 12-09-03, 07:25 AM
mpatheal
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Separate circuit for garbage disposal?

I am doing a Kitchen Remodel, and will be installing a new Gabage Disposal. We did not have one before so obviously I need power run to it. This may seem like a dumb question ( and it probably is) but can I run power from a nearby outlet to a new switch and then to the disposal, or do I new need to run a complete new circuit. The existing outlet is on a 15 amp circuit on a run of 3 outlets. Thanks for any advice, as I have just about enough electrical knowledge to be dangerous.
 
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Old 12-09-03, 07:47 AM
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Ideally, a garbage disposal should be on its own 15-amp or 20-amp circuit. Second choice is to have it share a 20-amp circuit with the dishwasher.

Putting the disposal on a circuit serving countertop receptacles is not an option. It is prohibited by code.
 
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Old 12-09-03, 07:49 AM
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The legal "code" answer to your question would depend on what else was on the circuit, specifically what other outlets.

However, the reasonable answer to your question is that you must run a new circuit for the garbage disposal. I say this for several reasons. You do not want your garbage disposal to trip the circuit breaker. On a 15 amp circuit this could very easily happen, depending on what other loads are in use at the other outlets.

I recommend a 20 amp circuit for your disposal. When running the circuit I recommend running the circuit first to the outlet and then to the switch. This way you can make half the duplex outlet switched and half always on. You may want this other outlet for a dishwasher or other under counter device that requires constant power.
 
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Old 12-09-03, 09:56 AM
mpatheal
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Thanks for the speedy response guys. If I were to tie in the disposal to the DW circuit, what are the potential problems, other than the obvious tripping of the breaker if both were running at same time. I ask this because my breaker box is full and running a complete new circuit could be cause for much additional work and expense.
Thanks, Mark
 
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Old 12-09-03, 10:17 AM
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What size breaker presently feeds your dishwasher, and how is it wired?

I have no problem with my dishwasher and disposal on the same circuit, and yes I do run them at the same time every so often.

I would think that if your circuit is 20 amps then you won't have problems unless your dishwasher or disposal is a real power hog.

As for your panel being full, there are tandem breakers that provide two breakers in the space of one. You may be able to use one of these in your panel, thus providing space for a new circuit.
 
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Old 12-10-03, 01:32 PM
mpatheal
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Thanks for the reply Bob. It is a 20 amp breaker for this circuit, and it is wired straight to an outlet which the dishwasher was plugged into. That outlet is the only thing on this circuit.

I have not purchased a disposal yet. What should I look for that would not be a power hog?

Mark
 
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Old 12-10-03, 02:10 PM
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Your disposal will be rated in horsepower. In other words, the motor will have a horsepower rating. The larger the motor, the more powerful the disposal, and the more power required to run it.

I have a model that is 1/3 horsepower, and according to the manual draws 4.0 amps. Typical home models are less than 1 horsepower.
 
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Old 12-10-03, 02:52 PM
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Normally a dishwasher and disposal will do fine on the same circuit. However, as time passes, dishwashers get more and more features (heat the water, sterilize the dishes, chop the scraps, etc.), and disposals get more and more powerful. A 1/3 HP disposal is kind of punky. I'd get a 1/2 or 3/4, but I'd might stay shy of the 1 HP if you are going to share a circuit with the DW. One nice feature I like in a DW is the time delay, whereby the dishwasher usually runs in the middle of the night. In that case, nobody is likely to be using the disposal at the same time anyway. But on family holidays, the normal rules don't apply and everything runs all the time.
 
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Old 12-10-03, 09:14 PM
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Most of the garbage disposal horsepower numbers are the fantasy variety. Check how many watts are on the nameplate. Only a motor alone is required to comply with the NEC motor section rules. They do not apply to an appliance like a dishwasher, washing machine, or garbage disposal.
 
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Old 12-11-03, 12:33 PM
mpatheal
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OK, thanks for the info guys. So if my dishwasher is rated at 8.9 amps and I purchase a disposal rated at 8.1 amps (3/4 HP), I'm assuming that my 20 amp circuit will suffice. Is that correct?

Sorry to be so elementary here but I have my wall open to the studs right now and I want to be sure I get this right now, and not have a problem after the drywall and cabinets go back in. Again thanks for all the responses!

Mark
 
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Old 12-11-03, 12:49 PM
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8.9 plus 8.1 equals 17.0 Technically you should not load a 20 amp circuit more than 16 amps (20% derating), but that is for constant loading. Your disposal will not be on for more than a few minutes, so you shouldn't have a problem.

If you are real concerned about this, and since you have the wall open you could add a new circuit and be safe all around, or you could make the incoming feedline 12-3 and not hook up the extra wire just yet, saving for a multi-wire circuit later on.
 
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Old 12-11-03, 01:43 PM
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Fantasy horsepower strikes again. The full load running current for a 3/4 hp motor is 13.8 amps. 8.1 amps is is almost exactly between a 1/3 and a 1/2 hp motor. And watch those shop vacs. If they really delivered 5 horses they would need 56 amps on a 120 circuit. Always ignore anything that says 'peak developed' and go by the wattage or current on the nameplate. It is harder to lie there.
If you have the NEC2002 see Table 430.148 Full-Load Currents in Amperes, Single-Phase Alternating Current Motors.
 
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Old 12-11-03, 02:13 PM
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Most California houses run the DW and disposal from the same duplex receptacle mounted under the sink. The receptacle is split and one side is switched for the disposal.
 
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Old 12-11-03, 04:55 PM
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As far as I know, there is no reason whatsoever that you would need to limit yourself to a total of 16 amps on this 20-amp circuit. There are many, many 80% rules in the code, but I do not believe that any of them apply to this situation. Neither appliance uses more than 50% or the circuit, there is no lighting on the circuit, the loads can't be considered continuous, etc, etc.

If anyone knows of a code that would restrict the total of the DW and disposal amps to 16, please post a reference.
 
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Old 12-11-03, 05:00 PM
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John, to me it's simple.

Article 100 - Definitions

Continuous Load- A load where the maximum current is expected to continue for 3 hours or more.

I don't think either appliance fits this description.
 
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Old 12-11-03, 05:06 PM
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While your interpretation of the continuous load rule is correct, there are many codes other than the continuous load rule that require you to limit yourself to some percentage of the circuit capacity. As I said, however, I don't think any of them apply to this case.
 
 

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