Calc Q

[Alwayslearningelec]

I know I'm beating this to death.

1. Wire size can be larger than OCPD??
2. OCPD can't be larger than wire current carrying capacity?

 

[infinity]

1) Yes of course otherwise you could never compensate for VD
2) In general yes, for motor circuits, AC units, Etc., no.

 

[augie47]

The basic rule I use in my classes:
Other than loads shown in 240.4(G) {air conditioners, motors, etc}\

1. Calculate your load (name plate, Art 200, etc)
2 Select a conductor with an ampacity adequate for that load (after any derating, etc)
3.Select an overcurrent device to protect the conductor and the load applying the rules in Art 240.4

Example" 59 amp load (non-continuous)
210.19 (and 215.2 for feeders) require the conductor ampacity be equal to or greater than the load so you need a conductor with an ampacity of 59 amps after any adjustments (fill, ambient, etc.)
For 59 amps that would normally be a #6 with an ampacity of 65 amps @ 75° (#4 NM due to the 60° rating)
You would then select an overcurrent device based on the 65 amps. 240.4would allow the "next size up rule" so a 70 amp breaker could be used.

 

[Alwayslearningelec]

The basic rule I use in my classes:
Other than loads shown in 240.4(G) {air conditioners, motors, etc}\

1. Calculate your load (name plate, Art 200, etc)
2 Select a conductor with an ampacity adequate for that load (after any derating, etc)
3.Select an overcurrent device to protect the conductor and the load applying the rules in Art 240.4

Example" 59 amp load (non-continuous)
210.19 (and 215.2 for feeders) require the conductor ampacity be equal to or greater than the load so you need a conductor with an ampacity of 59 amps after any adjustments (fill, ambient, etc.)
For 59 amps that would normally be a #6 with an ampacity of 65 amps @ 75° (#4 NM due to the 60° rating)
You would then select an overcurrent device based on the 65 amps. 240.4would allow the "next size up rule" so a 70 amp breaker could be used.

Ok thanks a lot but in this example wouldn't the wire overheat as the OCPD current rating is higher than the wire?

 

[augie47]

Ok thanks a lot but in this example wouldn't the wire overheat as the OCPD current rating is higher than the wire?

If the conductor is sized for the load there should be no problem.
In reality in the event the load increased there would not be significant overheating of the conductor as there is pretty much a safety factor in the ampacity ratings.
Note that the next size up rule only applies to circuits 800 amps or less so you are fairly limited as to the "overload" the conductor would see.