# voltage drop on motor starters



## Ceejandgeeg123! (Jan 22, 2020)

Hello,

I'm working on a new hammer mill made by valmetal, model # 12-1215 ( farm machinery). Unit comes with a control panel 120/240V

Motor is 240V 10HP @ 40A and it has 120V forward and reverse contactors.

When I use the 120V controls (start button) the contactor's pull in and only switch directions once stop button has been pressed

When I use the moter CCT at the same time as the control CCT, the contactors chatter!! But if I turn the control CCT off, I can manually press forward or reverse contactor and the motor will run...

I have a feeling could it be the voltage drop too low?

I pulled 3 #8 AWG with a ground about 90ft in a PVC outside and totally didnt factor the volt drop!!:vs_mad:

Ideas please?? 
Thanks!


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## dronai (Apr 11, 2011)

What is CTT ?

I think #8 wire should be fine for 10hp 3phase, you should be under 40A I think. Should be around 28A @ 240V

I would amp clamp it, and then test it for voltage drop while running
I had one last week, that was 10HP 208V 3 phase, but was fed with #10 wire ! Motor said amps should be 27A @208V, but the voltage was only 196V !!! It was running at 35A ! Tripped the OL's, but worked fine, after I adjusted


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## macmikeman (Jan 23, 2007)

If a motor nameplate says 40 amp breaker, you should put #6 wires as a habit. Always upsize one size for motors. It's not a rule, it's a rule of thumb. Make the motor happy. 


Unless ..... it's a motorized robot replacing the workforce and putting ten people on welfare. For those situations you undersize the feeders one size, so you would feed this one with #10's. F the new world order!


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## CMP (Oct 30, 2019)

Well, if your machine is single phase, the NEC call's for the 10HP motor circuit to be sized at 50A x1.25= 62.5A minimum


At a lower voltage like 208 it would want 55A x1.25



What is the size of the additional load? Is it 1 or 3 phase equipment?


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## Ceejandgeeg123! (Jan 22, 2020)

Sorry, electrical circuit abbreviation (CCT) its been a few months since I've been out in the filed but thanks for your input!!


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## Ceejandgeeg123! (Jan 22, 2020)

Its a 240V single phase motor, on the nameplate it says 10HP, 230V, 40A.
The control circuit is 120V

I did 40A x 1.25= 50A the Canadian Electrical Code #8AWG is rated for 50A
the next size up #6 is rated for 65A
The distance I pulled is about 90feet so I'am thinking I will up size the wire. I just wanted to double check before purchasing #6AWG


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## dronai (Apr 11, 2011)

Ahh single phase ! Yes 50A, but I still don't understand what you are saying here. The motor works fine, if you have the controls turned to manual, or off using start stop buttons ?

Manual: Forward, stop, then reverse, right ?

But in Auto, with the controls turned on, it chatters ? what turns it forward, and reverse in auto ?


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## Ceejandgeeg123! (Jan 22, 2020)

Inside the control panel there are two breakers. One breaker controls the control circuit, 120V which is 1 start and 1 stop button and the second breaker is only for the motor. 

If the control circuit is ON and the motor circuit is OFF, when I push the start button the forward contactor pulls in and holds. When stop button is press it releases the forward contactor and when I press the same start button the reverse contact pulls in and holds. Push stop then reverse contactor releases. Every time the stop button is pressed the direction of the contacts change.

When the motor circuit breaker is ON and the control circuit breaker is OFF I can only start the motor manually by pressing either forward or reverse contactor.

When both breakers are ON, once I push start button both forward and reverse contactors chatter.


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## gpop (May 14, 2018)

With out getting more details on the control panel its hard to tell whats going on. It sounds like a voltage drop is causing the reversing relay to reset. If it has a 1,auto,2 relay try taking the switch out of auto and see what happens.


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## 460 Delta (May 9, 2018)

You're gonna need a 260 Simpson or a real fast DMM to walk this one out. I'd start in reverse and begin at the coil reading voltage and go back from there, assuming that the control xformer isn't too small and sagging on you.


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## JRaef (Mar 23, 2009)

macmikeman said:


> ...
> 
> Unless ..... it's a motorized robot replacing the workforce and putting ten people on welfare. For those situations you undersize the feeders one size, so you would feed this one with #10's. F the new world order!


Speaking of which...




My daughter's old boyfriend was going to make his living as a barista at Starbucks. Looks like he has been made obsolete...


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## micromind (Aug 11, 2007)

There's no question that the starting current of the motor is pulling the voltage down to a level that causes the contractor to drop out and when it does, the voltage rises and it pulls in again, hence the chattering. 

The starting current of this motor will be somewhere around 200 amps, that will cause considerable voltage drop. 

Going to #6s will help but I'd also look at what feeds the panel where the #8s originate. There's a good chance the motor is causing considerable voltage drop at the panel, likely more than the #8s. 

The voltage drop with #8s will be about 30 volts. If it's 240 to begin with, a contactor should stay in at 105 volts unless it's one of those chintzy IEC models........lol. With #6s, it'd be about 20 volts drop, 10 at the contactor. I'd look at the system from the PUCO transformer all the way to the breaker that feeds the #8s. 

If you have a meter that has min/max, you can record the lowest voltage at any point in the system.

P.S. There's no need to have the neutral the same size as the hots. I carries only the control power, not the motor power.


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## triden (Jun 13, 2012)

Sounds like your control circuit is tied to the same circuit as the motor. Instead of running a #6 and wasting tons of money, just run a new #12awg 120vac branch circuit from the panelboard to feed your controls.

Alternatively you could use a good DC PSU (Phoenix Contact QUINT) and convert the contactor coils to 24vdc. This way the built-in buffering will keep the contactors closed during the short voltage drop. I think running a new control circuit would be the best option though.


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## JRaef (Mar 23, 2009)

> Its a 240V single phase motor, on the nameplate it says 10HP, 230V, 40A.
> The control circuit is 120V
> 
> I did 40A x 1.25= 50A the Canadian Electrical Code #8AWG is rated for 50A
> ...


Here in the US, for motor conductors, 10HP is the number that you use in picking the conductor size FROM THE TABLE IN THE NEC, _*not *_the actual motor nameplate. NEC table 430-128 for single phase AC induction motors says *50*A for 10HP 230V, so you would use 50A x 1.25 = 62.5A, requiring #6 wire. The reason you use the tables is precisely this issue. As far as I know, the CEC uses the same rules for sizing motor conductors.


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## glen1971 (Oct 10, 2012)

JRaef said:


> Here in the US, for motor conductors, 10HP is the number that you use in picking the conductor size FROM THE TABLE IN THE NEC, _*not *_the actual motor nameplate. NEC table 430-128 for single phase AC induction motors says *50*A for 10HP 230V, so you would use 50A x 1.25 = 62.5A, requiring #6 wire. The reason you use the tables is precisely this issue. As far as I know, the CEC uses the same rules for sizing motor conductors.


From the 2018 CEC for motor conductors:

28-106 Insulated conductors — Individual motors
1) The insulated conductors of a branch circuit supplying a motor for use on continuous duty service shall have an ampacity not less than 125% of the full load current rating of the motor.
2) The insulated conductors of a branch circuit supplying a motor for use on non-continuous duty service shall have an ampacity not less than the current value obtained by multiplying the full load current rating of the motor by the applicable percentage given in Table 27 for the duty involved, or for varying duty service where a deviation has been allowed in accordance with Rule 2-030 by a percentage less than that specified in Table 27.

Table 27 is duty factors of non continuous motors.

Where motor nameplate currents are not available:

28-704 Horsepower rated equipment
1) Horsepower rated equipment used for the control of motor-compressors and not having a locked rotor current rating shall be given an equivalent locked rotor current rating equal to 6 times the full load current rating.
2) Where the full load current rating is not marked, an equivalent full load current rating shall be determined from the horsepower rating by referring to Table 44 or 45 as applicable.

By Table 45 (single phase motors), a 10 hp motor at 230 volts, single phase is 50 amps. 50*1.25=62.5 amps, and by Table 2 (not more than 3 current carrying conductors) at 75C, it would need #6 wire. 

The OP used #8's, which are good for 50 amps at 75C. At 90', I'm going to guess it is beyond the voltage drop rules and the conductors should be increased, as others have said.


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## splatz (May 23, 2015)

JRaef said:


> My daughter's old boyfriend was going to make his living as a barista at Starbucks. Looks like he has been made obsolete...


They have had robotic coffee vending machines for a while...


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## varmit (Apr 19, 2009)

From your description, I would say that the voltage drop on motor start is causing the coil chatter or, the control wiring is incorrect and trying to pull in both forward and reverse at the same time. The reversing after a stop may be a mechanical alternator (of direction) on each start cycle.


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