motor control circuit origin

[xguard]

I have three motors (208V three phase, 5hp) that will be operated using a small pre-assembled control panel. The control panel does not contain starters, only relays. Each motor will have it's own individual combination motor starter. Should the control voltage (120 VAC) to each starter originate in the control panel or in each of the motor starters enclosures? It seems either way is allowed by the NEC, please correct me if I'm wrong.

Is one option more typical to see than the other? If the control voltage originates in the control panel, is an additional pole typically used in the combination motor starter disconnect to disconnect the control circuit?

Thank you for any feedback

 

[MD84]

I think that it depends where you are getting your 120v control power from. Whichever location is easier would be fine. The 120v signal from the control Enclosure would not need to be ran through an extra pole on the starter to disconnect it. The overload contacts are usually located on the starter. You would run the control circuit through the NC overload contact.

Many times for a start stop circuit a NO Aux contact on the motor starter is used to seal in the control circuit when the motor is started.

Perhaps you could layout a drawing of your installation or provide more details and we can provide better assistance.

 

[just the cowboy]

You have to mark it

You have to mark it

Yes you can have your control voltage originate from a separatesource; this is done all the time with MCC's with PLC control. You must markeach starter stating control voltage from different source. I think it is now orange wire suggested for separatevoltage ( may be yellow ).

 

[Johnnybob]

also, if you do have 2 separate control legs in the cabinet, (L1 and an X1 for clean power, for instance) it's a good idea to make sure they are from the same phase! Having control voltages that add up to 220 in the same cabinet is not a good idea, can be very costly if wires get crossed (I know, but it happens)!

 

[jumper]

Last edited by Johnnybob; Today at 04:38 PM. Reason: Age, possible senility

Okay, that made me chuckle.

 

[kwired]

If the control voltage originates in the control panel, is an additional pole typically used in the combination motor starter disconnect to disconnect the control circuit?

An additional "power pole" isn't necessary, you can use aux contacts to disconnect the control circuit when the power disconnect is opened. You shouldn't have to identify the control source if it is disconnected simultaneously with the power disconnect.

Those aux contacts may not be standard on many disconnects but most do have them as an optional accessory.

 

[petersonra]

An additional "power pole" isn't necessary, you can use aux contacts to disconnect the control circuit when the power disconnect is opened. You shouldn't have to identify the control source if it is disconnected simultaneously with the power disconnect.

Those aux contacts may not be standard on many disconnects but most do have them as an optional accessory.

I have discovered it is usually more cost effective to just buy a starter with the control xfmr than to add the interlock switch.

 

[kwired]

I have discovered it is usually more cost effective to just buy a starter with the control xfmr than to add the interlock switch.

But to interconnect two separate machines still means introduction of additional sources in an enclosure sometimes, but class 2 controls is one way around that without introducing some of the headaches that go with class 1 controls.

 

[Jraef]

But to interconnect two separate machines still means introduction of additional sources in an enclosure sometimes, but class 2 controls is one way around that without introducing some of the headaches that go with class 1 controls.

I agree. In fact in this case, he has 3 separate starters outside of the control panel so if he has CPTs in each starter, that means the control panel would have 4 separate power sources entering it; it's original control power source and separate sources from the 3 starters. THAT would be a nightmare in my opinion and for safety's sake would suggest some sort of multi-pole control disconnect device in the control panel that kills those foreign voltages when you open the door. Using the control panel source and interlocking the starter disconnects with aux contacts would be cleaner in my opinion.

Technically though, the NEC would not REQUIRE disconnection of the foreign voltages on either end, only clear indication via labeling (I think) warning anyone about to enter the enclosure that there ARE foreign voltages. Without checking I'm pretty sure NFPA 79 (the standard for industrial machinery control systems) would require disconnection of foreign sources, but whether or not your equipment falls under that jurisdiction is another matter.

 

[petersonra]

Technically though, the NEC would not REQUIRE disconnection of the foreign voltages on either end, only clear indication via labeling (I think) warning anyone about to enter the enclosure that there ARE foreign voltages. Without checking I'm pretty sure NFPA 79 (the standard for industrial machinery control systems) would require disconnection of foreign sources, but whether or not your equipment falls under that jurisdiction is another matter.

the point of orange wire in NFPA79 is to warn people who might be working in the panel that the orange wiring is not disconnected by the panel disconnect. It does have to have a disconnect somewhere though.

 

[Jraef]

the point of orange wire in NFPA79 is to warn people who might be working in the panel that the orange wiring is not disconnected by the panel disconnect. It does have to have a disconnect somewhere though.

OK, I checked this time. Again, IF NFPA79 applies...

5.3.1.2 The supply circuit disconnecting means shall disconnect
(isolate) the electrical equipment of the machine, including
all control circuits, from the supply circuit when required
(e.g., for work on the machine, including the electrical equipment).
Circuits that are not required to be disconnected by the
supply circuit disconnecting means shall comply with 5.3.5.

So there are "excepted circuits" that don't have to be disconnected, which if they are ungrounded, must be orange or yellow wires.

5.3.5 Excepted Circuits.
5.3.5.1 The following circuits shall not be required to be disconnected
by the main supply circuit disconnecting means:
(1) Lighting circuits for lighting needed during maintenance
or repair
(2) Attachment plugs and receptacles (plug and socket outlets)
for the exclusive connection of repair or maintenance tools
and equipment (e.g., hand drills, test equipment)
(3) Undervoltage protection circuits that are only used for
automatic tripping in the event of supply circuit failure
(4) Circuits supplying equipment that are required to remain
energized for satisfactory operation [e.g., temperature controlled
measuring devices, product (work in progress)
heaters, program storage devices]

It goes on to say that the excepted circuits, if originating somewhere else, must have a disconnect SOMEWHERE (makes sense) and there must be a label on the panel stating not only that there are foreign circuits, but where those disconnects are for them (that I didn't know).

 

[xguard]

Thanks for all the feedback!

I'm not familar with NFPA 79. This is for a water well system with a pressure tank. I'm assuming this would not fall under a standard for inductrial machinery......?

My preference would be to have one, 120 VAC, circuit feeding the control panel. Then the individual control circuits, that are each supplied and switched in the control panel, going out to each combination motor starter/disconnect. Also having labels as mentioned on the motor starters.