Adjustment factors and voltage drop calculations

[ctaylo360860]

I Have 4- 10hp motors being fed underground from service. Nec 430.250 gives me 14a for full load current. Since the motors are close to each othe, I plan to set an enclosure close to the first disconnect and branch out from there to the four motors. My spine decided Running two raceways 250’ back to the service sounds like a better plan than 8 individual runs….

I planned on running 12-#10thhn/thwn current carrying conductors inside 1-1/4 pipe.
I’m adjusting the #10 by 50% using the 90 degree column.

40a/50%=20 amps. Conductors are still protected at 1.25% of the flc.
14x1.25=17.5.

My 10 wire would be good for 20amps. I will land them on a 25a 3pole breaker. I believe 2dik/Kc mil is voltage drop formula. My longest run will be about 250ft away.

2x250x14x12.9/6530=13.82volts dropped on my farthest run.

480x.03=14.4volts 3%volt drop recommendation

I believe I’m allowed a total of 5% on my total feeder and branch circuit? so I should be good to go if I did this correctly.

I’m also curious if I would use the amperage the Nec gives in 430.250 as the flc or the amperage on the name plate of the motor?

 

[Dsg319]

Didn’t run the numbers myself but from looking at it looks correct and yes use NEC FLC tables for groundfault/short circuit protection. And nameplate data from motor for Overload protection.

Also correct on the 5%total VD of combined branch circuit/feeder.

 

[ptonsparky]

I would be inclined to send out one feeder. Max fuse = 100, Max CB = 150. Feeder amp needs to be about 60 so it could be less. Then use Outside Tap rules to each PP, if you want to reduce the conductor size.

I used #4 CU with a total load of 80 amps (Yes, higher than you have) @ 270' total. 2,4% VD,

 

[Dsg319]

I would be inclined to send out one feeder. Max fuse = 100, Max CB = 150. Feeder amp needs to be about 60 so it could be less. Then use Outside Tap rules to each PP, if you want to reduce the conductor size.

I used #4 CU with a total load of 80 amps (Yes, higher than you have) @ 270' total. 2,4% VD,

Good idea.

 

[xptpcrewx]

I believe I’m allowed a total of 5% on my total feeder and branch circuit? so I should be good to go if I did this correctly.

I’m also curious if I would use the amperage the Nec gives in 430.250 as the flc or the amperage on the name plate of the motor?

For the voltage drop calculation use the nameplate value (actual). Depending on what calculation method you are using for voltage drop, you can factor in the lowest service voltage, cable reactance, power factor, and temperature correction to get something more accurate.

 

[ctaylo360860]

I would be inclined to send out one feeder. Max fuse = 100, Max CB = 150. Feeder amp needs to be about 60 so it could be less. Then use Outside Tap rules to each PP, if you want to reduce the conductor size.

I used #4 CU with a total load of 80 amps (Yes, higher than you have) @ 270' total. 2,4% VD,

Good idea that’s how our concrete bunk is. I already have a ton of #10 coil packs and 25abreakers, so that’s why I chose that way…. Did you come up with your feeder calculation by taking 1.25 of largest motor and adding the rest of the flc of existing motors? 17.5+14x6= 101.5amps?

 

[ptonsparky]

I thought you had 4 10hp motors.
(1.25*14) +(3*14)= 59.5 amp feeder.

 

[winnie]

I would be inclined to send out one feeder. Max fuse = 100, Max CB = 150. Feeder amp needs to be about 60 so it could be less. Then use Outside Tap rules to each PP, if you want to reduce the conductor size.

I used #4 CU with a total load of 80 amps (Yes, higher than you have) @ 270' total. 2,4% VD,

This approach offers an additional benefit.

For motors you want to do _two_ voltage drop calculations. You want to calculate the VD for both running and starting.

If you use the feeder approach and these motors tend to start at different times, then you will see greatly improved voltage drop on starting versus running separate circuits.

-Jon

 

[ctaylo360860]

Good idea that’s how our concrete bunk is. I already have a ton of #10 coil packs and 25abreakers, so that’s why I chose that way….

I thought you had 4 10hp motors.
(1.25*14) +(3*14)= 59.5 amp feeder.

I’m sorry that’s my bad I’m gonna have Seven total motors. But yea I’ll have four being fed in one raceway and 3 from another raceway…

 

[ctaylo360860]

This approach offers an additional benefit.

For motors you want to do _two_ voltage drop calculations. You want to calculate the VD for both running and starting.

If you use the feeder approach and these motors tend to start at different times, then you will see greatly improved voltage drop on starting versus running separate circuits.

-Jon

So these fan motors are usually started individually. my pump panels will have a hand off auto switch. Usually motors are started in hand and switched to auto. We do this because we tarp our bunk. if we loose power we want all motors to come back on and suck tarp down as soon as power is restored….

 

[ptonsparky]

So these fan motors are usually started individually. my pump panels will have a hand off auto switch. Usually motors are started in hand and switched to auto. We do this because we tarp our bunk. if we loose power we want all motors to come back on and suck tarp down as soon as power is restored….

In that case, 125% of all the motors starting at the same time, minimum. I would go bigger as these fans most likely take a long time to start. Max fusing/cb. Don’t scrimp.

 

[ctaylo360860]

In that case, 125% of all the motors starting at the same time, minimum. I would go bigger as these fans most likely take a long time to start. Max fusing/cb. Don’t scrimp.

Yeah I think I’m just gonna go with my original plan, and use adjustment factors and individual feeders . It’s only gonna be two raceway instead of one… and that way the vd and in rush will be less on each circuit. If I’m I understood correctlywhat Winnie had said.