Possible NEC Violation?

[eytonxav]

Recently discovered that our builder has our builtin microwave and our gas range hood wired together on a single 20a breaker. I believe per NEC, the maximum recommend amps for a 20a circuit is 16a.

That said, my GE microwave shows 1.65 kw on the sticker inside the microwave, but their spec sheet claims 1200 microwave watts.

Since the hood is rated at 4.1a @120, if I use 1.65kw on the microwave, this would push these two appliance over 16a vs using 1200W which would not.

Wondering if any of our EEs knowledgeable in residential wiring can advise?

 

[ChiliPepr]

I am an amateur, so take with a grain of salt...

I believe per NEC, the maximum recommend amps for a 20a circuit is 16a continuous or 20a non-continuous..

amps = 1000 × kilowatts / volts
So, the microwave amps = 1000 * 1.65 / 115v = 14.34amps

For a total 4.1 + 14.34 = 18.44a

It is non-continuous, so I would think it is ok.

 

[Gus.]

The NEC is pretty clear on this topic but people still debate it. The first thing to do is to check what the installation instructions for the microwave and hood say. If either says that it must be on an individual (or dedicated) branch circuit, there is no argument - the installation instructions rule in that case.

Here's what the 2005 NEC says:

422.16(B)(4)Range Hoods.
Range hoods shall be permitted to be cord-and-plug connected with a flexible cord identified as suitable for use on range hoods in the installation instructions of the appliance manufacturer[FONT=inherit !important][FONT=inherit ! important][/FONT][/FONT], where all of the following conditions are met:
(1) The flexible cord is terminated with a grounding- type attachment plug.
Exception: A listed range hood distinctly marked to identify it as protected by a system of double insulationhttp://www.justanswer.com/topics-insulation-electrical/, or its equivalent, shall not be required to be terminated with a grounding-type attachment plug.
(2) The length of the cord is not less than 18 in. and not over 36 in..
(3) Receptacles are located to avoid physical damage to the flexible cord.
(4) The receptacle is accessible.
(5) the receptacle is supplied by an individual branch circuit.

If your hood will be plugged into an outlet, the outlet must be on its own circuit. Again, I think this is very clear but people argue that the installation instructions overrule this - if the instructions don't say that you need to use an individual circuit then (5) doesn't apply. That makes no sense. You have to install things in accordance with both the NEC and the installation instructions. The instructions only overrule the NEC when they are more restrictive, not when they are less restrictive. Otherwise the instructions would have to include every applicable section of the NEC.

 

[eytonxav]

Do you know how much run time separates continuous from non-continuous?

Its certainly possible both appliances could run simultaneously several minutes, but I don't know really know how long these appliances might be full on as that is DWs specialty.

Also, I was using 120v in my calculations, P=VI. We had the combination tripping the breaker recently and thats what lead me to believe there might be too much draw on one breaker. Replaced breaker and tightened lugs in panel and now everything is fine.

 

[eytonxav]

The NEC is pretty clear on this topic but people still debate it. The first thing to do is to check what the installation instructions for the microwave and hood say. If either says that it must be on an individual (or dedicated) branch circuit, there is no argument - the installation instructions rule in that case.

Here's what the 2005 NEC says:

422.16(B)(4)Range Hoods.
Range hoods shall be permitted to be cord-and-plug connected with a flexible cord identified as suitable for use on range hoods in the installation instructions of the appliance manufacturer[FONT=inherit !important][FONT=inherit ! important][/FONT][/FONT], where all of the following conditions are met:
(1) The flexible cord is terminated with a grounding- type attachment plug.
Exception: A listed range hood distinctly marked to identify it as protected by a system of double insulationhttp://www.justanswer.com/topics-insulation-electrical/, or its equivalent, shall not be required to be terminated with a grounding-type attachment plug.
(2) The length of the cord is not less than 18 in. and not over 36 in..
(3) Receptacles are located to avoid physical damage to the flexible cord.
(4) The receptacle is accessible.
(5) the receptacle is supplied by an individual branch circuit.

If your hood will be plugged into an outlet, the outlet must be on its own circuit. Again, I think this is very clear but people argue that the installation instructions overrule this - if the instructions don't say that you need to use an individual circuit then (5) doesn't apply. That makes no sense. You have to install things in accordance with both the NEC and the installation instructions. The instructions only overrule the NEC when they are more restrictive, not when they are less restrictive. Otherwise the instructions would have to include every applicable section of the NEC.

Neither the hood or microwave installation instructions say anything about electrical requirements

 

[Gus.]

A continuous load is one that is expected to run at maximum capacity for three hours or more. An electric heater is one such load.

If the breaker tripped a lot it may have worn out. If that's the case I would expect the replacement to wear out eventually too.

 

[travelover]

I sure wouldn't worry about it. Microwave ovens only run for minutes at a time.

 

[ERD50]

I installed a built in combo microwave oven with built in fan, so I can't comment on the separate units. But is sure seems odd to me to require that a fan would need to be on it's own circuit, when it is only ~ 4 amps?

As I recall, the NEC does specify a separate circuit for a built in microwave (but not a counter-top unit that could be the same power rating!). I may have overlooked this when I installed mine, but the other outlets on that circuit would not normally have a high current appliance plugged in, they are a bit difficult to reach we hardly use them at all. At any rate, we have never tripped a breaker in the kitchen. Though using a toaster on that outlet might trip it if both were on.

I think code also requires a separate circuit for a dish washer, ours shared the circuit with two counter-top outlets and a light. Never tripped those either (all 20A).

-ERD50

 

[eytonxav]

I installed a built in combo microwave oven with built in fan, so I can't comment on the separate units. But is sure seems odd to me to require that a fan would need to be on it's own circuit, when it is only ~ 4 amps?

As I recall, the NEC does specify a separate circuit for a built in microwave (but not a counter-top unit that could be the same power rating!). I may have overlooked this when I installed mine, but the other outlets on that circuit would not normally have a high current appliance plugged in, they are a bit difficult to reach we hardly use them at all. At any rate, we have never tripped a breaker in the kitchen. Though using a toaster on that outlet might trip it if both were on.

I think code also requires a separate circuit for a dish washer, ours shared the circuit with two counter-top outlets and a light. Never tripped those either (all 20A).

-ERD50

Although microwave is built in, I am pretty sure there would be a normal receptacle behind it, so no real difference from a countertop model. I also thought NEC 2011 required a standalone circuit for a microwave, but if not, the combined draw needed to be 16a or less. I was not aware of the continuous vs non-continuous use in terms of what is acceptable load on a 20a circuit, so as others suggested, it may be OK.

Some of the hot lugs in my panel needed tightening. I suspect since they were a little loose, that may have caused heating and premature breaker ware on the problem breaker.