CB Sizing

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Alwayslearningelec

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NJ
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Estimator
I don't understand how how you decide whether you would need a 175A or 200A breaker. Just the breaker rating?? Because your sizing your wire to the breaker size.

Example 1: 50A continuous load and 125A noncontinuous load.

OCPD = 100% noncontinuous load + 125% continuous load = (1.00 x 125A) + (1.25 x 50A) = 187.5A
Therefore, a 200A OCPD is needed. If a 100%-rated CB is chosen, a 175A rating (125A + 50A) is acceptable.
Click to expand...
 
Don't screw around with 80% breakers. They're not all that easy to come by when there are normal supply chains.

If this is a combination of motor and non motor loads you may find when you run the numbers that you can have a substantially higher rated circuit breaker compared to the ampacity of the conductors. You just have to run the numbers.
 
horsegoer said:
Example 1: 50A continuous load and 125A noncontinuous load.
OCPD = 100% noncontinuous load + 125% continuous load = (1.00 x 125A) + (1.25 x 50A) = 187.5A
Therefore, a 200A OCPD is needed. If a 100%-rated CB is chosen, a 175A rating (125A + 50A) is acceptable.
Click to expand...

Just figure on not using a 100% rated circuit breaker so 200 amps.
 
petersonra said:
Don't screw around with 80% breakers. They're not all that easy to come by when there are normal supply chains.

If this is a combination of motor and non motor loads you are you fine when you run the numbers that you can have a substantially higher rated circuit breaker compared to the ampacity of the conductors. You just have to run the numbers.
Click to expand...
So in summary for feeder sizing and OCPD. 125% of continuous load and 100% of continuous.

Say 200A cont and 100A non cont. so that would be 350A calculated load. So a 350A breaker with 350A wire. Correct?
 
TO piggy back on this calc I did above . How would one size the breaker (OCPD) for the 350A wire
infinity said:
200*125%+100=350 amps. That is the minimum feeder size.
Click to expand...
Thanks. Working backward to the source in my calc.
Panelboard has 350A breaker with 350A wire landing on that breaker. What's the calculation for the OCPD in the distribution panel feeding the panelboard with 350A MCB( yes with 350A wire)
 
horsegoer said:
TO piggy back on this calc I did above . How would one size the breaker (OCPD) for the 350A wire

Thanks. Working backward to the source in my calc.
Panelboard has 350A breaker with 350A wire landing on that breaker. What's the calculation for the OCPD in the distribution panel feeding the panelboard with 350A MCB( yes with 350A wire)
Click to expand...
If you're panel is 350 amps and the conductors are 350 amps then most would use a 350 OCPD at the origin of the feeder. It could be smaller if the load calculation called for less than 350 amps of load.
 
infinity said:
If you're panel is 350 amps and the conductors are 350 amps then most would use a 350 OCPD at the origin of the feeder. It could be smaller if the load calculation called for less than 350 amps of load.
Click to expand...
Thank you! So your origin breaker could be rated less than the ampacity of the wire in some cases depending on connected load?
 
horsegoer said:
Thank you! So your origin breaker could be rated less than the ampacity of the wire in some cases depending on connected load?
Click to expand...
It depends.

It's not entirely clear to me what you have here. Do you have two motor loads one with 200 FLA and the other with 100 FLA? If so the feeder to these two motor loads would need to be a feeder with 350 amps worth of ampacity.

The cb overcurrent protection for the feeder could be a max of 250% of the 200 amps plus the 100 amps or 600 amps.
 
petersonra said:
It depends.

It's not entirely clear to me what you have here. Do you have two motor loads one with 200 FLA and the other with 100 FLA? If so the feeder to these two motor loads would need to be a feeder with 350 amps worth of ampacity.

The cb overcurrent protection for the feeder could be a max of 250% of the 200 amps plus the 100 amps or 600 amps.
Click to expand...
loads are not motors
 
horsegoer said:
Thanks. Working backward to the source in my calc.
Panelboard has 350A breaker with 350A wire landing on that breaker. What's the calculation for the OCPD in the distribution panel feeding the panelboard with 350A MCB( yes with 350A wire)
Click to expand...
You said the load is 200A continuous and 100A non-continuous. The panelboard has a a 350A breaker and 350A wire...

The calculation for the ocpd in the upstream panelboard is the same....350A.
 
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