100% Rated Breaker and its Wire verses 80% Rated Breaker and it Wire

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stuartdmc

Senior Member
My question is this;
If we?re forced to use and install 80% rated circuit breaker then shouldn?t we be able to install the wire rated for 80% of the circuit breaker?

And if we use and install 100% rated circuit breaker then shouldn?t we be able to install the wire rated for 100% of the circuit breaker?

Example:
A 200 amp OCPD rated at 80% (160 amps) The wire should be sized for 160 amp.
And a 200 amp OCPD rated at 100% (200 amps) The wire should be sized for 200 amp.
Right or wrong?
 

infinity

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Staff member
Location
New Jersey
Occupation
Journeyman Electrician
My question is this;
If we?re forced to use and install 80% rated circuit breaker then shouldn?t we be able to install the wire rated for 80% of the circuit breaker?

To call it an 80% circuit breaker is somewhat of a misnomer. You can load the breaker to 100% if the load is non-continuous. You can also load it to 80% if it is continuous.
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
You are looking at the problem backwards. You start by figuring out the load. That process includes adding 25% to continuous loads and 25% to the largest motor. That gives you the required ampacity of the wire. Then you select a wire that has an ampacity at least that high. Finally, you select a breaker that will protect the wire at that ampacity. So if the calculated load (including the extra 25% for the continuous part) is 200 amps, you can select a 200 amp conductor and use a 200 amp breaker.
 
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