10:27AM | 02/26/02
Member Since: 07/15/01
13 lifetime posts
Hi all:

The circuit breaker on the line that supplies to my convection microwave trips intermittently.

The problem is very sporadic and an annoyance. Yes, there are lights on the same circuit but it is not as if the problem goes away if I switch off all the lights when using the microwave.

Someone suggested that I use a time delay circuit breaker of same capacity as existing one. What such a breaker apparently does is that it allows occasional spikes to cope with high starting currents. Home Depot does not carry such breakers and when I asked for it, their assistant in the electical aisle recommended that I run a separate circuit for microwave.

What are the possibilities?

1. Run another circuit?
2. Look for a time delay breaker?
3. Faulty microwave?
4. Lose wiring?



11:00AM | 03/11/02
Member Since: 11/14/00
333 lifetime posts
If this problem just started occurring out of the blue, the most likely problem is that the breaker is simply failing. Breakers do not last forever; they trip more easily as they begin to fail.

Just replace it and see if that solves your problem. Make sure you turn the power off at the meter (or the main breaker switch), so that your entire circuit breaker box is dead before you replace the breaker. Take the old one to the store with you to match it because there are different sizes. You also will want to make sure that you replace it with a 20-amp breaker, not a 15-amp breaker, assuming your wiring can handle it (at least 12 gauge wire; see below)

If the problem arose because you just installed the microwave or the additional lights, then your post does not include enough information. It might still be a faulty breaker or you might be overpowering the circuit. The fact that it is a convection microwave is almost irrelevant: the wattage/amperage it pulls is the relevant fact. (I assume it is high, but it could still be under 1000 watts.) The other facts are the number of light fixtures and the wattage of the bulbs or other appliances on that circuit. The Ampacity (Wattage divided by volts) of the breaker also might be too small for the load. (It might be a 15 amp breaker, whereas a microwave will typically need a 20-amp breaker).

Finally, the amperage of the wire the circuit is on is relevant, too. A 10 gauge wire can pull more amps than a 12 gauge wire. Overpowering the wire will cause the breaker to trip excessively, as well. Assuming a 120 volt circuit, here are the ampacity ratings for different wire. Note that the safe ratings are lower than the maximum ampacity so as to give slack for surges and spikes.

10 gauge: 30 amps, (3600 Watts max) Safe at 2880 Watts
12 gauge: 20 amps, (2400 Watts max) Safe at 1920 Watts
14 gauge: 15 amps, (1800 Watts max) Safe at 1440 Watts

That said, it is standard practice if not code for new construction in most jurisdictions to run a seperate circuit for microwaves. In my remodel, I avoided wiring a new circuit by sharing the microwave circuit with the garbage disposal because I have a low-amp disposal and I can make a mental note to not use the two at the same time. (They typically do not get used together in normal use, anyway). However, that is not the preferred way, and would not fly in new construction in my area.

You could also try to re-wire the lights onto another circuit.


02:10PM | 03/11/02
Member Since: 11/05/01
98 lifetime posts
First you should run a new 20 amp circuit for the micro only. If that breaker trips the problem is with the micro unit, get appliance repair. Also a note on the previous post. Overloading a wire will NOT cause the breaker to trip. If you have wire rated at 15 amps and it is fed from a 30 amp breaker it will work fine. If you start to draw high amperage it will still work fine. The wire will heat up and burn down the house. The breaker will only trip when IT is overloaded.


04:56AM | 03/12/02
Member Since: 03/13/00
1675 lifetime posts
If your microwave is now on a 20-amp breaker, and if there is another, identical 20-amp breaker in your box, you can do some problem determination by swapping the 2 breakers (or swap the hot wires on them). Make sure power is off first.
Which one fails now? Is it the breaker or something on the circuit that's causing the problem?
(As Electricmanscott said, the wrong size wire won't cause a breaker to trip.)


05:19AM | 03/12/02
Member Since: 07/15/01
13 lifetime posts

Thanks all.

After considering all replies on this board, from electricians and from friends, I have decided to run a separate circuit (20 AMP breaker) for the Microwave first.

Seems to me that that is the bare minium that needs to be done whatever be the problem.

Regards, Sanjay


09:34AM | 03/14/02
Member Since: 11/14/00
333 lifetime posts
I came back to correct myself, and I see that it was already done for me. Overloading the wire will not trip the breaker. However, the original breaker might have been selected according to the ampacity of the wire. If you replace the breaker with a higher amp breaker, make sure you do not get a breaker with a higher ampacity than the wire.

Running a new circuit can be easy in a home with an exposed-beam basement and easy, direct runs to the kitchen; but it is often far more complicated than that. I would still recommend replacing the breaker and seeing if doing so solves the problem. Running an entirely new circuit in order to merely diagnose if that is the problem seems a bit much. If the problem is with the old breaker or a short in the appliance, then you will have done a lot of unnecessary work. It will improve things, but the cost/bother outweighs the benefit.


12:26AM | 03/16/02
Member Since: 11/05/01
98 lifetime posts
Running a new circuit is just the right thing to do, regardless of the work involved. It is more than just diagnosing the problem. The micro should be on its own circuit.


02:59AM | 03/18/02
Member Since: 03/13/00
1675 lifetime posts
OK, so now you've run a new circuit. You still have the original problem.
1) the original circuit breaker is going bad. And remember, it's still in use.
2) the microwave has a problem. A new circuit won't fix that.
3) something else in the original circuit is bad. You could still have a fire.
Diagnosing AND FIXING the problem is still a good idea. Then, if you want a nice, new circuit for the microwave, that's cool too.

[This message has been edited by rpxlpx (edited March 18, 2002).]


05:36AM | 03/24/02
Member Since: 07/15/01
13 lifetime posts
Thanks to all who replied with very practical, safe and cost effective tips. Here is the scoop - I waited till a quiet weekend to investigate deeper

Fact 1: The circuit breaker is rated 15AMP.

Fact 2: The wire is 14 gauge, which means I can not just replace the breaker with a 20AMP one.

Fact 3: There are other lights on the same circuit some also in the kitchen although there is another circuit in the kitchen with outlets.

Fact 4: Microwave is rated 120 V, 1000W -- 13 Amps (Microwave), 13 Amps (Heater).

Code / Safe Practice 1: Microwave should be on its own circuit.

Code / Safe Practice 2: This circuit should be run using wire rated 20AMP (12 gauge).

Code / Safe Practice 3: The breaker for circuit should be 20AMP.

What Next 1: Run new 20 AMP circuit with 20 AMP breaker for the microwave.

What Next 2: Replace the 15 AMP breaker to diagnose problem - although without using microwave this breaker has not trippd ever before.

Thanks again. Hope everyone agrees that this is the safe thing to do.


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