derating wire capacity

[rich41scout]

If I run a number of wires in a single conduit. and have to derate the allowable ampacity to 40 percent .. then what size breakes is used ??

There are 12 motors, all have name plate 21 amps .. they are 3phase . that gives 36 wires in the conduit .. and if single each circuit would be wired with #10s and on a 30 amp breaker. now I run the circuits all in one conduit, that reduces the allowable ampacity , 40% , so I upsize the wire to bring it back up over my 21 amps for each motor, if the wire size is now #8 and my allowable ampacity is 21 amps , what size breaker is needed .. 30 amp?
or is the breaker sized top the reduced ampacity of the gang of wires ..

some one else advised that ... these 36 #8 conductors in a single conduit would only be 22 amps and not adequate for a 30 amp feed. #6 conductors would be correct for a 30 amp requirement.

what is right ??

 

[augie47]

we need to drop way back and punt for a second.
for purpose of these calculations, nameplate amps is not the number to be used. Look at 430.6 and it will refer you to the tables 430.250, etc.
Once we get a ampacity from the tables, then apply your required 1.25 % for your conductor ampacity (430.22)
Then go to 430.52 and ascertain your breaker (short-circuit/ground fault) protection.
Now use the tables to select you conductor.
36 #8's would have an ampacity of 22 amps . If that is suitable for the numbers obtained by the method shown above then you wold be fine.
The breaker would be based on 430.52

If you nameplate is 21, it is almost a given that 1.25 times table 430.250 will be higher than 22 and your #8 will be too small, but the above procedure should be followed to make sure.
(assuming a 7-1/2 HP @ 240 Table 430.250 shows 22 amps , 1.25 would be 27.5. #6s would be required)