24vdc Control


Thread Starter


Good afternoon guys and gals,

So we are running outputs from our micrologix 1100 to a terminal strip, then out to some relays. I have an 24vdc 8amp power supply

Here's my question, I'm looking at a multi conductor cable going to the junction box 12c 18awg, roughly 300 feet.

From there I want to run 2c 18awg 200 feet to a relay coil.

Will I be able to fire a 24vdc relay coil that distance? What is the standard resistance on a coil, if there is such a thing. I know the coil will pull in even with a decent voltage drop, but I want to understand the load I'm putting on the coil and the plc.
If you're talking about a normal ice-cube relay, you'll be fine.

Typical coil power is 1/2 - 1w, so 20-40ma at 24V. I have no idea what the resistance per mft for 18ga is, but I know it'll carry 5A or so, so here you're at 1% or less of the max capacity of the conductor, so voltage drop won't be a problem (don't forget to calculate the voltage drop in the return conductor, which is probably several times that in one of the individual coil wires because they probably share a common return, but even then you're fine).
The spec sheet on my desk for a Weidmuller 17333230 24Vdc relay says it pulls in at 16.7Vdc. It also gives current and wattage specs.

Check the spec sheet on yours.
[... I have no idea what the resistance per mft for 18ga is...]

18AWG copper is 7.77ohms per 1000'.

Vd = I x 7.77ohms x (distance in feet) ÷ 1000'

Great point on mentioning the voltage drop could be substantially greater in a common return section. Calculate sections of differing current or any other parameter separately, then add them up for the total.
Thanks Steve, I am looking at a IDEC cube relay. Initially I was going to run from our micrologix 1100 OW16 card to the junction box, then from the terminal blocks to the relay mounted near the light.

Looking at it again and being worried that I will damage the plc, I'm wondering if it would be better to run from the outputs, trigger a slim din rail mount relay (like the phoenix PLC-RSC-24dc/21-2966171) then out to the junction box and my cube relay. This way I could separate and fuse the output and coil power independently.

or is that just being paranoid? lol
This is VERY unlikely to harm the PLC output. Any excess voltage drop will more than likely REDUCE the current, which is what would harm the output. The only exception would be if the voltage drop was so severe that the relay stayed in inrush mode indefinitely and the current never came down, but you're nowhere near that scenario.

Just run your 1000' or whatever and drive the relay, you'll be fine. Maybe use two cores in parallel for the return or run a separate 16AWG wire for the return (even that's unnecessary but would help the voltage drop while avoiding adding a new relay that could fail).
>...micrologix 1100 OW16 card ...or is that just being paranoid? lol<

It's being paranoid.

Ignorance, as in the lack of knowledge, can induce paranoia. So let's work on the knowledge part...

The relay contacts of a micrologix 1100 OW16 card are rated [email protected], 28VA (voltampere).

Footnotes say...

(1) Surge Suppression - Connecting surge suppressors across your external inductive load will extend the life of the relay
contacts. For additional details, refer to Industrial Automation Wiring and Grounding Guidelines, Allen-Bradley publication

(2) For DC voltage applications, the make/break ampere rating for relay contacts can be determined by dividing 28 VA by the
applied DC voltage. For example, 28 VA/48V DC = 0.58A. For DC voltage applications less than 14V, the make/break rating
for relay contacts cannot exceed 2A.
Oh, and my previous answer was assuming you had a solid state DC output. Since you have a relay (W) output, you REALLY REALLY have nothing to worry about. You can drive decent sized motor starters directly with an Allen-Bradley relay output; driving an ice cube relay, even 1000' away, is WAY toward the trivial end of what that output will withstand.
OR...voltage drop becomes less of a problem as the SQUARE of the nominal voltage.

Since you're using relay outputs, which will happily switch 120, and since the load on the other end of the wire is a relay anyway which are readily available with whatever coil voltage you want, let's say you switched to 120. While you now can tolerate 5 times the voltage drop, you've reduced your current by a factor of 5, so the 120 is actually 25 times less sensitive to voltage drop than the 24. That's why for transmission lines, they increase the voltage rather than reducing it.

But this whole discussion is talking about stuff that's unnecessary and irrelevant because you don't have a problem in the first place.
Your wire run is 1000 ft and 18AWG has a resistance of 6.385m-ohms/ft; so wiring resistance of 6.4 ohms.

There is a wide range of current draws out there depending on size, duty and contact rating of the relay.

Weidmeuller and Pheonix TB interposing relays draw 11 mA @ 24VDC.
An IDEC RH relay will draw 32-62 mA @ 24VDC depending number of poles.
A AB IEC control relay 3 W @ 24VDC.

You need to know these things when designing any I/O loop.
It looks like you are not current limited.

BUT, some things to consider:
1- your Electrical Code will derate a cable with multiple conductors because convective cooling is reduced and adjacent conductors heat each other (18 AWG max current in jacket 2.3A).

2- every output circuit should have a fuse to protect the I/O circuit and the field wiring. Your PLC relay output has a max. load of 1.2A; that is a hint about your fuse size.

3- the standard for control wiring gauge of any voltage from 24-125VDC and 120VAC is 14AWG. You could find that the control relay that you must energise draws 2 - 5A.

good luck
Thanks guys, lots of information there. I have been combing through the code book and internet searches trying to find something that tells me its ok to put a relay with 24vdc and 120 ac in the same utility/control box. But i'm coming up short. Obviously these voltages are thrown in together all the time for control circuits, but where's the rules for that? I'm using the Ontario electrical code book because that's what we have here. By early next week I hope to have some sketches/material lists to post and discuss.

Thanks again!
I have been combing through the code book and internet searches trying to find something that tells me its ok to put a relay with 24vdc and 120 ac in the same utility/control box.

The NEC does not distinguish what conductors can be installed in the same box or raceway by voltage.

It does require all conductors have insulation exceeding the highest voltage found in the raceway.

There are also some restrictions on how article 725 circuits are handled. Depending on exactly what you are doing and how, this could be a class 1 circuit.
Interposing relays for 24VDC control of 120VAC or 125VDC are used everywhere. Wire type and insulation must exceed the working voltage. The different voltage levels must be separated and identified:\
-in different wireways

-wiring on one side of relay will be 24VDC and the other side will be the higher voltage

-wireways labeled as to voltage

-different wire colours for different voltages

-terminal blocks for different voltages separated with dividers as necessary.
One reason I've heard for preferring low voltage control is to limit the number of enclosures where you have to put on rubber gloves and leather outer gloves and a face shield, blast trousers, etc. etc. That doesn't help much for motor buckets, but the other end can be made more convenient to work on. I don't do much with big motors but that's what I have heard. It goes along with separating the arc-flash liabilities into sub enclosures.