24VDC max. wire length

I want to energize a relay with 24VDC on CAT5e cable over approx. 130ft from a PLC. My concern, before actually running the cable, is that the VDC will not carry that far.

If this is the case, I may have to use remote IO, or 120VAC output.
thanks

To answer your question you need to know how much current the relay pulls when energised, and what gauge cat 5 cable you are using.

Armed with this information you will be able to work out the volt drop of the cable for the length of run you have.

You could probably expect 7ohms per 100ft of resistance for most cat 5 cables.

So we have 130ft at 7ohms per 100ft = 7*1.3 = 9.1. so the total resistance is 9.1 ohms. Let's say the relay pulls 500mA, that will give you a volt drop of 4.55V. (Ohms law E = IR, or Volt drop = 0.5 x 9.1). The voltage availiable to the relay will be 24 - 4.55 = 19.45v, is that enough for your relay?

The above is for an illutration only, you need to find out the exact figures.

Paul

There is a formula for voltage drop... I don't have it at hand.

In general, the smaller the wire and the longer the run the more the voltage drop.

Off the top of my head, I should think that you would do fine with 130-foot of #16 AWG. I seem to recall that CAT is on the order of #22 AWG or less. That might be marginal. It also depends on what you are trying to drive.

Typically, a relay spec indicates a certain amount of current to guarantee operation. The relay spec should also indicate the relay coil resistance.

Minimum current times coil resistance = minimum voltage required.

If the minimum voltage required is less than 5% below nominal voltage then you should be OK.

I see that Paul beat me to it... but none the less, as a general guide, here ya go.

I hope you're not planning on using this cat-5 for communications at the same time as running power through it. Even if it is on a different twisted pair, you can expect enough induced noise on the line to cause you problems.

No communications will be used on the CAT5 cable. It will be closing contacts on a paging system from a machine.

Thanks everyone for the feedback,
Brad

Greetings Brad,

you said:

My concern, before actually running the cable, is that the VDC will not carry that far.

Click to expand...

well ... you don't really have to RUN the cable to test it out ... you could just use a 130' length of cable and hook up the relay for a "bench test" ... that might be a "low tech" way to answer the question ...

This thread has some guidance.

http://www.plctalk.net/qanda/showthread.php?s=&threadid=8504&highlight=voltage+drop

Ron is always trying to simplify....
Taking the fun out...

Ron, hope the weather is nice down there next week...I'll be at Edisto Island

hope the weather is nice down there next week

Click to expand...

I can't make any promises about next week ... but today is beautiful ...

Ref info from Alpha Wire and Cable site

http://www.alphawire.com/PAGES/379.cfm?partner=0&part=0

Keep in mind that you have a wire 130' out and 130' back, calculate accordingly.

Bob K is right. Circuit is there and back. Length is allways x 2.

vd = 2xkxlxa / cm or to pick wire size cm = 2xkxlxa / vd

k=constant 12.6 for copper and 18 for aluminum

typical UL test at +/- 5%. NEC recommends no more than 3% vd for branch circuits with 5% total from service entrance to end device.

24vdc x 5% = 1.2 vdc dropped max (relay will function properly)

Pick wire size or CM. 2x12.6x130x.5? / 1.2 = 1365 cm

table 8 conductor properties shows 16 awg at 2580 cm

It worked, and I appreciate everyones assistance.
thanks,
Brad