Reader Question: Many over-range microwaves say they require a dedicated 20-amp branch circuit, but include a plug that can easily be plugged into a 15-amp receptacle. Is it safe to plug the microwave into a 15 amp circuit? — John
15-Amp vs. 20-Amp Receptacles
John, before we answer the question (which is a great one by the way), let’s review receptacle types. A standard 15-amp grounded receptacle is the one most of us are used to seeing throughout our house. It has two vertical slots–one slightly larger than the other–and a circular grounding slot at the bottom. A 20-amp receptacle, which is more commonly found in office buildings, looks just like the 15-amp version but has an extra horizontal slot, making the left slot look like a sideways T. (pictures available here from wikipedia)
When the microwave says it requires a dedicated 20-amp branch circuit, you would expect it to have a plug that can only plug into a 20-amp receptacle — a plug with a horizontal tong to fit into the horizontal slot.
But it doesn’t. Why?
15-Amp Receptacles on 20-Amp Branch Circuits
The National Electric Code (called the NEC) defines the receptacles that can be installed on various levels of branch circuits. For 20-amp circuits (e.g., a circuit protected by a 20-amp breaker), the NEC allows both 15-amp and 20-amp receptacles to be installed on the same circuit.
Even though both receptacle types can be installed on a 20-amp circuit, the circuit wiring will differ. A 15-amp circuit is usually served by 14 gauge wire, while a 20-amp circuit must be served by12-guage wire. The thicker gauge required by 20-amp circuits ensures the wires do not overheat under a 20-amp load.
Why (Some) Microwaves Require a 20-Amp Circuit
Microwaves require a 20-amp branch circuit due to constant load and spike issues. Under strenuous use, a microwave could draw nearly the maximum 15 amperes of current regularly for hours. Under this maximum load scenario, 14- gauge wiring could heat up beyond safe levels. Also, high power microwaves can temporarily spike over the 15-ampere limit. If the circuit were governed by a 15-amp breaker, the spike would cause the breaker to trip.
Microwaves include the standard 15-amp plug because these are the plugs most often found in homes in the U.S. If 20-amp receptacles were common on 20-amp branch circuits in modern homes, high power microwaves would likely sport 20-amp plugs.
How to Determine Whether a Circuit Is 15 or 20-amp
The easiest way to determine whether the circuit is a 15 or 20-amp circuit is to find the corresponding breaker or fuse in the breaker panel. Note, however, that it is possible that a prior homeowner or electrician made a mistake, so this isn’t 100% reliable.
What’s the Danger?
A microwave plugged into a 15-amp circuit could cause the wires in the wall to overheat and present a fire hazard. More likely, however, the microwave will trip the 15-amp breaker and pose a regular nuissance.
Note that you should never just replace a 15-amp breaker with a 20-amp breaker. This will present a fire hazard since the wiring is not suited for a 20-amp breaker.
What do you think? Have you installed a microwave with this requirement? Did you run the separate 20-amp circuit?
(Appropos Photo: srbyug)
Great post Fred and a great explanation of a confusing topic. Today’s electrical code actually requires built in microwaves (above the range) to be wired with a 20 amp circuit. I wouldn’t suggest relying on the size of the breaker to determine the circuit as many people have swapped breakers in the past without understanding the issue. The real test is checking the wire. Open the receptacle box or the breaker panel and look at the wire. In newer homes the 20 gauge wire typically has a yellow sheath. You can also use a wire diameter gauge to check the size.
Nice job Fred!
Todd – excellent point on checking the wire gauge on the circuit… scary to think how many houses might have breakers that exceed the max rating for the wires…
Also, I didn’t realize that the code now specified a 20-amp circuit for over-range microwaves, but it makes sense since many (if not all) new models require it.
During the inspection of our first home the inspector found several 20 amp breakers plugged into 15 amp circuits. I think it’s quite common with circuits that get tripped due to power tools, hair dryers, etc. People don’t stop and think about the cause, they just want to fix it…quickly! Anyway, great post!
As far as the plug/receptacle combinations are concerned, a NEMA configuration chart makes it easy to see what is possible. Below is a link that ought to cover most residential situations:
The installation manual for my Whirlpool microwave says to use 15A or 20A fuse/circuit breaker. The owner’s manual says to use 20A. I have no problem because my home has been rewired with #12 wiring (20A). Most of the homes in my area, however, have the original #14 (15A) wiring. This is probably true for most older homes throughout the country
According to your article, this microwave could cause #14 wires to overheat. It seems that if you are going to buy a microwave oven, you also have to consider rewiring the circuit branch. Pretty expensive.
Murray, if a house has #14 wire, they’ve got to keep a 15A breaker on that circuit. As long as the breaker is functioning properly, the wire wouldn’t get an opportunity to overheat as the breaker would trip. Just have to be sure not to try to install a 20A breaker on a #14 wire – that would be dangerous.
If the owners manual and installation manual aren’t in sync, I’d call the manufacturer and let them know.
I’ve seen overrange microwaves installed on 15A breakers – they blow every now and again since the microwave can draw nearly all 15 amps (and most breakers will trip under max constant load for any period of time).
Great info. on circuit amps. Clear and detailed.
Checking proper amps for existing wiring is a great idea.
I have worked on many older homes as a handyman and
have turned down jobs because of unsafe conditions that
the home owner was not ready to address.
I installed a microwave that required a 20 amp breaker that was placed on a 15 amp breaker with 12 AWG. According to the 2011 NEC code book., a 20 amp breaker requires a #10 awg. I this safe and what are the consequences.
I believe you have misread. A 20amp breaker requires #12awg.
The consequence of installing a microwave requiring a 20amp breaker on a 15 amp circuit is that you are far more likely to trip the circuit.
It is also dangerous to install a load on a circuit that exceeds the over-current protection. If you truly have #12awg on the whole circuit, the breaker could be switched out for a 20 amp. Note: You must be sure the ENTIRE circuit is 12 gauge.
No don’t do that even if its 12awg 15amp breaker if done right the 15amp breaker is there to keep max breaker down. 100amp breaker supplying 3 20amp breaker 1 25amp 1 15amp 100 right there, put in 20amp now u at 105 enough said. All theses are outdated with lack of knowledge to this a 15amp can and will supply a basic microwave not oven types if its the only 1 on the circuit then the risk of fire, is if u cook something in it for 3 hour or so who does that, if done by code book only time breaker will flip is if you stop or it stops by being done you restart it within that 3sec (2.6 most 15amp breaker)window then breaker will trip as it should if everything is wired right end of that, now for 14awg to 12awg by code we have to use 14awg with 20amps in hot areas when done right there is no risk of fire and for more or less info your fans,tvs don’t use 120v when on the max can be 120v but all I tested came back from 30v to 80v some fans use 26 watts. This is just to inform people for personal use only I see a lot of take this out put this in but not much or none state not to go over main breaker in main junction box and better yet if its a 2nd sub panel or just sub panel,best I can say is call someone that knows what there doing. Be safe stay powered
On 15Amp or 20Amp circuits, Couldn’t one use a #10 AWG wire from the panel through the circuits to respective rated outlets, light fixtures, or appliances? Wouldn’t that be better in an older home with an older code?