Location: Sizing a Breaker

Circuit Breaker Sizing and Breaker Derating

Breakers come in different amp ratings, but the need to be derated for a variety of reasons. Remember, be safe. Breakers do not work under all circumstances because electricity does funny things under very high current conditions, very high altitudes, underwater, in exceptionally dry conditions, etc.. Therefore there are limits to where a breaker can operate properly. Assuming your application for a breaker is satisfactory, let's move into selecting the right breaker for your needs.

AC / DC - Choose a Current Type for the Breaker

Not all breakers are made equally. Some are designed for AC, phased, electrical current. Other breakers are designed for a continual direct or DC current. Make sure you choose the right type of breaker for your circuit.

Voltage - Choose a breaker in the Voltage Class

The next significant thing you need to know when selecting a breaker is the voltage. Do not try to place a 120 VAC breaker in 240 VAC service unless it is marked saying it can. Let's just say bad things WILL happen, and fire is the least of your problems...

Amperage - Choose a breaker in the Amperage Rating Of your Circuit

The next significant thing you need to know when selecting a breaker is the amperage. The whole point of a circuit breaker is to limit the maximum amperage. However, here is where the elegance of the installer really shines. There are a number of things environmentally that can negative or positively effect the breaker in operation. For these reasons we derate breakers, depending on their installation.

Derating for Amperage - The Real Nuts and Bolts of Breaker Selection

There are maximum current ratings for wires, motors, etc. The breaker is designed to be well under these maximums to protect the system. However, if we run a circuit at the maximum level of the breaker for a long enough period, the breaker will trip anyway.

WHY?
Sustained use at 19 amps on a 20 amp circuit will often trip because the heat builds up in the bi-metallic stip over time.
To compensate for this, we need to derate the breaker to handle the load, but stay well within the maximums for the wire, equipment, etc. Usually AC home and office 120 VAC circuits are derated by 20%. The same 20% seems to work well for DC applications as well, but is less well tested. If the ambient temperature, or altitude changes significantly, please derate additionally for these factors. Click foraltitude breaker derating and temperature breaker derating tables respectively.

So to show how this works, we have two examples below.

Assuming the Photovoltaic array is inside the greenhouse (to keep ambient temperatures and avoid that derating) we need High Voltage DC Breakers. We need to be careful to have high enough DC voltage breakers to satisfy the high voltage here. Many Solar Panels have a max voltage over 500 volts, but short circuit at 16 amps. Breakers for these solar installations should then be 15AMP breakers of adequate voltage rating. The 15 amp breaker derates to 12 amps, which is well under the 16 amp short circuit for the string of solar panels.

Let us assume your kitchen runs a microwave at 800 watts, and a 500 watt fridge. When the family comes over for the holidays, they all plug in their appliances in the kitchen. Two crock pots (300 watts each), an electric skittle at 180 watts, and a 200 watt carving knife. They want to know why the 20 amp circuit in the kitchen blows ALL THE TIME!

First a 20 amp fuse runs at 16 amps continuously after derating. Now we need to convert amps and volts to watts to compare apples to apples per se.
16 amps x 120 volts = 1920 watts
800 watt microwave + 500 watt fridge = 1300 watts normally
When the family arrives....
800 watt microwave + 500 watt fridge + 600 watts crock pots + 180 watt electric knife + 200 watt knife = 2280 watts
2280 watts > 1920 watts by 360 watts ... they need to run those crock pots off another circuit in the house.
Try the living room ;)

Electrical How To and Why Guides:
Breakers and System Protection
Fuses and Electronics Protection
Which Wire Should I use
Solar Charge Controllers, What, How, and Why.
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