When should a 15-amp breaker trip?

kylie_mFebruary 5, 2007

We have a laser printer, a ceramic heater, and a few other things plugged into a 15 amp circuit in our home office. We noticed the overhead lights dimmed when our printer surged, so we did some testing.

Specifically, the printer runs at 14 amps, but will peak at 24-26 when it surges. The floor heater pulls 14 amps when on "high". Both of these together do not trip a 15-amp breaker. We even added the television and paper shredder and all of them failed to trip the breaker.

This is a brand new house, and we had asked for an extra 20-amp circuit for this room because we knew we'd have a lot plugged in. The builder failed to do that, so the electrician was out last week and ran another wire plus the 20-amp breaker. We asked him why the printer + heater + television didn't trip the breaker, and he said there's a cushion built in to handle surges, but we'd have to exceed 15 amps consistently and continually for the breaker to trip. This doesn't make sense to me. Can somebody explain why a 15 amp breaker doesn't trip when ya pull 16 amps?


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There are two controls that determine if and when a breaker trips. One is heat. If the breaker passes too much current, the heat buildup in the breaker will cause it to open just like a bi-metaalic strip in a thermostat. The second control is magnetism. AC produces a magnetic field. The more current, the stronger the magnetic field. If not enough heat or not enough magnetic field are present, the breaker won't trip. 16A wasn't tripping your breaker because of exactly what your electrician told you. You may need to see 20+ amps continuously to trip that 15A breaker. That's normal.

    Bookmark   February 5, 2007 at 10:32AM
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Thanks bigbird1. My husband asked me to add an additional question to my post... thanks in advance for any replies!

The National Electrical Code specifies that a circuit should not pull more that 80% of the rated load of the circuit. This is 12.5 amps on a 15 amp circuit. If a circuit with a 15 amp breaker on 14 AWG wire must pull 20+ amps to trip the breaker, then it must run at almost twice what the NEC allows before it trips. Is this safe?

    Bookmark   February 5, 2007 at 11:49AM
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    Bookmark   February 5, 2007 at 12:00PM
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In re-reading your original post you state that the printer draws 14A. If you also have a heater operating at full current at the same time, and you're not tripping a 15A breaker, then I'd say you have a faulty breaker.

    Bookmark   February 5, 2007 at 3:38PM
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Where did you get 14 A for the printer?
They are a little power hngry for the fuser, but not usually that high.
15 A for a heater is not uncommon, but still at the upper end.
Watch out for voltages on heaters.
Many rate the wattage at 130 V (instead of the NEC mandated value for calculations of 120 V).
The NEC does not directly apply to cord-and-plug connected appliances except for the cord material.
NEMA has voluntary standards that are used for appliances.

    Bookmark   February 5, 2007 at 4:29PM
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We actually measured each item. The printer is a Brother MFC 9700, a known power hog (not known when we bought it 5 years ago, however!)

Actual amps pulled were:

14 - 17 when printing
24 - 26 when surging

    Bookmark   February 6, 2007 at 11:00AM
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The overload must continue for a significant period of time to trip the breaker. It's designed that way for user convenience. It's entirely safe.

When you overload a wire, it gets warm. There are other factors to consider, but the one that concerns us here is that the greater the overload, the hotter the wire gets, and the faster it gets hot. If the overload lasts only a few seconds, there is no time for the wire to get hot and thus no real danger. Depending on circumstances, a large overload on the order of double or triple capacity may be sustained for several seconds without excessive heating. A moderate overload (50% or so) will most likely be OK for a few to several minutes.

Circuit breakers are designed to take this into account. It depends on brand and type, but IIRC a breaker can typically carry double its rated current for anywhere from several seconds to a few minutes.

Unless you're printing a major American novel, your printer is idling most of the time. I'd be very surprised if it were consuming 14 amps when idle - more like 2-3 amps at most, and possibly less than an ampere. This information may be in your owner's manual. (If it really does use 14 amps at idle, chuck that energy pig and get something more reasonable!)

Thus your only continuous load is the heater (and the television set, if it's on all the time). This probably doesn't add up to an overload.

I doubt that your situation poses any danger. But as a test you might try running a second space heater in this circuit. Based on my own experience, I'd say that the breaker should trip in something between 2 and 10 minutes.

    Bookmark   February 6, 2007 at 1:01PM
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"14 - 17when printing"

That is 1680 to 2040 Watts (technically V-A since we do not know the power factor) but it is way out of line.
How did you measure the draw?
Plug in adapter and a clmap on ammeter?
Did you use the 10X hole by accident?
Did the meter actually read zero before you clamped it on?

    Bookmark   February 6, 2007 at 7:42PM
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That does not seem so out of line for a decent size laser printer. An ink jet yes, but laser....??

Kylie, your quote of the NEC is not at all accurate in general terms. It IS accurate for a continuous load. That said, VERY little in a home is considered a continuous load. I'd venture to say nothing in your home office is.

A continuous load is a load where the maximum current is expected to continue for 3 hours or more.
The key words being "maximum current" and "expected".

    Bookmark   February 6, 2007 at 9:51PM
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Something's not right here. I took a look at the manual for the MFC9700 and it says:

Power Consumption
Average - 350 W
Peak - 940 W
Standby - 75 W or less

You've either got a defective printer or you're not measuring correctly...

    Bookmark   February 6, 2007 at 11:52PM
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We are not the only users who have experienced problems with this printer. This is from a reviewer at Amazon (not me):

Third, as someone else noted in another review, Brother printers and MFC devices are power hogs. They make the lights dim and will trip your uninterruptible power supply if you are on a 15 amp circuit (20 amp circuits don't have the problem). This is actually a problem with most Brother printers and MFC devices, so its not unique to this problem. The real problem is that Brother gives no indication in the user manual of the real power draw involved with this product and the published specifications are highly dubious given what my electrician has told me.

As far as how we tested, we plugged a Kill-A-Watt electricity monitor into the wall and plugged the printer into that. My husband verified it by using some thing-a-ma-jig he has (sorry, don't know what it is but I'll find out, lol).

Again, we used this printer for years at our previous home and never had reason to question it-- most likely because it was not on the same circuit as the overhead lights. It was the dimming of the lights in our new home that started us researching this to begin with. First, because the builder had told us the room would be wired with either two 15-amp circuits or a 15 and a 20 (it's a home office and we requested that and obviously it wasn't done - the electrician apologized and ran the 20 amp circuit last week). Second, we are still under the one-year home builder warranty period and if there are any issues of concern, we want to get them corrected now while we have the chance.

Here is a link that might be useful: Printer Review at Amazon

    Bookmark   February 7, 2007 at 9:40AM
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The thing in most laser printers that sucks power is the fuser. Even the high voltage supplies are pretty small.

The fuser is just a straight resistance heater used to 'melt' the toner onto the paper ('fuser' just sounds fancier).

As noted above the regualr breakers in a residential setting are thermal-magnetic.
The thermal portion creates the 'slow blow' on transient overloads function, while the magnetic trip operates in milliseconds and protects against short circuits.
The thermal portion is influenced by temperarure, and if the panel is in a cool place the curves can get rather generous.
The breaker might carry 200% of rating for a pretty decent interval.
This is not really a hazard since the protection on #14 (& #12) wire is held low to add margin to the safety of the system. #14 is actually rated at 20 amps and up, while #12 is 25 amps and up (actual rating depends on what insulation is used).

Diming lights are more an annoyance than anything else.
Placing a completely seperate circuit for the computer system is generally a good idea anyway.
It should have no other receptacles except the ones used for the computer, and there should be a surge protector strip betwenthe equipment and the receptacle.
By running a single outlet circuit you create a single ground path (helps with electrical noise) and minimze the chances of picking up transients from other things on the circuit (like the vacuum).

    Bookmark   February 7, 2007 at 12:12PM
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Thanks Brickeyee. For the record, we have a lot more in this room than one computer and one printer. DH is a software engineer and does a lot of work at home. We have 3 computers plus 2 printers plus a television and a few little things like a modem and router. And we run the small portable floor heater on really cold days to add a little extra heat to the room. So, all this was on the same 15-amp circuit as the overhead light that has four 60-watt bulbs and a fan. That is why we specifically requested the second circuit when we bought the house, while it was still under construction and it was easy to drop the wire.

    Bookmark   February 7, 2007 at 2:37PM
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I still think a laser printer drawing 945W peak, 3 computers, a second printer, a TV, 4X60W bulbs + a 1500W heater all running continuously should trip a 15A breaker. I still say faulty breaker. Does no one else concur? Maybe I should go back to electrical school :-)

    Bookmark   February 7, 2007 at 4:50PM
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Yeah, that sounds weird to me too. I think that it's a good idea to try running two space heaters on the circuit - 2800W should DEFINITELY tripp a 15A 120V breaker. If it doesn't, yeah, defective or incorrectly connected breaker.

    Bookmark   February 7, 2007 at 5:52PM
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"I think that it's a good idea to try running two space heaters on the circuit - 2800W should DEFINITELY tripp a 15A 120V breaker.

Why does the word "Chernobyl" keep coming into my mind when I read Perel's post?

    Bookmark   February 7, 2007 at 9:16PM
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I still think a laser printer drawing 945W peak, 3 computers, a second printer, a TV, 4X60W bulbs + a 1500W heater all running continuously should trip a 15A breaker

I bet the laser printer doesn't draw more than 50 watts when idle. Unless it's printing a big job the fuser will run for only a few seconds at a time. So, unless those are very efficient computers, we have a total of perhaps 2250 watts. This is about a 25% overload. Most breakers will hold for quite some time minutes at 25% over, and probably even longer if the breaker were located in a cold basement. But I agree now that I add up the loads - it probably should open at least once in a while.

    Bookmark   February 8, 2007 at 4:29AM
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DH thinks it's a faulty breaker. We're going to put it to the test... by plugging in two of the little floor heaters on "high" plus add a blow dryer to the mix. If that doesn't trip it, he'll swap out breakers and we'll try again. I'll report back :)

Thanks for all the help guys!

    Bookmark   February 8, 2007 at 9:53AM
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Looking at the Square-D tripping curves mentioned earlier is surprising (to a non-electrician like myself). To trip the 15A breakers in 4-5 seconds requires 3-4 TIMES the rated current. To trip the breaker "immediately" requires something like 10 times the rating.

Clearly the panel breakers are about preventing heat build-up.

I think I want more GFCI and AFCI.

    Bookmark   February 8, 2007 at 2:14PM
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I guess my comment about "Chernobyl" was too obscure. Let me be more direct: The cause of the Chernobyl disaster was that they shut off several safety systems and deliberately created a fault situation in order to test one of the safety systems. We all know what the result was.

Deliberately overloading a circuit in order to trip a breaker is a VERY BAD IDEA. People have in fact burned buildings down by doing exactly that. While the breaker is supposed to safely protect the wiring, if there are other problems in the wiring, such as a poor splice or damage to a wire somewhere, it is very possible to start a fire. (This is doubly true when it is suspected that the breaker doesn't trip properly.)

And it may not occur immediately. In one case I am aware of, the wire and the wood around it smoldered for several hours before it finally burst into flames and started a major fire.

    Bookmark   February 8, 2007 at 3:24PM
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My CB tester limits the time for testing based on what the breaker should respond to.
You could dial in a longer time if you wanted, but there is no reason to.
Find a curve for the equipment you have before you start playing overload games, and recognize the risk of this testing.

    Bookmark   February 8, 2007 at 7:13PM
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You are probably better off just swapping out w/ a new breaker ($10 or $12) & seeing if you experience the same phenomenom.

You could easily damage the wiring & connections doing it the way you described. You want to use a NON-destructive test for this situation. Don't assume this potentially non-working breaker will be the weakest link in your circuit. While it is designed to be if working properly you have already raised concerns that it may not be working properly. So why do this test that might result with the wiring getting to hot & catching the walls on fire. Of course, I guess that would prove the $10 breaker was not funtioning properly.

This destructive test methodology has too many variables so is not a good idea.

$10 new breaker

10 minutes of your time or $75 to an electrician.

Watching your house go up in flames. Not exactly priceless.

    Bookmark   February 9, 2007 at 11:18AM
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Oh, I should have added your "destructive" test may not result in the walls going up in flames immediately. It could be weeks or months down the road depending on the extent of the weakening of the circuit components.

    Bookmark   February 9, 2007 at 11:38AM
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Just quickly checking in to let everyone know we heeded your advice and did NOT put the potentially faulty breaker to the test. We are planning on replacing the breaker this weekend.

    Bookmark   February 9, 2007 at 1:44PM
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Glad to hear it - that was a drastic idea and not really a solution.

Since you're putting a new circuit in (here in BC I don't think you're allowed to use a 15a circuit for power, it's only allowed for lighting) I'd suggest you put the printer on the 20a and keep the computer on the 15.

I too wonder about the accuracy of the printer's draw but at the same time suggest you do consider replacing it, if you can verify that excessive draw.

The 80% you were mentioning is more a thing of good housekeeping - for safety's sake, a circuit should always be de-rated as it were - just as with electronic components like capacitors, they're usually rated far above the operating voltage you should actually use. For example, I got a collection of capacitors from a surplus store very cheap because they were odd values - one is rated at 100,000 microfarads which is huge but rated only at 15 volts dc - I couldn't use it say, in a car as a noise suppressor in my stereo, as car alternators can put out as much as 13.8 to 14 volts, which would be too close to its peak rated voltage....its absolute limit would probably be 12 volts, but in practice, it'd be better for 6 or 8 volts.

The other thing to note in the life of circuit breakers is they do not have an indefinite life, I've heard talk they should be replaced as soon as they've been tripped a few times, since they can start to act up like yours is, and not trip when they should....also, if you're running them at or above their peak, the likelihood of them failing to trip is increased. A really safety-conscious person probably ought to replace ALL their breakers every few years, although I doubt many do.

    Bookmark   February 9, 2007 at 2:55PM
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A really paranoid person probably ought to replace ALL their breakers every few years.

I fixed that sentence for you pjb. ;)

Circuit breakers are designed to trip many many many times. Otherwise they would just be expensive fuses. Changing them every few trips is extremely excessive.

    Bookmark   February 9, 2007 at 3:19PM
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"here in BC I don't think you're allowed to use a 15a circuit for power, it's only allowed for lighting"

I doubt this is true for BC. It's not true anywhere else in Canada. 15a cc'ts are just as common for receptacles as lighting.

"The 80% you were mentioning is more a thing of good housekeeping - for safety's sake, a circuit should always be de-rated"

Derating in Canada is only required for devices that will draw a continuous heavy load, as in a baseboard heater running many hours straight. It's not necessary for any other type of cc't. If it gives you peace of mind, great, otherwise you're wasting your money by derating every cc't to 80%.

    Bookmark   February 9, 2007 at 3:47PM
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One thing on this situation that has me puzzled is why you need a portable heater in a new house. If you are under a house warranty and that room doesn't heat well, won't they correct the problem for you? Electric eaters are a pretty expensive way to heat.

    Bookmark   February 9, 2007 at 4:12PM
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For coolvt... why we run a portable heater: Our house heat works fine, I just don't set the thermostat hot enuf to suit him because I am a woman with hot flashes, lol, and I cannot stand the house too hot! So hubby will put this little heater (a small Holmes heater) by his feet if he's working on the computer for maybe an hour or so.

    Bookmark   February 9, 2007 at 6:19PM
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hubby will put this little heater ... by his feet ...

Methinks what he really needs is a dog. ;-)

    Bookmark   February 11, 2007 at 9:43AM
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