OCPD Sizing Question

Status
Not open for further replies.
fifty60

fifty60

Senior Member
Location
USA
I know that 100% noncontinuous load + 125% continuous load is used to size a branch circuit breaker if it is an 80% rated breaker, and 100% noncontinuous + 100% continuous if the circuit breaker is rated for 100%.

I also believe that I remember something in the NEC that used the actual size of all the circuit breakers to determine the size of the branch Circuit breaker. I'm not sure where I seen this, and it is driving me crazy. Is it for sizing a feeder and not a branch? The feeder has to be sized to not exceed the sum of all the breakers? If so, where is that located in the code?
 
240.92(C)(2)

240.92(C)(2)

240.92(C)(2) talks about sizing of circuit ampacity based upon 6 disconnect rule.
 
fifty60 said:
The feeder has to be sized to not exceed the sum of all the breakers? If so, where is that located in the code?
Click to expand...

There is no such rule in the Code.

Feeder OCPD's are also sized using 100% of the non-continuous load plus 125% of the continuous load, per 215.2(A)(1)...(with the same exception for 100% rated assemblies.)
 
fifty60 said:
I know that 100% noncontinuous load + 125% continuous load is used to size a branch circuit breaker if it is an 80% rated breaker, and 100% noncontinuous + 100% continuous if the circuit breaker is rated for 100%.

I also believe that I remember something in the NEC that used the actual size of all the circuit breakers to determine the size of the branch Circuit breaker. I'm not sure where I seen this, and it is driving me crazy. Is it for sizing a feeder and not a branch? The feeder has to be sized to not exceed the sum of all the breakers? If so, where is that located in the code?
Click to expand...

This is not true so it is not in the code
 
What I am looking at is involving the almost continuous loop that is created by 430.63 and 430.62 of the 2017 NFPA 70. I think it makes sense. For a load that is just Motors, then 430.62 only lets you size based of the largest branch OCPD setting plus the sum of the the other motor loads.

But, if it not just motor loads, then per 430.63 you cannot size LESS than the non motor loads plus the motor load calculated from 430.62.

So with just motor loads, you are given a no greater than, but if is a mixed load you get a no less than...seems to contradict but it makes sense...
 
(2) The sum of the overcurrent devices at the conductor
termination limits the load to the conductor ampacity.
The overcurrent devices shall consist of not more than six
circuit breakers or sets of fuses mounted in a single enclosure,
in a group of separate enclosures, or in or on a
switchboard or switchgear. There shall be no more than
Click to expand...
What i understand from code section 240.92(C)(2), the conductor ampacity after derating shall not be less than sum of over-current devices, am i right?
 
m sleem said:
What i understand from code section 240.92(C)(2), the conductor ampacity after derating shall not be less than sum of over-current devices, am i right?
Click to expand...
For 240.92(C)(2)(2), yes (except if derating is higher than termination temperature limitation, then the latter sets the ampacity limit).

For those that are not familiar with 240.92, it concerns feeder taps in supervised industrial installations, and 240.92(C)(2) is about OCP at the end of the taps.
 
fifty60 said:
I know that 100% noncontinuous load + 125% continuous load is used to size a branch circuit breaker if it is an 80% rated breaker, and 100% noncontinuous + 100% continuous if the circuit breaker is rated for 100%.
Click to expand...

Not exactly. You use 100% of noncontinuous load + 125% of continuous load to determine the minimum ampacity of the conductors required. You then select the appropriate OCPD to protect those conductors.
 
jrohe said:
Not exactly. You use 100% of noncontinuous load + 125% of continuous load to determine the minimum ampacity of the conductors required. You then select the appropriate OCPD to protect those conductors.
Click to expand...
Actually, you use 100% of noncontinuous load + 125% of continuous load to determine the minimum OCPD rating required also. But just as you have to upsize the conductor to match a recognized size, you can or have to upsize the minimum OCPD rating to a standard OCPD rating [240.4(B) & (C)]. If the actual minimum conductor size is not protected by the actual OCPD rating, the actual minimum conductor size must be increased such that the actual OCPD protects it at its ampacity.
 
fifty60 said:
100% noncontinuous + 100% continuous if the circuit breaker is rated for 100%.
Click to expand...

To be clear, there is no such thing as 100% rated breaker. There is great area and ventilation openings in the gear around the breaker to achieve 100%. It is a “100% rated assembly” that we are discussing.
 
publicgood said:
To be clear, there is no such thing as 100% rated breaker. There is great area and ventilation openings in the gear around the breaker to achieve 100%. It is a “100% rated assembly” that we are discussing.
Click to expand...
True... but the breaker must be rated for use in a 100% rated assembly.
 
m sleem said:
I cannot understand the reasoning behind.
Click to expand...
Let's say you are using #4 THHN with 3 current carrying conductors in a conduit at an ambient temperature of 90°F.

No adjustment per Table 310.15(B)(3)(a).
A 0.96 correction applies per Table 310.15(B)(2)(a).

95A × 100% × 0.96 = 91.2A derated.​

However, because the terminal temperature limitation is 75°C, the circuit is limited to 85A, the value in the 75°C column of Table 310.15(B)(16).
 
Status
Not open for further replies.
Top