SIZING A FEEDER

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mstrlucky74

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I have a 3P 60A breaker from a 120/208v panel feeding a 100A MLO 120/208v panel.

I guess they don't think the continuos load will be greater than 60A?
I need to take 125% of 60A and size my feeder based on that before any correction or adjustment factors?

Thanks.
 
The continuous load can not exceed 48 amperes with a 60 amp OCPD protecting the feeder.

215.3 requires the that OCPD protecting a feeder be sized not less that 100% of the non-continuous load plus 125% of the continuous load.

You need to determine the load, then size the conductors to that load and the OCPD to protect the feeder conductors.

Chris
 
raider1 said:
The continuous load can not exceed 48 amperes with a 60 amp OCPD protecting the feeder.

215.3 requires the that OCPD protecting a feeder be sized not less that 100% of the non-continuous load plus 125% of the continuous load.

You need to determine the load, then size the conductors to that load and the OCPD to protect the feeder conductors.

Chris
Click to expand...

Thanks. The OCPD is 60A and what it is feeding is a 100A panel. So if the OCPD is for sure 60A then like you said...the load can't exceed 48amps. So i guess number 10's...hmmm:?
 
raider1 said:
Unless the load is a continuous load then the OCPD and feeder conductors must be sized at 125%.

Chris
Click to expand...

So the loads the the 100A panel is feeding needs to be determined whether they are continuous or not? If so how would Davis be able to make that statement?
 
mstrlucky74 said:
Thanks. The OCPD is 60A and what it is feeding is a 100A panel. So if the OCPD is for sure 60A then like you said...the load can't exceed 48amps. So i guess number 10's...hmmm:?
Click to expand...

The wire terminals (lugs) of both the 60A circuit breaker and the 100A panel are probably rated 75 deg C. So you would need to use #6 Cu THHN (or similar), which is rated 65A at 75 deg C. See 2011NEC table 310.15(B)(16), 75 degree column.

I'm assuming the 60A CB is already sized properly for the loads that are connected to the panel.
 
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raider1 said:
Unless the load is a continuous load then the OCPD and feeder conductors must be sized at 125%.

Chris
Click to expand...

I think you missed part of the question...

mstrlucky74 said:
I have a 3P 60A breaker from a 120/208v panel feeding a 100A MLO 120/208v panel.
Click to expand...

The max. load can be 60A non-continuous, or 48A continuous, or some combination thereof. There would be no need to apply 125% to 60A to determine the feeder conductor size protected by the 60A c/b.
 
david luchini said:
I think you missed part of the question...



The max. load can be 60A non-continuous, or 48A continuous, or some combination thereof. There would be no need to apply 125% to 60A to determine the feeder conductor size protected by the 60A c/b.
Click to expand...

Correct.

You would not need to apply the 125% to the 60 amp breaker, just make sure that the load falls within the 48 to 60 amp range.

Chris
 
raider1 said:
Unless the load is a continuous load then the OCPD and feeder conductors must be sized at 125%.

Chris
Click to expand...
That still amounts to sizing the feeder conductors to 60A when using a 60A breaker.

Technically, because the OCP-rating requirement (excluding the 100%-rated equipment exception), feeders must be at least 60A before adjustment and correction, not less than 51A after adjustment and correction, and always equal or exceed the supplied load... and comply with other requirements, such as 110.14(C).
 
No need to do any calculations for the feeder size if the OCPD was already sized, correct? The feeder, as stated before, should be sized for 60a. THanks.
 
mstrlucky74 said:
I have a 3P 60A breaker from a 120/208v panel feeding a 100A MLO 120/208v panel.
Click to expand...

I'm bored, so nit pick time.:)

The usual way to write that voltage is 208Y/120 volts.

Voltage, Nominal. A nominal value assigned to a circuit or
system for the purpose of conveniently designating its voltage
class (e.g., 120/240 volts, 480Y/277 volts, 600 volts).
The actual voltage at which a circuit operates can vary from
the nominal within a range that permits satisfactory operation
of equipment.
 
jumper said:
I'm bored, so nit pick time.:)

The usual way to write that voltage is 208Y/120 volts.

Voltage, Nominal. A nominal value assigned to a circuit or
system for the purpose of conveniently designating its voltage
class (e.g., 120/240 volts, 480Y/277 volts, 600 volts).
The actual voltage at which a circuit operates can vary from
the nominal within a range that permits satisfactory operation
of equipment.
Click to expand...

You must be bored... :D Hope all is well.
 
If the main concern here is that the 60 amp circuit is feeding the 100 amp panelboard, consider how many panelboards are available that are rated 60 amps and you will see why a 100 amp panelboard was chosen. If you find one with a 60 amp rating it may well be designed for only one pole per phase or is otherwise limited to very few poles anyway. Otherwise most with more than 8 spaces will usually be 100 or even 125 amp rated minimum. Nothing wrong with protecting them below their rating.
 
kwired said:
If the main concern here is that the 60 amp circuit is feeding the 100 amp panelboard, consider how many panelboards are available that are rated 60 amps and you will see why a 100 amp panelboard was chosen. If you find one with a 60 amp rating it may well be designed for only one pole per phase or is otherwise limited to very few poles anyway. Otherwise most with more than 8 spaces will usually be 100 or even 125 amp rated minimum. Nothing wrong with protecting them below their rating.
Click to expand...

Thanks. The size of the OCPD made me curious compared to the panel....but you clarified.
 
Rewire said:
I always started with the load then sized the feeder to the load and then sized the OCPD to the feeder.
Click to expand...

Ok, stupid question which I'm pretty sure of the answer. So if are given the OCPD size and the load the conductor should be sized off the OCPD? Thanks
 
mstrlucky74 said:
Ok, stupid question which I'm pretty sure of the answer. So if are given the OCPD size and the load the conductor should be sized off the OCPD? Thanks
Click to expand...
As Rewire said... but I have to add, that is the overly simplified version and not always true.
 
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