Electrician Talk banner
1 - 8 of 8 Posts

·
Registered
Joined
·
32 Posts
Discussion Starter · #1 ·
I have an air compressor with dual 7 1/2 hp single phase 240v motors. I will have 4 conductors for the compressor (separate circuits) and a 120v lighting circuit in 1 conduit. I will need to de-rate by 80% due to 4-6 conductors in the conduit, #8 @ 90degree is 55amps by 80% is 44amps. The motor table list a 7 1/2 hp @ 40 amps is there another multiplier I have to add to the 40amps or is the #8thhn sufficient. The air compressor has a device that during each run it switches motors but in a rare case they both could run at the same time.
 

·
Registered
Joined
·
5,512 Posts
I have an air compressor with dual 7 1/2 hp single phase 240v motors. I will have 4 conductors for the compressor (separate circuits) and a 120v lighting circuit in 1 conduit. I will need to de-rate by 80% due to 4-6 conductors in the conduit, #8 @ 90degree is 55amps by 80% is 44amps. The motor table list a 7 1/2 hp @ 40 amps is there another multiplier I have to add to the 40amps or is the #8thhn sufficient. The air compressor has a device that during each run it switches motors but in a rare case they both could run at the same time.
By what rule are you using the 90C rating of the conductors?
 

·
Coffee drinking member
I pretwist and then use wire nuts. Solder pots rule.
Joined
·
13,958 Posts
CES said:
I have an air compressor with dual 7 1/2 hp single phase 240v motors. I will have 4 conductors for the compressor (separate circuits) and a 120v lighting circuit in 1 conduit. I will need to de-rate by 80% due to 4-6 conductors in the conduit, #8 @ 90degree is 55amps by 80% is 44amps. The motor table list a 7 1/2 hp @ 40 amps is there another multiplier I have to add to the 40amps or is the #8thhn sufficient. The air compressor has a device that during each run it switches motors but in a rare case they both could run at the same time.
Size them to account for both to start/run at the same time. Sooner or later they will. Wire sizing should be in the compressor package. If its a Quincy hit their site and they have just about every package waiting as a PDF.
 

·
Registered
Joined
·
1,802 Posts
Size them to account for both to start/run at the same time. Sooner or later they will. Wire sizing should be in the compressor package. If its a Quincy hit their site and they have just about every package waiting as a PDF.
regardless of manufacturer the all usually carry the info you would need for their equipment
 
  • Like
Reactions: Wirenuting

·
Coffee drinking member
I pretwist and then use wire nuts. Solder pots rule.
Joined
·
13,958 Posts
gnuuser said:
regardless of manufacturer the all usually carry the info you would need for their equipment
I wanted to just mention Quincy as they have a good library. They even still put up items that long ago went obsolete.
 

·
Registered
Joined
·
1,802 Posts
I wanted to just mention Quincy as they have a good library. They even still put up items that long ago went obsolete.
god i know what you mean :eek:we have a couple quincy's at the plants air building and weve had to look up info there before:laughing:
 

·
Bilge Rat
motors and controls.........
Joined
·
8,403 Posts
You're right about 40 amps each.

430.21(A) states that the table values need to be multiplied by 1.25 for a single motor, so 40 X 1.25 = 50.

If you're using THHN, 310.16 lists #8 at 55 amps at 90ºC; 55 X .8 = 44. But 50 is needed.

#6 is 75 amps at 90ºC, so it will derate to 60. This is the minimum size needed.

Just for info, Table 430.52 allows a basic standard circuit breaker to be 250% of the table current. In this case, 40 X 2.5 = 100. This is the maximum breaker, there is no minimum.

If you're using fuses, it's be 40 X 1.75 = 70. This is the maximum size for basic time-delay fuses. Again, there is no minimum.
 
1 - 8 of 8 Posts
Top