24VDC controls

I'm working out the controls on a new conveyor system and some of my photoeyes are about 300 feet from the Motor Control Panel. I've usually worked with 120 VAC controls but we want to use 24 VDC on this system. Am I going to have any problems with voltage drop over a 300' run? We will run the 24 to the photoeyes with #12 wire. Thoughts?

You shouldn't have any problems at all. Even #18 AWG is only about 6.5 ohms/1000 feet.

the Voltage drop calculation is as follows :Vd = 2K x L x I over Cm (sorry, can't fit all that in a fraction)
Vd = Voltage Drop
K = resistance in Ohms of one Circular mil foot of conductor
L = length from point A to point B
I = amps
Cm = circular mils of the conductor

If you know the amps of the circuit, which in the case of photoeyes...less than 2A, the math is easy.

roughly a loss of .36 Volts, unless I flubbed the math (it is only 8AM & 1 cup of coffee:/ ) ,it'shardly enough to hinder operation.

24V is a nominal figure...it's like saying 110 V/120 V or 460V. Actual voltages will either be higher or lower.

Thanks guys--I'd done the math but wanted some confirmation.

Easy. Unless there's some unusual load this should not be a problem.

Mark, I have miles of conveyor that I use 24 VDC for over 1200 Photoeyes and Prox switches. My furthest ones from the panel are 800 feet away and I use #16 wire for all my field devices. A couple of hints that I thought I might add for you. Spend the extra money to get quick connects on the photoeyes, it has paid off for me. I also run all the wire in wireway instead of conduit which is great if you have to add something somewhere. Lastly, I have remote "power panels" in certain locations. I run 1 +24VDC and 1 -24VDC from the control panel to several remote boxes and then wire from the box to the field device. I know you didn't ask for any of this, but I thought I would add a few more pennies with my 2 cents.

Possibility of interference?

Only other concern I would have is if there would be any noise sources that might exist, e.g. running the 24V in the same conduit with 120VAC or even 460VAC. By applying good wiring practices, this should not be a problem.

Thanks again, everyone. I haven't designed a 24VDC system running this far out before and I thought I'd be okay--but I trust experience over the "book" anyday.

Mike and Bruce--I'll be running my 24VDC stuff part of the way through wire tray and part of the way in EMT. BOTH routes will be strictly Blue wire. The 480 and 120 will run in separate wireway/conduit. We'll be running VFDs to the motors. The wiring done before I started to work here was run together. We tried to add VFDs to some of those motors and had to run new pipe--the VFDs were inducting enough voltage to trigger false input conditions on some of our pushbuttons.

I appreciate the feedback.

MikeW said:

Only other concern I would have is if there would be any noise sources that might exist, e.g. running the 24V in the same conduit with 120VAC or even 460VAC. By applying good wiring practices, this should not be a problem.

Click to expand...

NEC says that you can run mixed voltages in the same raceway/conduit as long as it has the same insulation type (all THHN, or all MTW, etc.,). I've done this lots of times without interference from noise...

I would try to seperate the high current 480 wires from the 24vdc siganal wires to avoid interferance. If the 408 is for a small load (like a 1 hp motor) it may not be a problem, but with a large motor the current is likely to induce a current/voltage on the signal wires. On short runs you may be able to get away with it..

Russian roulette anyone?

stasis,

Yes, as long as the insulation class is sufficient, it's per NEC code. But it's like playing Russian roulette, the longer you play, the more of a chance you'll eventually loose and not like the consequences. Been there, done that. (with regards to having to troubleshoot why 24VDC failed, that is. It had "floated" to over 200V and wonder why relays were doing random things on someone else's design.)

I'm stuck now in an application where I have to run some 24VDC through a disconnect switch that will have up to 400A 460/60/3 and at times it will be switched over to an inverter to run the motors from 43 to 65 hz. I'm going to only cross the power wires at 90 degrees and am using shielded 600V class cable as an insurance policy.