Voltage drop to small, 120 VAC control panel

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Isaiah

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Engineering wants to install a new local control panel approximately 700 feet from the MCC. The schematic shows 120VAC power to the panel from a 130VA CPT within the associated ‘combo-starter’ MCC bucket. The panel contains 3 ‘ice cube’ control relays, 2 limit switches and 2 LEDs.
My question is, will there be enough voltage to energize these small devices at 700’ from its 130VA source using #14 AWG wire, XHHW-2, 600V insulated CU?


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Just do a standard voltage drop calculation.

Roughly 1A of load, 1500 feet of wire, 2.5 ohms per 1000 feet. A bit less than 4V of drop.

But unstated in your question: what is getting controlled by the ice cube relays; is there a separate circuit which also needs a voltage drop calculation?

Do you have any long control (not power) wires which might be subject to inductive pickup causing false operation?

Jon
 
Are the switch loads only the relays? If not, low-coil-current relays could be used to minimize the control currents.
 
Larry the control relays are vendor supplied - not sure if they’re low coil current or not.
The panel has been UL 508 listed. Also I think I’ll need a disconnecting means per NEC 409, right?
The conductors are coming off the CPT with only the secondary fuse as protection and no disconnect shown.


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There is also the potential for capacitance in a long control circuit preventing the coil from dropping out when the control circuit tells it to.
This is from an Square D Document
In addition to series impedance, the control circuit wires also exhibit a distributed capacitance. The effect of this shunt capacitance is particularly important when the relay, contactor, or starter circuit is opened and the device is to drop out. If the arrangement of the control circuit components is such that the control wire shunt capacitance is in parallel with the STOP button, limit switch, or other disconnect means controlling the relay, contactor, or starter, a large enough amount of capacitance will prevent the device from dropping out even though the control circuit was opened. This is a very serious condition and must be prevented.
Click to expand...
Tables in that document indicate that this condition occurs at a lesser length than the series impedance of the control circuit that may create enough voltage drop to prevent the coil from pulling in.
 
winnie said:
Just do a standard voltage drop calculation.

Roughly 1A of load, 1500 feet of wire, 2.5 ohms per 1000 feet. A bit less than 4V of drop.

But unstated in your question: what is getting controlled by the ice cube relays; is there a separate circuit which also needs a voltage drop calculation?

Do you have any long control (not power) wires which might be subject to inductive pickup causing false operation?

Jon
Click to expand...

There are two limit switches just outside the panel approximately 6 feet from the control panel. They will monitor torque associated with a large ‘rake’ inside a water tank and provide 24VDC discreet output (from the ‘dry’ relay contacts) feedback to the PLC, also approximately 700 feet away


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suggest you consider using DC from the MCC. You won’t have the issue Don mentions, and less voltage drop. We used to have 120 VDC fire alarm call boxes that used 19 gage copper phone wire that ran for miles.
 
don_resqcapt19 said:
There is also the potential for capacitance in a long control circuit preventing the coil from dropping out when the control circuit tells it to.
This is from an Square D Document

Tables in that document indicate that this condition occurs at a lesser length than the series impedance of the control circuit that may create enough voltage drop to prevent the coil from pulling in.
Click to expand...

What if I bring power into the panel from a local lighting panel @ 120VAC using number 12’s? This power source is around 400’ feet from the Control Panel.


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Isaiah said:
What if I bring power into the panel from a local lighting panel @ 120VAC using number 12’s? This power source is around 400’ feet from the Control Panel.


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Click to expand...
The shunt capacitance issue gets worse with larger conductors. Tom's suggestion of DC is a good idea.
 
don_resqcapt19 said:
The shunt capacitance issue gets worse with larger conductors. Tom's suggestion of DC is a good idea.
Click to expand...

The only DC source is the 120VAC/24VDC power supply at the PLC. They don’t have a battery bank and DC Panelboard like I’m used to seeing at 125VDC


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Isaiah said:
The only DC source is the 120VAC/24VDC power supply at the PLC. They don’t have a battery bank and DC Panelboard like I’m used to seeing at 125VDC


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Click to expand...
Count me in as one in favor of using DC.

You don't need a DC power supply. Bridge rectifiers are easy to come by and could be used to convert the circuit voltage to DC.
 
ActionDave said:
Count me in as one in favor of using DC.

You don't need a DC power supply. Bridge rectifiers are easy to come by and could be used to convert the circuit voltage to DC.
Click to expand...

Is that going to destroy the panels
UL listing?


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ActionDave said:
I don't know. Who says the UL has to be involved in at all? At my old shop we built all sorts of control panels. I don't have any idea the context of your situation but I do know that not everything that electrical needs UL listing.
Click to expand...

Unfortunately I’m not having it built by an external shop. It’s a vendor supplied Panelboard.

The client requires a NRTL to be involved, either UL, CSA etc.


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You are over thinking it. The only "load" here are the coils of the ice cube relays. Most of those are very low current, which is why they are used. I'm looking at one now where the INRUSH is 3.3VA. At 120V, that's less than 3/10 of an amp per relay. LED lamps on pilot lights is even less than that. VD is not going to be an issue here.
 
I may of misunderstood your application.
120 volts to a control panel for limit switches, relays and pilot lights. Are there any inputs-outputs from the MCC?
 
Jraef said:
You are over thinking it. The only "load" here are the coils of the ice cube relays. Most of those are very low current, which is why they are used. I'm looking at one now where the INRUSH is 3.3VA. At 120V, that's less than 3/10 of an amp per relay. LED lamps on pilot lights is even less than that. VD is not going to be an issue here.
Click to expand...

Thanks Jraef.
That’s what I thought. I just wanted to be sure.


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tom baker said:
I may of misunderstood your application.
120 volts to a control panel for limit switches, relays and pilot lights. Are there any inputs-outputs from the MCC?
Click to expand...

Not really. The starter M coil is energized off the CPT like a ‘normal’ motor control configuration but the 120VAC (hot/neutral) extends (700’) to the control panel and associated relays.

Thanks to all for this great input
 
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