electrical question

[mikejm79]

I recently added an outlet in my kitchen for a under cabinet microwave off of an existing circuit. Before mounting the new cabinets and covering up everything, I was unsure if the new setup could handle the load of the new microwave. I borrowed a friends' clamping ammeter and saw that with microwave running and exhaust fan blowing on high, it uses 16.8 amps. The outlet and wire (romex) I bought was only for

15amp circuit. The breaker didn't switch, I'm assuming it's on a 20amp circuit. It measured only 14.5amp with microwave running only. Although, I'm not exactly sure about what else is running off the circuit.

Is the fix to just update the wire and outlet to handle 20amps? Or do I need a dedicated line for this. All other outlets on circuit are GFCI outlets, 15amp max (I believe).

If it requires > 15amp circuit, then why doesn't the plug have the one T shaped pin?

Please advise.....I will call electrician to inspect after it's complete. But I want to understand it first.

[Chris Friesen]

Around here, a built-in microwave requires its own circuit, while the kind that sits on the counter does not.

Assuming the ammeter is accurate, this requires a 20A circuit. I'm assuming the microwave/fan is a single unit? If so, what is the nameplate current rating?

Also, if it's a 20A breaker then putting #14 wiring on it is generally a code violation. It's also possible that it's a 15A breaker and you didn't run it long enough to trip.

Probably the safest is to run a separate 20A circuit.

So that it will still "work" if people plug it into 15A circuits? I don't know.

Chris

[JoeSpareBedroom]

You might want to ask the electrician what it costs for him to upgrade the circuit. Since lots of tradespeople charge the bulk of their fee just to show up, it might not cost you much more to read a magazine while he does all the work.

[scott21230]

Your microwave should really be on it's own 20A circuit. From what you have wrote, I suspect that you used wire intended to be used on a

15A circuit on a 20A breaker. I hope you keep a fire estinguisher handy.

[M Q]

Is this a plug and cord connected microwave, or is it intended to be permanently wired in? If it is intended to be plugged in, 16.8 amps sounds too high. I have not found clamp-on ammeters to be very accurate. What is the nameplate rating on the microwave? I would go with that, or at least get a third source for the current.

Circuit breakers don't necessarily trip when the current is just a little over their ratings. There are published curves of time vs. current above which it must trip, and a separate one below which it must not trip. In between, it may or may not.

A typical trip characteristic might be something like: no trip at 100% load, 1 hour trip time at 135% of rated capacity, 5 seconds-40 seconds at 200%, down to tenths of a second at 1000%

[Mordred9]

[hallerb]

a

best to put the microwave oon its own 20 amp breaker since the existing circuit has other outlets that may be in use on it.

otherwise cooking times may vary dramatically depending on what else is being used, besides the hassle of resetting the breaker, which is more likely to trip with longer runs

[RBM]

You need to use 12 gauge cable and run a dedicated circuit to do it properly

[lee h]

If the wiring for the circuit is 14 gauge, you'll likely be seeing a voltage drop. This, in turn, would require more current. What is the voltage supplied to the uwave while under load? With 12 ga wiring, you'll likely see a current drop as you restore higher voltage to the uwave. A dedicated 20 amp circuit is the right way to go.

[hallerb]

is the voltage

I am sorry to inform you.....

true there will be a voltage drop, but that also causes a current drop. lower voltage equals less current for the device, since current is wasted heating the wiring and running other devices on the circuit

[M Q]

I am sorry to inform you.....

true when your load is a simple resistive device (such as a light bulb), but an electronic device with a switching regulator power supply probably would go the other way.

However, I don't know what the behavior of the microwave in question is, nor do I believe that current reading could be due to a voltage drop on a 14ga wire.

[JB]

I don't think I've seen a "permanently wired in" microwave. Even over- the-range "built-ins" have a plug that gets fed into the cabinet above and into a normal outlet.

But I agree that the specs plate should be used for calculating load.

--Jeff