SIZE OF CABLE COMPARED TO BREAKER

[m sleem]

I want to confirm that:

for example: Breaker size is 80 AT

feeder size should carry out 80A

OR feeder size should carry out 90A

OR feeder size should carry out 70A

 

[infinity]

You need to first determine the type of load and whether or not is will be continuous or non-continuous.

 

[m sleem]

could you please explain for the both cases...

 

[augie47]

Your feeder should be sized to your load.... 100% of non-continuous plus 125% of continuous.
Then select the overcurrent to protect the feeder.
(There are execptiosn for motor loads, etc)

 

[infinity]

could you please explain for the both cases...

If you had a non-continuous resistive load then the 80 OCPD in your example could be loaded to 80 amps. The same resistive load that is continuous could only be 64 amp load (64*125%) on the same 80 amp OCPD. Motors change the scenario completely because the OCPD is only providing short circuit and ground fault protection.

 

[kwired]

First thing is to do what Augie said. Size conductor to load. Then size overcurrent device - certain cirumstances may require larger overcurrent device than conductor ampacity - especially when supplying motors.

You could have conductor larger than the overcurrent device because of deratinig of the conducotr - but derating is an adjustment of ampacity and in that particular case conductor is technically no longer higher ampacity.

You can also have larger conductor as a result of compensating for voltage drop.

 

[resistance]

In a nut shell: What they said, and I recommend you pick-up a study guide?as there is no one answer, because it will depend on the installation.

 

[templdl]

As they said, you must size the cable to carry the load first. Have you established the cable size yet?
Then, the breaker is sized to protect the cable.

 

[m sleem]

240.4(B) Devices Rated 800 Amperes or Less.

The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

(1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.

(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed 800 amperes.

Please can we discuss 1 & 2

 

[augie47]

Please can we discuss 1 & 2

Not sure what there is to discuss.
What part of the text do you not understand ?

In (1) you can't go to the next size up in the conditions stated. For example if you derate #12s to a amperage less than 20 and you are feeding multiple receptacle outlets on a multi-wire branch circuit you can't upsize to a 20 amp breaker.

In (2) you can't go above the next "standard size" OCP from 240.6.
For example if your derated factor is 69, or 70, you can't go beyond 70 since that's a standard size.

 

[m sleem]

Not sure what there is to discuss.
What part of the text do you not understand ?

In (1) you can't go to the next size up in the conditions stated. For example if you derate #12s to a amperage less than 20 and you are feeding multiple receptacle outlets on a multi-wire branch circuit you can't upsize to a 20 amp breaker.

In (2) you can't go above the next "standard size" OCP from 240.6.
For example if your derated factor is 69, or 70, you can't go beyond 70 since that's a standard size.

Many thanks.........clear now