# 80% amperage rule for continuous loads?

I'm trying to understand how an electrical load affects a circuit.

Let's say, for example, I have a setup that is on a 120 V, 15 A circuit:

• 120 V x 15 A = 1,800 W max
• if we have a continuous load (3+ hours), then we should aim for 80% amperage; so 15 A becomes 12 A and 120 V x 12 A = 1,440 W max
• if we maintain a continuous load of less than 1,440 W, there is nothing to worry about
• if we ever reach 1,801 W, the circuit breaker turns off immediately to prevent any damage to the circuit

My question is: If we have a system running continuously at somewhere in between (let's say 1,600 W), what happens?

• Does it do long-term damage to either the circuit or the electrical device(s)?
• Does it do short-term damage to either the circuit or the electrical device(s)?
• Does it make no difference at all until a new load is introduced that pushes it over 1,800 W?

And does it make a difference whether the voltage or amperage changes?

• If we have 240 V x 20 A = 4,800 W max and 240 V x 16 A = 3,840 W (continuous), and we run the system somewhere in between (like 4,200 W continuous), would the same concept apply (ie, same level of damage/non-damage) or would it be less or more damaging due to more power being involved?

Edit: Thank you for all your answers. I regret being able to only vote 1 answer as being correct. :[

• Isn't the 80% rule just a regulatory thing? I know in the US you're not allowed to sell appliances that draw more than 1500 watts continuously if they're meant to be plugged into a normal outlet (this is why every space heater ever is 1500 watts). Outside of regulatory things, the "rule" is only a rule of thumb. Commented Dec 5, 2021 at 19:19
• In reality, wire heating is dependent upon many factors that are often too variable, unknown, or complicated to calculate in practice. Commented Dec 5, 2021 at 19:20
• I'm going to answer it anyway, but this is actually a better question for diy.stackexchange.com, because it has to do with the intersection between home wiring practices, engineering, human stupidity, and the regulations they engender. Commented Dec 5, 2021 at 19:36

Assuming that this is someplace where the US or Canadian electrical codes apply, a 15 amp circuit is designed to be loaded at 15 amps 24/7 with no damage and only long-term (many years) deterioration. The 80% rule says that no individual load shall exceed 80% of the branch circuit current rating. That reduces the probability of nuisance circuit breaker tripping. Nuisance tripping may tempt users to use extension cords on a continuous basis or otherwise engage in less safe usage practices.

Here is a trip curve for typical residential circuit breakers in the US.

• Pennies behind the fuses, even. If pennies were still made of copper, and fuses were still screw in. Commented Dec 5, 2021 at 19:52
• That is exactly the behavior that may have prompted the rule. Today, not many of us are likely to have heard of that.
– user80875
Commented Dec 5, 2021 at 20:08

Disclaimer: the question implies North American wiring standards, and that's what I know. I suspect that anywhere that has decent standards still deals adequately with the underlying issues, and some probably have similar standards. But I don't know.

if we ever reach 1,801w, the circuit breaker turns off immediately to prevent any damage to the circuit

Actually no, and that's part of the reason for the recommendation.

Breakers work because the current heats up a bit of metal, and when that metal gets above a certain temperature it trips the breaker. They're not precision instruments, and because the trip happens at an absolute temperature while the temperature reached by the active element depends on the ambient temperature, a breaker will carry more current in a really cold room than it will in a really hot room.

Probably the biggest reason for the standard that says you can't sell a 15A appliance with a 15A plug is that if the circuit breaker were perfect, and someone plugs in that hair dryer (or whatever) and a couple of other things, then the circuit will exceed 15A and the breaker will trip. If that starts happening on a regular basis, then someone may be tempted to replace that 15A breaker with a 20A breaker, and then the house might catch on fire.

Or the wiring is old and faulty, and develops hot spots where it shouldn't, even when the breakers aren't tripping. Here again, the regulations ask us not to tempt fate.

I don't know what the allowable amount of variation in current there is for a "15A" household breaker, but there's something. Some internet research here will help you get a handle on it.

I also don't know what the allowable amount of variation is in the actual power draw of a "1500W" 120V device -- but given the simplicity of construction, and the fact that power line voltages vary, I suspect it's a lot.

So you circle back to not wanting to plug in your device and have breakers pop, and that means margin, and someone decided that a 20% margin would be good.

• Not all breakers are thermal, plenty of them are magnetic or even active electronic current-monitoring for some really fancy ones. None of them are very precise, however (intentionally so in the case of the really fancy electronic ones). Commented Dec 5, 2021 at 19:55
• I believe that all circuit breakers used for 120V distribution in USA & Canada have both thermal and magnetic trip.
– user80875
Commented Dec 5, 2021 at 20:24
– user80875
Commented Dec 5, 2021 at 20:44

if we ever reach 1,801w, the circuit breaker turns off immediately to prevent any damage to the circuit

In the real world, breakers are nowhere near that precise, nor that fast. The breaker would probably never trip. Even if it did, it may take a long time. The bigger the overload, the faster the breaker trips.

Does it do long-term damage to either the circuit or the electrical device(s)?

Maybe. You can't tell, but it could overheat the wiring, shortening its life. The device won't care.

Does it do short-term damage to either the circuit or the electrical device(s)?

I'm not sure what short term damage even means. If it's damaged, it won't fix itself.

Does it make no difference at all until a new load is introduced that pushes it over 1,800 W?

Adding more load increases the risk of overheating the cable. The breaker may trip if the overload is great enough.