Voltage drop for Motors

[Grouch]

Hi,
What's the appropriate way to calculate the branch circuit wiring for a motor when taking into account voltage drop? I know the wires are sized at 125% of the FLC. So to upsize the wires due to voltage drop, what number do you use for the amps in the voltage drop calculations? The FLC, or the FLC x 125%? I would assume it's the latter since you have to take into account the 125%.

Thanks!

 

[Smart $]

Calculating running voltage drop, use FLA*.

Calculating starting voltage drop, use LRC.

Also, for a large motor on a comparatively small service or SDS, voltage sag can be a factor in starting the motor.


*After reading kwired's post, I agree you can use nameplate FLA or actual running amps. Use Table value to be conservative.

 

[kwired]

Depends on what you want to cover. If you want less voltage drop while starting the motor you may want to use locked rotor current in your calculations. This may be more of a concern with a feeder that supplies other loads in addition to your motor. For the motor circuit itself - usually you only need to consider the normal running amps - even if it is not the same as nameplate amps.

 

[Grouch]

ok, I'm somewhat following. my circuit is only for the motor itself. Usually my motors are on dedicated circuits and feed nothing else. I'm not concerned with the starting voltage drop, but more with running voltage drop. I'll use the amps from Table 430.250 instead of nameplate amps. But how do I take into consideration the 125% multiplier that is required for the motor branch circuit wiring? When I plug my amp value into the voltage drop calculation, do I use amps OR amps x 1.25? (again, amps from table 430.250). That's the part where i'm unsure.

 

[Flapjack]

Use the FLC value in Table 430.250 in the voltage drop calculation. The 125% x amps is used to size your conductor per 430.22. Using Chapter 9, Table 9, you can get the appropriate resistance and reactance values to use in the voltage drop calculation based on the conductor size.

Note - using Table 9 is assuming the motor is 3 phase, 60 Hz, AC, 600 V or less. Since you are using Table 430.250, I am making the assumption that is the case.

 

[Grouch]

Use the FLC value in Table 430.250 in the voltage drop calculation. The 125% x amps is used to size your conductor per 430.22. Using Chapter 9, Table 9, you can get the appropriate resistance and reactance values to use in the voltage drop calculation based on the conductor size.

Note - using Table 9 is assuming the motor is 3 phase, 60 Hz, AC, 600 V or less. Since you are using Table 430.250, I am making the assumption that is the case.

yes, i'm using a 3 phase motor, so table 430.250 would apply. ok, so I use the FLC value from the table in the voltage drop calculation... but what happens to the 125% multiplier from section 430.22? Do I disregard it?

 

[don_resqcapt19]

yes, i'm using a 3 phase motor, so table 430.250 would apply. ok, so I use the FLC value from the table in the voltage drop calculation... but what happens to the 125% multiplier from section 430.22? Do I disregard it?

You use it to size the conductor, but you don't use that value to calculate the voltage drop. In fact, because 430.22 requires 25% more copper than the load requires, you can go a long way before the running voltage drop becomes an issue.

 

[Grouch]

You use it to size the conductor, but you don't use that value to calculate the voltage drop. In fact, because 430.22 requires 25% more copper than the load requires, you can go a long way before the running voltage drop becomes an issue.

Don, my fault... I think I worded my question wrong actually... I'm using a calculation from the Cooper Bussmann book, "Selecting Protective Devices". There's a section in it called "How to Select size of wire". The calculation gives me a larger wire size due to voltage drop. So for motors, I use my amp value from table 430.250 to calculate the larger wire. But, where does the 125% multiplier come into play from section 430.22?

 

[Smart $]

... But, where does the 125% multiplier come into play from section 430.22?

The minimum permitted conductor size. It must have an ampacity not less than Table FLA × 125%, correlated with the terminal temperature limitation [110.14(C)].

When you calculate for voltage drop, the resulting conductor size could be smaller than this minimum permitted conductor size.

 

[Grouch]

The minimum permitted conductor size. It must have an ampacity not less than Table FLA × 125%, correlated with the terminal temperature limitation [110.14(C)].

When you calculate for voltage drop, the resulting conductor size could be smaller than this minimum permitted conductor size.

NOW I got it! So only amps is used in the wire upsizing calc. Your result just cannot be smaller than amps x 125%. Great!