Voltage drop for Motor Feeder 3

[cherryg]

I have a simple question. Does any Code or Standard or any guideline ever specify that voltage drop of motors needs to take into consideration: a. Full load Amps (Nameplate rating) and b. Resistance of the conductor needs to be at the max temp of its insulation i.e 90degC? . The reason I ask this question is though for 99% motors, the general rule applies and we do use the Nameplate rated FLA and the standard resistance at 90degC, there will be that 1% motor in any plant where circuit lengths exceed 700m to 1km. In such cases, a closer attention is warranted so that we don't end up with insane size (or parallel runs) of cables and a qualified electrical engineer can easily figure out how the specific case can be treated. I would love to hear from all you experts.
Note: We can always check the motor characteristic with respect to the load curve and see that the accelerating torque is healthy and the motor torque (proportional to square of V) always exceeds the load torque by 10%

 

[waross]

The Canadian Electrical Code specifies the maximum allowable voltage drop on circuits and systems.
Motor voltage drop calculations are based on motor full load current.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cherryg]

Hello Bill... Yes you are correct. But CEC unlike NEC also allows variances and a qualified engineer can use the shaft kW value to reduce the load amps to actual value, and show that the permissible voltage drop is maintained. This as I said above is one of a case, not a general practice.

 

[waross]

What edition of the CEC? I don't have a current edition.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cherryg]

Hello Waross I have the 2021 version of CEC which covers in Rule 8-102, the requirements of voltage drop.

In Rule 8-102 (2) the Code recognizes the need to size the feeder or branch circuit as per the actual load seen by the circuit (and not the nameplate rating):
"Notwithstanding Subrule 1), where overcurrent devices are selected in accordance with other Sections of this Code, the voltage drop shall be based on the calculated demand load of the feeder or branch circuit"

This clarity in CEC is however not present in NEC.

 

[waross]

Sub rule 2, This rule addresses feeders and circuits such as motor circuits where the circuit breaker does not provide overload protection, only Over-current (fault current) protection.
The voltage drop for such feeders is calculated based on the demand.
The demand is calculated based on the nameplate rating of the motors and equipment.

But the customer says that the motor will never be loaded to more than 50%
The customer's lying.
They don't intend to lie, they are trying to save money.
But, when the time comes to increase the load, the customer will have completely forgotten the assurances given that the load will never be increased.
To add insult to injury, the customer will expect you to remedy the problem. (Without spending any more money)

I doubt that you will find an inspector that will agree with your interpretation.

Example:
Calculated for 10% voltage drop in the motor circuit conductors.
Motor starting at 6 times full load current, times 10% voltage drop equals 60% voltage drop when the motor starts.
Good luck with that.
Actually there are a few reasons why the voltage won't drop that far, but you get the idea.

.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Padlock01]

Hi All,
SANS spec states voltage drop should be based on 5% of the cable voltage from the TX source to the motor terminals. A motor running voltage drop of 2.5% VD. Starting VD of the motor between 15%-20%VD.

 

[resqcapt19]

Other than a couple of specific cases, the NEC does not have any voltage drop requirements. There are Informational Notes, for both feeders and branch circuits, that have a maximum recommended voltage drop, but those are only recommendations and not code requirements.

210.19 ....Informational Note:
Conductors for branch circuits as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, provide reasonable efficiency of operation. See 215.2(A)(2), Informational Note No. 2, for information on voltage drop on feeder conductors.

One thing that helps with the voltage drop is the NEC requirement that the conductors have an ampacity of 125% of the full load currents as shown in Tables 430.247 through 430.250.

 

[7anoter4]

Theoretically, the torque produced by the motor must overcome the resistance of the moment of inertia and cause sufficient angular acceleration to limit the time that the starting current ,well in excess of the rated current, can produce dangerous heating.This torque depends on supplied voltage and inverse proportional on slip.
So,if the voltage drop will be big, the remaining voltage does not produce a sufficient torque and the motor start will fail.
Usually at start 15% voltage drop will be o.k and 5% at rated speed will be o.k. also.