Education time here. I installed a dedicated 120v 20amp Microwave circuit for a coffee shop some time ago. This circuit gets heavy use during the morning hours more than at any other time during the day. It’s protected by a Square D QO 20Amp SP breaker.
Customer was complaining that recently the breaker was tripping constantly, in particular during heavy use AM times. Microwave draws around 17Amps according to the label and is 4yrs old. Seeing no OBVIOUS problem, and having had experience in the past with breakers going bad, I recommended we first try a new breaker. So, that’s what I did, a new QO breaker. Client said for first few days no nuisance trips, now it’s starting up again with intermittent, though not constant, tripping.
How do I test if the micro is going bad and maybe pulling more amps than needed? I have a clamp style amp meter but since it’s intermittent how do I avoid testing the amperage draw when all is well and I show no problem ? I can’t stand there all day having them abuse the micro to try and make it happen.
Ideas, suggestions?
Replies
What's the nameplate value of the current (and at what voltge) on the microwave?
CP
CAP, I'm assuming you're asking what does the sticker on the micro say the amp draw is? It says 120V and I'm going by memory here but I am pretty sure it said 17A. I'll verify that for sure when I go see it tomorrow afternoon.
What I'm thinking is--if the load is 17A @120V, and the voltage is drooping (yea, that's a real technical term), your amp draw may be going up to a point where after a while, the breaker opens on overload.If measuring the draw while the micro is working doesn't reveal anything, I'd monitor the voltage and amperage at the receptacle outlet. A meter with min-max recording (like the Fluke 16) and an amp-clamp with "max hold" would do the trick and are a lot less expensive than a Fluke 189 or similar with full datalogging.Hey, speaking of which--does the dedicated circuit end at a duplex outlet? Anything else plugged into that outlet?Cliff
Edited 2/1/2005 12:22 pm ET by CAP
It ends at a duplex, but nothing else pluggedin it.
At least there's nothing plugged into it while you're there...
I'd replace it with a simplex (single) receptacle.
Cliff
That was the first thing that crossed my mind. Microwave and, perhaps, a coffeemaker or other appliance. Wouldn't take much. A lot of microwaves, particularly the older commercial units, drew a lot of power.The plug should control the circuit that they can be plugged into but an older unit could have had its 20A/120v cord cap replaced with a 15A/120v unit. Making you think you have more head room available than you actually do.I have also seen one or two old 30A/120v commercial units. They can draw 25A or more and will trip a 20A breaker if used for more than a few minutes.
Yep, I was thinking the same thing--a 30A unit that's had the cord cap replaced. That's why I asked pyrotechie to check the nameplate specs.
Off topic, but are things going O.K. in Florida? Here in Calif it's pretty damn funny, Gov Arnie appears to have double-crossed the teachers--got 'em to take a budget cut last year w/promises that there'd be no cuts for a couple years. And this year, he's proposed deep cuts in schools. Why is it that the kind of person that goes into politics, is the one you'd rather not have running the government? (With apologies to Groucho).
Cliff
Florida has Jeb. Means the FEMA payments go through, at least before the election, pretty swiftly. Even the once like dentures and TVs damaged 60 miles from where the hurricanes were. Jeb is laying low. He says he isn't going to run for CinC. Of course both Reagan and Flight-suit-boy said they would cut government spending. Heritage Foundation, those liberals, pointed out that they did exactly the opposite compared to Clinton by almost two to one. Point being never go by what they say. I think Jeb has more sense and has had to swallow hard on some of the GOP lines. He is a lame duck, can't run again. Unless the GOP gives him a whole lot of massaging and cheering I suspect he will gracefully retire to the Bush homes in Maine/Texas. Sometimes he looks at the camera and I see a glimpse of disgust and quiet irritation that he can't say what he thinks. That he has to play politics and bow to those with money. I suspect he is a whole lot less ideological than his brother.Checking the nameplate is a good idea. Some older units might not have one left. Still a good place to start. Comparing the nameplate with ammeter readings at both the receptacle and panel would be revealing if compared to the nameplate.
Use clamp on Ammeter to check to circuit load while the microwave cooks a bowl of water. Some appliences as they get older will gradually draw more current. An internal short could draw more current also.
If it is below 20A average replace the breaker. You likely got a bad one or batch.
Edited for clarity.
Edited 2/1/2005 12:48 am ET by 4LORN1
Thanks 4LORN, will do.
This is a is a long-shot that I am about to relate... but stranger things have happened (I guess).
Neighbor's restaurant had a 20A circuit (dedicated) for a commercial fridge. The plate on the fridge said it pulled 17A. It kept tripping the breaker. Had repairmen out... electicians... you name it.
I looked at it (not an electrician, but I looked anyway), just as a friend. I noticed a small amount of rust on the edge of the galvanized box. We ripped the box out, and there was the problem.
Behind the fridge was a plaskolite-type wall covering (sealed spaces). It backed up to a dead space that had water pipes, heat ducts, etc running through it. The space was, equivocally, not heated either. Long story, shortened... there was condensation dripping down on the box; dripping around the box onto the outlet; and shorting the breaker.
The only reason I thought of this.... when we "backtracked" after we had found the problem... it only occurred during busy times (when water was flowing through the pipes to the dishwasher). It would sweat... drip... hit an old 2x4 brace... roll down the brace to it's "drip point"... down to the box... outlet... TRIP.
Like I said... long-shot... but maybe it will give you a different perspective when you look at it.