Breakers

[Skyline]

Walmart actually has 2000 watt 110 volt portable electric heaters for sale on the website. I would think they would have 20 amp plugs.

plugs.

 

[FISTEP]

I'm not an electrician and I definitely don't know the codes where you may live so I readily admit that I could be off base on this, but...
I know there are rules, though I think they are actually guidelines, that suggest that circuits be limited to 80% of the rating of the breaker for continuous or sustained loads. They also recommend that loads of 100% of the rating be limited to some period of time, i.e. three hours.
However, the breakers themselves are designed to sustain 100% of their rated current indefinitely without tripping, within design parameters. Elevated ambient temperatures for example, can de-rate the breaker to below it's rating. For this reason, I was always conservative when specifying power supplies, wiring and circuit protection when designing electrical systems. My experience is mainly in DC systems.
Most of this applies to resistive loads, which are more predictable than inductive loads like motors. But, I would agree that an 1800W heater is a very heavy load, considering that it could be considered to be on virtually permanently if it is cold, and every part of the circuit; breaker, wiring, plugs will be pushed to it's design limits with each a point of failure. However, an 1800 watt kettle will boil a quart of water in about four minutes, and a blow dryer can dry whats left of my hair in less than that.

 

[Skyline]

Im not an electrician. The way I’m understanding this a single dedicated 20 amp outlet would safely operate an 18,000 watt heater. It would be using 15 amps, at 120 volts, 75% of 20 amps. A 2000 watt heater would be using 16.667 amps, at 120 volts, 16.667amps is 83.3% of 20 amps close to the recommended 80% maximum.

 

[craigav]

Breakers such as in an RV and at home are rated to handle 100% if the load is intermittent and 80% for continuous loads. For example if you plugged in a 1800W heater to a 15A breaker protected circuit with no other loads it will work for a while, and if it is turning off and on with the thermostat setting, it may work without ever tripping the breaker however, if it stays on (the heating element) continuously, the breaker will trip in time, right around 3 hours under test conditions.

 

[Skyline]

There are breakers rated for 100% load continuous use. They are not in RVs as far as I know. “100% load breakers” changes a lot.

 

[Skyline]

 

[FISTEP]

Breakers such as in an RV and at home are rated to handle 100% if the load is intermittent and 80% for continuous loads. For example if you plugged in a 1800W heater to a 15A breaker protected circuit with no other loads it will work for a while, and if it is turning off and on with the thermostat setting, it may work without ever tripping the breaker however, if it stays on (the heating element) continuously, the breaker will trip in time, right around 3 hours under test conditions.

Craig, you have me going down the rabbit hole again. I see lots of references to this and I believe you but I cannot find a curve that shows this working. I see curves that show breakers holding at 1X rated current for 10,000 seconds, something like three hours, but the time scale doesn't continue. I cannot find the NEC code that states that or when it came to be a rule. Do you have that reference? I don't mean waste time on research, but if you have that, I'd like to see it. If not, I'm tapping out on household breakers.

 

[craigav]

Craig, you have me going down the rabbit hole again. I see lots of references to this and I believe you but I cannot find a curve that shows this working. I see curves that show breakers holding at 1X rated current for 10,000 seconds, something like three hours, but the time scale doesn't continue. I cannot find the NEC code that states that or when it came to be a rule. Do you have that reference? I don't mean waste time on research, but if you have that, I'd like to see it. If not, I'm tapping out on household breakers.

Interesting question

I didn't see this until late and may not be on tomorrow. Kevin Cooper may be able to better explain this with more time and more detail.

Quick answers are that the 80% rule isn't in the NEC code. The 80% rule that you may hear about originates in NEC 210.20. (My brief info following here is regarding continues loads but the code further expands on both continuous and noncontinuous loads), with that the code states that the breaker is not to be less than 125% of the continuous load. For an example here we will take a 15A breaker and the math would be "15A / 125% = 12A"

The 80% rule is simply the inverse which is easier for most to calculate in their head "15 * .80 = 12A" For the sake of others and keep it simple 12A (which is 80% of the breaker) and at 120V (120V x 12A) would equal 1,440 watts available and at say 110V that would be 1320 watts (for a continuous load, and it is common that the voltage should be very close to 120V but many times the voltage is a bit lower when electrical demand is high and in some campgrounds).

In addition to the above which wasn't your exact question outside of not finding anything in the NEC codes, is that most trip curves I have seen are based on an open air conditions and a specific temperature environment. Generally the curve charts stop at 10,000 seconds as past that is considered continuous usage (in other words the curve becomes straight).

Other thoughts are (as I remember) there is around 15~20% trip current tolerance allowed on common household style breakers... and this is just a memory, a residential style breaker is in of itself capable to hold 100% of its rating in open air and 40C (104f) (or less). However, the trip rating is derated when the breaker is in a panel of breakers which brings it back down to around the 80% maybe a little more depending on the temperature in the breaker panel and specifically at the breaker (there isn't much air flow around the breaker once in the panel). ~CA

 

[craigav]

Just another quick thought to add, a residential breaker is tripped by Thermal or Magnetic conditions. The magnetic trip amperage rating is very high and designed more for short circuit protection. The thermal aspects are where the trip timing gets more tricky so to speak, as the breaker isn't actually measuring the current flowing and instead acts similar to a thermostat (that doesn't reset although many 12v breakers do reset when cooled back down) and as the current increases through the breaker so does the temperature inside of the breaker and at a certain temperature, the breaker trips. If the breaker was in a panel in direct sunlight on a hot summer day, it would trip much sooner (perhaps even less than 80%) than it would if it was outside at night in subfreezing temperatures (which could be even above 100% of its rating, but not a lot more).

When it comes to purchasing residential breakers (such as in an RV as well), they are not sold or labeled as 100% or 80%, at least not as I have ever seen. (My career was more industrial related and not residential.) ~CA