Contactor control and load, same power source? Yay or Nay

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titan1021

titan1021

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Northern California
I need to install a small 3 pole contactor to allow On/Off switching of a 2.7 amp 3phase motor with a 2-pole switch. This will be a non-typical installation, ie.. custom for this client.

The load is 3 phase 208v. I was planning to use a 3 pole contactor with a 208v coil then basically pig-tail off of two of the line legs, terminating one set at the coil and one to the line side terminals of the contactor. I haven't done this before, but I don't see why it wouldn't work.

Just wanted to run it by a few folks and see if there are any considerations or potential issues that I may be overlooking here.

Thank you
 
titan1021 said:
I need to install a small 3 pole contactor to allow On/Off switching of a 2.7 amp 3phase motor with a 2-pole switch. This will be a non-typical installation, ie.. custom for this client.

The load is 3 phase 208v. I was planning to use a 3 pole contactor with a 208v coil then basically pig-tail off of two of the line legs, terminating one set at the coil and one to the line side terminals of the contactor. I haven't done this before, but I don't see why it wouldn't work.

Just wanted to run it by a few folks and see if there are any considerations or potential issues that I may be overlooking here.

Thank you
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I don't know about that where you are but it is commonly done here, and in the other countries I've been, for motor starters.
Usually, there is no aux supply for the contactor coil unless it is part of a larger control system.
 
cpickett said:
Wouldn't you need a 2 pole switch to break both ungrounded conductors?
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For control purposes only no. For a disconnecting means yes.

Two pole switch opening two supply leads to a typical three phase motor is acceptable to NEC as a control means, with only one lead still energized there is no circuit path allowing it to operate even at partial capacity. Some equipment instructions do say to break all lines though.
 
kwired said:
For control purposes only no. For a disconnecting means yes.

Two pole switch opening two supply leads to a typical three phase motor is acceptable to NEC as a control means, with only one lead still energized there is no circuit path allowing it to operate even at partial capacity. Some equipment instructions do say to break all lines though.
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Thanks for the clarification! Makes sense.
 
430.84 Need Not Open All Conductors.
The controller shall not be required to open all conductors to the motor.


Exception: Where the controller serves also as a disconnecting means, it shall open all ungrounded conductors to the motor as provided in 430.111
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Not finding anything on motor control circuits about opening multiple conductors, but such a rule would complicate things dramatically in some schemes.

Basic start-stop with holding contact wouldn't work if you had to switch both lines of the control circuit. Overload control contact normally only interrupts one conductor of the circuit as well. Control limits also usually only switch one line of the control circuit.

The more complex the process is the more likely you might see 120 or even 24 volt control circuit, but for simpler processes/machines with limited number of motors - the chance of those motors having control circuit derived from the motor circuit increases. Goes up even more with a single motor application.
 
kwired said:
Not finding anything on motor control circuits about opening multiple conductors, but such a rule would complicate things dramatically in some schemes. ...
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That's correct. From the standpoint of switching a load on and off, all that is required is to break enough circuits that it cannot operate. So on a single phase load, that is one pole and on a 3 phase load that is 2 poles.

But to disconnect a load for servicing, you are required to break all ungrounded conductors and to protect a circuit you must install the protective devices on all ungrounded circuits.
 
Jraef said:
That's correct. From the standpoint of switching a load on and off, all that is required is to break enough circuits that it cannot operate. So on a single phase load, that is one pole and on a 3 phase load that is 2 poles.

But to disconnect a load for servicing, you are required to break all ungrounded conductors and to protect a circuit you must install the protective devices on all ungrounded circuits.
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Short circuit and ground fault protection needs to be in each ungrounded conductor but motor overload protection only needs to be in one conductor of a single phase motor though. Long ago NEC used to let us put motor overload in two lines of a three phase motor, and it generally did kind of work, but they have wanted devices in all three lines for some time now.
 
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