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)