Single phase manual motor starters

[mjc1060]

Does the NEC allow the grounded (neutral) conductor to broken (with an overload) in a motor circuit? My question is when installing a motor starter for a 1-phase motor 120V should a single overload (for the hot) be used?

 

[augie47]

430.37 lists a OL unit in the ungrounded conductor as the minimum.
In practice, a single overload is what I see installed.

 

[ActionDave]

You only need one heater for a single phase motor 120 or 240V. For a 120V motor it is okay to run both the hot and the neutral to the motor starter but the heater has to be on the hot.

 

[Aleman]

I have seen 3 phase starters used for single phase circuits. Wired with line and neutral and the line doubled back
through the spare phase. As far as I know this is legal, and the overload doesn't know the difference.

 

[ActionDave]

I have seen 3 phase starters used for single phase circuits. Wired with line and neutral and the line doubled back
through the spare phase. As far as I know this is legal, and the overload doesn't know the difference.

It's legal and on starters with electronic overloads it is necessary.

 

[Aleman]

It's legal and on starters with electronic overloads it is necessary.

That's true. They need current through all poles to function correctly.

 

[kwired]

You only need one heater for a single phase motor 120 or 240V. For a 120V motor it is okay to run both the hot and the neutral to the motor starter but the heater has to be on the hot.

Not fully certain about that, if the "heater" opens the circuit then yes it need be in the ungrounded conductor, but these "heaters" typically only create heat that causes something else to open the control circuit to a contactor. As long as the contactor opens all ungrounded conductors simultaneously it shouldn't hurt to have a device whose only purpose is current level detection in the grounded conductor.

I have seen 3 phase starters used for single phase circuits. Wired with line and neutral and the line doubled back
through the spare phase. As far as I know this is legal, and the overload doesn't know the difference.

It's legal and on starters with electronic overloads it is necessary.

With overloads that are designed to detect phase loss or phase unbalance - you must use all three poles somehow or it will trip on phase loss.

For melting alloy overloads - you may need to use different overload selection for one heater element, then you use for two elements, and a different one for three elements as well. Each situation will introduce different heat levels into the enclosure an will change the trip level even though the motor current is the same.

 

[GoldDigger]

But if the thermal overloads share a common heat envelope (which would properly model overall motor heating) you could still loop the same single line current through both (if two) or all three (if three phase) heaters to get protection at the nominal current.

 

[kwired]

But if the thermal overloads share a common heat envelope (which would properly model overall motor heating) you could still loop the same single line current through both (if two) or all three (if three phase) heaters to get protection at the nominal current.

Correct.

You need to look at overload selection tables carefully to get the right chart for your application with thermal trip type units Usually a two step process, first step selecting the right selection chart, second step selecting the correct element from that chart. Selecting the right chart can involve consideration of number of thermal elements in your overload unit, size of enclosure the overload unit is installed in, other heat producing components within that enclosure. If you use a chart designed for three elements, then when using a single phase motor you still need to run current through all three elements or you throw the accuracy off.

This shouldn't be an issue with electronic overloads but they still often need current through all three current sensors or it will trip thinking we have a phase loss condition.