NEC 2017 Exam Prep - Question regarding feeder conductor for multiple motors.

[Electricallearner]

Hi All -
I came across 2 examples in the 2017 prep material that kinda contradict themselves.

In qty 1 motor - 208V - 3 phase - we see that the amps are just going through 2 lines.
But in 3 motors - 208V - 3 phase - the amps are shown to be going through all three lines (phases).

Is it shown correctly?

Thank you

 

[Eddie702]

Any single-phase motor running on a 3-phase feed will only draw power from 2 wires for each motor. A three phase motor will draw power from all three phases.

So if you feed single phase motors and 3 phase motors from the same three phase feeder. The 3 phase motor will draw equal power from a three hot legs.

If you only have 1 single phase motor running with a three phase motor you connect this motor to any 2 hot legs. If it runs at the same time as the three phase motor 2 hot legs will draw mor power than the third leg which is only loaded by 1 leg of the three phase motor.

If you have 2 single phase motor and 1 three phase motor on the same feed, again the three phase motor draws equal power from all three legs.

The 2 single phase motors should be wired in "ATTEMPT" to balance the power on the 3 phase feed

So the three phase motor would be wired to L1, L2 & L3

1 single phase motor should be wired to L1 and L2

The 2d single phase motor should be wired to L2 & L3. In this example L2 connected to all motors and will carry the most power.

If a third single phase motor is added it would be wired to L1 & L3

Basically the single phase motors especially different sized are added to the feeder by wiring to any two phases in an attempt to have equal load on all three wires or as close as possible.

Probably not the best explanation.

 

[Electricallearner]

Any single-phase motor running on a 3-phase feed will only draw power from 2 wires for each motor. A three phase motor will draw power from all three phases.

So if you feed single phase motors and 3 phase motors from the same three phase feeder. The 3 phase motor will draw equal power from a three hot legs.

If you only have 1 single phase motor running with a three phase motor you connect this motor to any 2 hot legs. If it runs at the same time as the three phase motor 2 hot legs will draw mor power than the third leg which is only loaded by 1 leg of the three phase motor.

If you have 2 single phase motor and 1 three phase motor on the same feed, again the three phase motor draws equal power from all three legs.

The 2 single phase motors should be wired in "ATTEMPT" to balance the power on the 3 phase feed

So the three phase motor would be wired to L1, L2 & L3

1 single phase motor should be wired to L1 and L2

The 2d single phase motor should be wired to L2 & L3. In this example L2 connected to all motors and will carry the most power.

If a third single phase motor is added it would be wired to L1 & L3

Basically the single phase motors especially different sized are added to the feeder by wiring to any two phases in an attempt to have equal load on all three wires or as close as possible.

Probably not the best explanation.

Yeah I get that about single phase motors - but as you can see in 3 phase, 208V, qty 1 motor, only 2 lines are being used, instead of 3 phase as you would expect. Correct?

 

[Tulsa Electrician]

I see what your saying. See pic.
What it does not do is change the answer.

 

[Tulsa Electrician]

I would have layed it out.
L1. L2. L3
30.8 30.8 30.8
24.2 24.2 0
0 0 16

L1 L2 L3
55 55 46.8

(30.8 x 1.25)+ 24.2 = 62.7

 

[wwhitney]

See pic.

Actually L3 is 24.2 + 16.

Cheers, Wayne

 

[wwhitney]

Is it shown correctly?

If you are referring to the table in the first picture in post #4, which you did not include in your OP, then no, that's an error, the table should show 24.2A on L3 on the first line.

Cheers, Wayne