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In a normal, properly-wired home, what are the possible causes of a tripped breaker? At 110-120V is there a maximum wattage throughput on a single breaker?

I'd like to understand the system to be able to know how best to lay out the electrical system in a brand new house. I'm not an electrician, but I do understand certain basic concepts. (I'm also not planning necessarily on doing the work myself, but I'd like to understand the whole line of thinking behind electrical system layout.)

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When planning circuits, you'll typically only want to load the circuit to 80%. So for a 15A circuit @ 120V, you'll only want to have ~1440 Watts. You also have to consider that this is in a perfect world, where the voltage is always 120V. In the real world, a good supply will be +- 5%, so you could be anywhere from 114V to 126V (@80% = 1368 - 1512 Watts). – Tester101 Jun 3 '13 at 12:04

Short-circuit Protection

A breaker that offers short-circuit protection, will trip if there is a short somewhere in the circuit. A short-circuit is characterized by a very high current increase in a very short period of time. This type of protection is usually provided by an electromagnetic device; commonly an electromechanical solenoid, which physically opens the circuit when a short-circuit is detected. Most residential short-circuit protection devices trip at 10 kA (10,000 A).

Overcurrent Protection

Breakers that offer overcurrent protection, trip when a current higher than the rated current is drawn through the device over a specific amount of time. An overcurrent situation is usually detected using heat, and so is slower acting than short-circuit protection. Overcurrent protection is usually provided by a bimetallic strip, which trips the breaker if it is heated too much for too long.

Most typical residential circuit breakers are combination devices, and offer both short-circuit and overcurrent protection. Wikipedia has a great image of the inside of a circuit breaker, which shows both short-circuit and overcurrent protection mechanisms.

Circuit breaker cross section

  • 5 - Bimetallic strip providing overcurrent protection.
  • 7 - Electromechanical solenoid offering short-circuit protection.

Ground-fault Protection

Ground-fault protection is a recent addition to circuit breakers, which offers protection to people and equipment. Breakers that offer ground-fault protection, trip when the current flowing out to the circuit is higher than the current returning from the circuit. A current transformer (CT) inside the breaker, is a common way used to measure current imbalances. The CT will be wrapped around both conductors, and if a large enough current is induced on the CT the breaker will open the circuit. Residential ground-fault circuit breakers are almost exclusively combination devices offering short-circuit, overcurrent, and ground-fault protection.

Arc-fault Protection

Arc-fault protection is the most recent addition to circuit breakers, and offers protection against dangerous electrical arcs. They use complex electronics to analyze the circuit looking for tell tale signs of dangerous electric arcs, all the while ignoring non-hazardous arcs. As with ground-fault circuit breakers, arc-fault breakers are almost always combination devices as well.

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Breakers trip when an overcurrent condition occurs. That's it.

With some caveats we can ignore, overcurrent = load greater than the breaker's rating. For example, if you draw 50A on a 15A breaker, it will trip.

However, overcurrents can be considered in two groups.

Some are effectively infinite - an infinite draw on a breaker of any size is always an overcurrent. This happens when there's a short circuit, for example if a rodent chews through the hot wire in the wall, and it touches ground. If you're getting a short circuit in your home wiring, a repair will be required.

The other kind is typically within the same order of magnitude of the breaker's rating, for example if you put three tea kettles on the same circuit in your kitchen. You want the breaker to trip here, too, but it's annoying. This is the user's error, and can be remedied by putting devices on separate circuits, or only running 1 at a time, etc.

In all cases, the purpose of the breaker is to protect the in-wall wiring for that circuit. If the breaker did not trip, then the wire would act as a fuse, heating up, and eventually melting to break the circuit. A fire might have started in the meantime.

Usually a breaker is at the beginning of a circuit, but the main breaker for a panel (or subpanel) is often at the end of the circuit. That means that flipping the breaker off will not de-energize the wire it is protecting.

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So, TL;DR: A breaker trips when there's more amperage draw than is allowed/rated by the breaker itself? – Naftuli Tzvi Kay Jun 2 '13 at 20:00
Exactly right. *** – Jay Bazuzi Jun 2 '13 at 20:01
Unless we're talking about a GFCI or AFCI breaker. – BMitch Jun 2 '13 at 20:05
Most breakers trip just shy of infinity, usually around 10kA. – Tester101 Jun 3 '13 at 11:02

Circuit Breakers are rated in Amps and fail open when the rating is exceeded.

Excessive current causes heat, leading to fire danger.

If you're going to base your calculations on wattage, you have to know the formula.

Power (Watts) = EMF (Volts) & I (Current)

P/E = I or 3000W/120V = 25A

Plugging two 1500 watt hair dryers into a single circuit and using both on high will do it. 3000 Watts @ 120V = 25A and your circuit breaker will usually be 20A. You're 5A over limit, so pop.

Same goes for using a 2250W (3 hp) motor on the circuit, but the problem here is the inrush current during motor start will pop the breaker as will bogging the motor down (current increases as back-emf decreases)

So, too many devices on the circuit, or not having the appropriate dedicated circuit for a high power device will lead to current drawn exceeding the breaker's rating.

Maximum Wattage for common house wiring circuits, split phase:

240V @ 50A = 12000W
240V @ 30A =  7200W
240V @ 20A =  4800V

120V @ 30A =  3600W
120V @ 20A =  2400W
120V @ 15A =  1800W
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Is there a reason why using a lower-amperage breaker would be appropriate? Is there a certain type of wiring which is necessary for higher-load applications? ie: why wouldn't I wire my entire house with 30A breakers? If it's because of heat, can't I use better insulated wire to work around the problem? – Naftuli Tzvi Kay Jun 2 '13 at 21:54
You need to break out the NEC and start studying why things are done the way they are. Yes, you could oversize the wire on everything, no, it wouldn't pass code inspection is the short, succinct answer. Explanation would run into tldr; we've arrived at doing things so they fail safe, excessive copper and amperage available where it shouldn't be allows things to fail disastrously. – Fiasco Labs Jun 2 '13 at 22:00
@TKKocheran Breakers are sized to protect the wire in the circuit from overheating. You could install 30A breakers on every circuit, but then you'd have to use the appropriate sized wire. This is almost always cost prohibitive. There is also the problem that you'd have to modify all of your cord-and-plug devices (Radio, TV, etc.), to allow them to be plugged in to a 30A receptacle. – Tester101 Jun 3 '13 at 11:10
@FiascoLabs NEC does not give you the why, it only gives you the how. The why according to NEC, is because they say so. – Tester101 Jun 3 '13 at 14:50

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