Motor control cable lenght 1

[MABB]

HI,

I have an application where the start-stop buttons for a motor will be 4000 feet from the starter. We planned to install relays in the starters that will be driven by the buttons. We will use #14 AWG wiring (Teck armored cable) for this application. Voltage drop is ok to drive the relays.

Should I be concerned with any other phenomenon that could trigger these relays because of the cable length ?

Thanks,

Michel

 

[dpc]

You left out what the control power voltage will be. If it is 120 V ac, this will almost certainly NOT work. The capacitance in the cable will prevent the contactor from dropping out, assuming the voltage drop will allow it to pickup in the first place.

There have been a few earlier discussion about this in this forum. Try doing a search on control circuit distance, or something similar - maybe in the Power engineering forum as well. Anything over about 1000 feet will be a problem.

You can do it with a dc control circuit.

 

[MABB]

The control voltage is 120 Vac.

 

[dpc]

Then it won't work. Even if you increase the wire size to reduce the voltage drop, the capacitance to ground of 4000 ft of conductor will hold the coil when you try to de-energize it.

Square D has a paper that gives maximum recommended circuit lengths at different ac control voltages. They are surprisingly short.

Google up Square D Data Bulletin M-379F or try this:

http://ecatalog.squared.com/techlib/docdetail.cfm?oid=0900892680079349

 

[MABB]

Thanks,

But we use Teck armored cable with explosion proof sealed connectors at both ends. The risk of water entering the cable is almost nil. I wonder what the values for distances for shunt capacitance be, considering a dry cable....

 

[dpc]

Just put in a dc interposing relay and move on to something else. You'll sleep a lot better. It's really annoying when you hit the STOP button and nothing happens.

A dry cable will work better, but there is still capacitance.

 

[itsmoked]

Yep DPC is correct you should not use 120VAC over that distance. Nor even half that distance.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[controlsdude]

general rule of thumb. We try to keep any control cable run below 500ft runs.

If your controlling something that far away, you need to have a power drop, run a comm cable between the plc where the pushbuttons are tied to and the motor area. Even the comm cable would have to have repeaters due to the distance.

Why 500 ft? Most reasonable priced wire is in 500ft runs. Good luck getting a manufacturer to produce a 4000ft spool.

 

[Thedroid]

We use Phoenix Contact RF relays to control our well pumps. They are over a 1/2 mile from the plant, and they have worked reliably for the past 3yrs.

 

[stephenw22]

I took a quick look at a few drawings, and the longest 120VAC control cable run we have (that I could find) is just over 1700ft. It's a 5C#14 TECK cable, and it's been in service for about 20 years now without any problems.

At that distance though, I'd be really tempted to try and use DC control. If the motor starter needs to be AC control in the cabinet, you could use a small DC power supply and an interposing relay for your control circuit.

A 4000' length of TECK should be pretty easy to find - we order up 5000' reels on a semi-regular basis for our site.