Max surge load on a 15amp breaker

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DanielG

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Technically a 15amp breaker should support a non continuous load of 1800 watts and a continuous load of 1440w. Now, how fast the breaker trips is dependent on a lot of things, amount of surge, heat, age of breaker, condition. If a line had a continuous load of say 1000w, but a surge a few times a day to 2000w for up to 3 seconds(compressor on the fridge), most 15amps should not trip? I suppose really only testing the circuit will tell for sure. Personally I think it should hold as the surge is less than 2 amps above rating and the duration is so short that it could not build up enough heat. I might be able to move something if it gives me trouble. Any thoughts?

-Dan
 
180114-1550 EST

DanielG:

You should be OK.

Here is a very rough example.

I have a DeWalt radial arm saw. I believe the motor is rated about 1.5 HP. I am not where I can look at it. A ballpark full load current would be about 12 A. This was loaded with a 12" saw blade with carbide teeth. Thus, a high inertia load. The motor was connected for 120 V, and that at 1000 VA per HP is how I am estimating 12 A in my head.

This saw was connected at the end of about 120 ft of #12 copper on a Sq-D QO 20 breaker. RMS current for starting was about 80 A for about 4 seconds because of the high inertia load and high voltage drop during starting.

Note: 80 A is 4 times the breaker rating or 16 times the power being dissipated in the breaker compared to that at full rating.

Breaker panel was at about room temperature of 70 F. If the breaker was allowed to cool with no load for possibly 30 to 60 seconds, then I could always start without tripping the breaker. If only waiting a few seconds to try a restart I would almost always trip the breaker.

What you are proposing would always allow lots of cooling (just several times per day, probably about 12 to 24 times). Your increase in breaker power dissipation for a few seconds, probably less than 1 second, from 15 A to 17 A would only be about 1.3 times in power. But I don't think that is what you are proposing.

Modern refrigerators are running longer duty cycles and smaller motors. Average power under 100 W. Running power possibly 200 W.

Look at the trip time curves for QO breakers, standard blow fuses, and slow blow fuses.

.
 
Last edited:
Dan, my thought is that a refrigerator ought not be on a 15-amp circuit if that circuit is supplying other loads.
 
Welcome to the forum, Dan.

Imho, you are grossly overestimating the surge/start-up time on the refrigerator compressor. They are small hermetically sealed units with low inertia or start-up draw; 3 seconds.... more like 15 cycles or a quarter second. something like a bench grinder swinging two heavy stone discs may take a solid 5 seconds to bring up to speed.

No properly functioning 15A breaker is going to trip carrying 16.7A in either a quarter second or three seconds. If you look at this breaker trip charts:

http://static.schneider-electric.us...100-400 A Frame FA-LA/FA-FC-FH/0600DB0105.pdf

Notice that an overload of 10% doesnt even put the breaker into the trip range within the 10,000 second high end of the chart. Sure, they can still trip on thermal overload, but a 15A breaker could carry 16.7A indefinitely.

If you look at your refrigerator, there is a fairly good chance it draws less than 3A, probably half that.

In other words, if adding the frig causes problems, your 15A breaker was already loaded past 12A in the first place.
 
2000W “surge” is only about 17A if it’s a heating load, correlating to only 113% of the trip rating. A 15A breaker may never trip at 17A load, that’s not even in the pick-up percentage.

Here is a GE trip curve. 113% shows as anywhere from 10 minutes to no trip. Even at 200% (30A), it could be anywhere from 16 seconds to 2 minutes.

https://i.stack.imgur.com/CwAqH.png
 
gar said:
180114-1550 EST

DanielG:

You should be OK.

Here is a very rough example.

I have a DeWalt radial arm saw. I believe the motor is rated about 1.5 HP. I am not where I can look at it. A ballpark full load current would be about 12 A. This was loaded with a 12" saw blade with carbide teeth. Thus, a high inertia load. The motor was connected for 120 V, and that at 1000 VA per HP is how I am estimating 12 A in my head.

This saw was connected at the end of about 120 ft of #12 copper on a Sq-D QO 20 breaker. RMS current for starting was about 80 A for about 4 seconds because of the high inertia load and high voltage drop during starting.

Note: 80 A is 4 times the breaker rating or 16 times the power being dissipated in the breaker compared to that at full rating.

Breaker panel was at about room temperature of 70 F. If the breaker was allowed to cool with no load for possibly 30 to 60 seconds, then I could always start without tripping the breaker. If only waiting a few seconds to try a restart I would almost always trip the breaker.

What you are proposing would always allow lots of cooling (just several times per day, probably about 12 to 24 times). Your increase in breaker power dissipation for a few seconds, probably less than 1 second, from 15 A to 17 A would only be about 1.3 times in power. But I don't think that is what you are proposing.

Modern refrigerators are running longer duty cycles and smaller motors. Average power under 100 W. Running power possibly 200 W.

Look at the trip time curves for QO breakers, standard blow fuses, and slow blow fuses.

.
Click to expand...
Standard 15 and 20 amp single pole QO breakers do have lower magnetic trip setting as well then other breakers, even a 15 or 20 amp 2 pole QO breaker.

If you do have high inrush loads and have a lot of nuisance tripping when starting they do make ones with "HM" suffix in cat no, which stands for high magnetic. They have a higher magnetic trip setting then the standard breaker has. Probably out of luck at this point if you need AFCI also though.
 
180114-2400 EST

kwired:

What I described was almost certainly thermal tripping because there was insufficient time for the thermal element to cool sufficiently. In a QO the trip actuator is a bimetal u shaped mechanism, and because of the u shape it is also a magnetic actuator, but at much higher current than the thermal actuating point.

.

.
 
gar said:
180114-2400 EST

kwired:

What I described was almost certainly thermal tripping because there was insufficient time for the thermal element to cool sufficiently. In a QO the trip actuator is a bimetal u shaped mechanism, and because of the u shape it is also a magnetic actuator, but at much higher current than the thermal actuating point.

.

.
Click to expand...
Probably is thermal tripping in your case.

I did want to mention not all breakers trip at same point, and that QO is one that has low mag setting on single pole 15 and 20 amp units. Since you mentioned QO - you just sort of the one that got replied to.
 
Thx for your replies. I don't think it will trip. Continuous load on the circuit is 950w with the potential for 300w washer for a few hours most. The fridge is an old one. Tested it and it drew 865w when the compressor kicks on, then drops to about 180w running. So really the max the line would take for a couple seconds is 950+300+865 or 2115w which should be within tolerances. Probably since the washer is only on for 2 hours a day max, it is most likely that it would only hit that once per day for 2 seconds. And the line will not exceed a continuous load above 1440w.
Ideally would like to have the fridge on its own circuit. Just not possible. I know when I was doing some running wire on new homes a few years ago the fridge was on a dedicated circuit with the hardwired smoke alarms the only other thing allowed, as people would notice their fridges off before they might notice another breaker in a large panel.
 
Interesting thread....

So i've a related Q, does a standard exist for magnetic trip levels in the residential OCPD (IIRC, referred to as 'miniature' breakers) manufacturing universe?

~RJ~
 
romex jockey said:
Interesting thread....

So i've a related Q, does a standard exist for magnetic trip levels in the residential OCPD (IIRC, referred to as 'miniature' breakers) manufacturing universe?

~RJ~
Click to expand...
I'd say yes, but appears to be a pretty wide lane to drive through.
 
There is a NEMA design standard of sorts, albeit "voluntary". No less than 400% (4x) the stated trip value, no more than 1,000% (10x). Most molded case breakers have a separate hard stop at 10x anyway because that's usually the mechanical limit of the breaker's interrupting capability. So I'd say the majority will start tripping at around 7-9x, absolutely at 10x. Test values however are +25%, -25% as well, so it's really kind of nebulous.
 
Interesting stuff.

I'll wager many here have read Fred Franklin's old (40 yr)piece on this.

It makes me wonder that, if mag trips were such a problem, why didn't some formal standard come out ?


~RJ~
 
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