Another Motor circuit design question

[MIEngineer]

Hello All,

I am wondering if anyone could help me out. I have 3 motors I would like to place on the same feeder 240V 1 phase.

I have the following information on them.
Compressor (M1): 7.5 hp, 240V 1 phase.
Air Dryer (M2): 1.22kW, 240V 1 phase
After Cooler (M3): 2.5 FLA, 240V 1 phase

I will feed them with an 80A CB from the panel using #8 wire.
M1 will then branch to a disconnect with 60A dual-element fuses, M1 also has integrated overload protection.

Finally to the question. What would I branch off with to feed M2 & M3. According to 430.32 I don't need overload protection. Would an unfused disconnect work?

Thank you for any and all assistance.

 

[davidr43229]

What would I branch off with to feed M2 & M3. According to 430.32 I don't need overload protection. Would an unfused disconnect work?

You still overload & Shortcircuit protection for the wire., If you fused the individual M2 & M3 to no more than 125% their individual FLA, the NEC allows you to use the fuse for overload protection.
just my $.02

 

[augie47]

If my figures are correct, you need to rethink your #8 wire.
7-1/2 HP 240 v is 40 amps (Table) x 1.25 = 50
Thats your #8's capacity before you add the other loads

and, as david said, you still need to address short/circuit GF protection on the other loads

 

[micromind]

This is a very common set-up, here's what I would do.

The 80 amp breaker is about right for a 7 1/2HP single phase motor. #8's are ok as well, but it's about the upper limit. The disconnect doesn't need to be fused, the wire is protected by the motor overloads. If the breaker is in sight and less than 50 feet away, you don't need a disconnect at all.

If possible, run a 20 amp circuit along with the 80 amp, and use 20 amp 250 volt receptacles to power the dryer and aftercooler. Most dryers have a compressor and a fan built into them, and usually a switch as well. If not, simply switch the recept. If you use a 2 pole switch, it can be the disconnecting means, and you don't need the recpt's at all. These units are almost always self-protected, so you don't need to worry about overloads.

The aftercooler is likely a fan blowing across a rediator, it can be connected on the same circuit as the dryer. If it has a built-in overload (alot of them do), then just a 2 pole switch or a recpt is ok. If not, you'll need to provide some sort of overload protection.

 

[tryinghard]

I have 3 motors I would like to place on the same feeder 240V 1 phase.

Compressor (M1): 7.5 hp, 240V 1 phase.
Air Dryer (M2): 1.22kW, 240V 1 phase
After Cooler (M3): 2.5 FLA, 240V 1 phase

I will feed them with an 80A CB from the panel using #8 wire.

430-24, 430-6(A), & (1)
M1: 7.5 HP @ 40A x 1.25 = 50A
M2: 1.5 HP @ 10A
M3: ? HP @ 2.9A
Total 63A = #6?s
Short circuit & ground fault protection device, breaker: 160A = 63 x 250% (430-52)

 

[tryinghard]

430-24, 430-6(A), & (1)
M1: 7.5 HP @ 40A x 1.25 = 50A
M2: 1.5 HP @ 10A
M3: ? HP @ 2.9A
Total 63A = #6?s
Short circuit & ground fault protection device, breaker: 160A = 63 x 250% (430-52)

Oops! I need to correct the above

Feeder: #6 cu (430-24, 430-6(A)&(1))
M1: 50A = 7.5 HP @ 40A x 1.25
M2: 10A = 1.5 HP
M3: 2.9A = ? HP
Total 63A

Feeder protection, breaker: 125A (430-62(A) & 430-52)
M1: 100A = 7.5 HP @ 40A x 250%
M2: 10A = 1.5 HP
M3: 2.9A = ? HP
Total 113A

Tap conductors sized as per 430-28
Overload protection as per 430 part III

 

[MIEngineer]

Oops! I need to correct the above

Feeder: #6 cu (430-24, 430-6(A)&(1))
M1: 50A = 7.5 HP @ 40A x 1.25
M2: 10A = 1.5 HP
M3: 2.9A = ? HP
Total 63A

I see now where everyone is going with this. Thanks for the info.

Tryinghard, where did you come up with the 10A for M2. This data I have from the submittal sheet simply has 1.22 kW @ 240V. I will have get more information about internal overloads on each unit.

 

[tryinghard]

Tryinghard, where did you come up with the 10A for M2. This data I have from the submittal sheet simply has 1.22 kW @ 240V. I will have get more information about internal overloads on each unit.

430-6(A)(1) The values given in Tables 430-247, 430-248, 430-249, and 430-250, including notes, shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to that corresponding to the value given in Tables 430-247, 430-248, 430-249, and 430-250, interpolated if necessary.

In other words we can only use these listed tables in NEC for full load amperes and not the actual nameplate, the only time we can use the nameplate amperes is for sizing the overloads. With M2 the wattage needs to be corresponded/interpolated to the amperes.

The M2 criteria you gave is 1.22kW this is understood as output power, so: HP = Output watts/746 meaning 1.5HP = 1,220/746. Now go back to Table 430-248 notice 1.5 HP at 230V = 10A

 

[tryinghard]

...M1 will then branch to a disconnect with 60A dual-element fuses, M1 also has integrated overload protection.

Finally to the question. What would I branch off with to feed M2 & M3. According to 430.32 I don't need overload protection. Would an unfused disconnect work?

Actually you?ll be tapping from your feeder to branch 430-28 &(2) and you?ll provide OL protection as per 430 part III, at a minimum you must provide OL protection for motors NOT labeled TP meaning they have thermal protection integrally built in, your circuit should be as follows:
M1: 7.5HP = 40A, 40 x 125% = 50A, tap conductor = #8?s to a disconnect w/70A DE fuses (40 x 175% = 70A), to OL?s sized at nameplate FLA x 125%.

M2: 1.5HP = 10A, 10 x 125% = 12.5A, tap conductor (cannot be smaller that 1/3 of the ampere rating of the feeder, 430-28(2)) = #12?s to a disconnect w/18A DE fuses (10 x 175% = 17.5A), to OL?s sized at nameplate FLA x 125%.

M3: 1/4HP = 2.9A, 2.9 x 125% = 3.6A, tap conductor (cannot be smaller that 1/3 of the ampere rating of the feeder, 430-28(2)) = #12?s to a disconnect w/5A DE fuses (2.9 x 175% = 5A), to OL?s sized at nameplate FLA x 125%, 430-32(D) you probably do not need to provide OL for this motor but you do need to fuse it at the local disconnect because of the feeder.