Voltage drop 1

[GTodd]

I've got a 120V relay that pulls 5.5 mA at rated voltage. The specs say it will operate at 80% of rated voltage. My circuit length is 1100 ft (one way). I've done the voltage drop calculation for #14 AWG. The loss is negligible, but I'm concerned that simply following the formula might be leading me astray. Thoughts?

 

[itsmoked]

Yup. AC or DC?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[GTodd]

AC

 

[waross]

With that length of cable I would double check my voltage drop in a voltage drop table.
The induction on AC may give you a reactive voltage drop considerable greater than the calculated voltage drop due to simple resistance. With your very low current I doubt that there will be a problem. However it is due diligence and good practice to check the impedance drop for AC circuits. In many instances the reactive voltage drop is significantly greater than the DC resistance drop.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

You may find this thread interesting.
thread237-102363

There was a paper written on this subject by a big company. SquareD or AB ?? Danged if I can find it now.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

Probably way too long for an ac control circuit especially if it may be subject to being wet. Suggest you switch to dc control.

Capacitance of the cable on ac can lead to problems.

The circuit arrangement makes a big difference. If the initiating contact is close to the power source and the relay is 1100 feet away, your problem will be getting the relay to pickup. If the relay is close to the power source and the contact that operates the relay is 1100 feet away, your problem will be getting the relay to drop out.

 

[logbook]

Hang on, the reason why the "must operate" voltage is typically 75% of the nominal is to allow for temperature.

Copper has a TC of +0.4%/degC. At 70degC you get significantly less coil current at the nominal operating voltage. Don't use up the margin by operating at 80% of nominal or you will find that the relay may pull in then get hot. It won't drop out, that's fine. You turn it off. No problem, but it won't then pull back in when you try to turn it on when it is still hot!

 

[GTodd]

Thanks EVERYONE for the input.

Let me add some clarification to my situation. I have five (5) relays located in the control panel housing a PLC. I have an air compressor located 1100 feet away. Three (3) of the relays are for control signals TO the compressor. Two (2) are for feedback signals FROM the compressor.

My control voltages on either end are 120VAC. I assume by DC you mean 24VDC. Correct? Do I need to design appropriate 24VDC control relays to provide the signals I need both ways?

 

[hcooperCO]

Interesting thread. So if what is being said, there is a larger voltage drop using 120VAC vs 24VDC considering the same length and gauge of wire? I always thought it was better to run AC over long distances ?

 

[VEBill]

Higher voltage provides a squared advantage. 1) Presumably less current (for a given power) and thus less IR drop. 2) And any Vdrop is a smaller fraction of a higher voltage. Advantage^2.

DC has less peak current so less peak Vdrop.

But as mentioned above, there may be other considerations than just Vdrop.

 

[peebee]

I believe the paper that itsmoked was looking for is:

http://ecatalog.squared.com/pubs/Ma...s/Relays, Industrial Control/8501X/M-379F.pdf

Given the very long URL, and the frequency with which Square D reorganizes its website (making it nearly impossible to find that old documentation you've come to rely upon), you might do better to Google for "M-379.pdf".

 

[itsmoked]

Five huh...

The typical relay 3Pole KU style Potter & Brumfield 24VDC, that plugs into a socket is 1.2W

1.2W/24V = 51mA

It is guaranteed to operate at 75% of nominal voltage.

0.75 x 24 = 18V
24V - 18V = 6V

This means you can tolerate a 6V drop over the round trip circuit at the relay's current.

That equates to 6V/51mA = 117 Ohms that can be the circuit's drop.

The drop is a function of the circuit wire gauge and its resulting resistance per foot.

1100ft x 2 = 2200ft.

117 Ohms / 2200ft = 0.053 ohms/foot
Any wire with LESS THAN 0.053 Ohms/foot will work.

Looking at:

http://www.epanorama.net/documents/wiring/wire_resistance.html

Down just a ways shows "Wire Gauge Resistance per foot".

This shows that even paltry 28AWG would allow the relays to operate in 24VDC land.

Certainly this is too small from a purely mechanical durability standpoint. But you can use low voltage door bell, heater thermostat, limited energy type cable to run the relays over this distance. 24AWG or 22AWG or 20AWG. The smaller the gauge the less likelihood of wire theft. The less expensive. Going with 120VAC can require much larger wire for code minimums costing substantially more and raising the theft aspect, conduit needs, etc.

I am not sure if you would have problems with an AC link on that distance. I can't find the document that gave the formula/table. I do know that distance is near the ragged edge.

At the high price of copper these days I suspect that needing maybe an extra set of relays/sockets if your sources are 120VAC, (and not dry contacts)and maybe two cheap power supplies is possibly less expensive than all the code required wire needed for the AC solution.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

From experience, I would do this using dc interposing relays. Probably 24 vdc is fine. I think 120 V ac will be very marginal at this distance and I would not bother even trying.

 

[itsmoked]

peebee; Thank you! Thank you! Thank you! kiss kiss er... ah(never mind)

That's it! I never saw your post for some reason. Maybe you ducked in while I was writing. Anyway I was just looking for Mac's tag line and randomly picked this thread. And noticed your post.

Belated Thanks again.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

We have been experimenting on a real pain-in-the-ass circuit where a 24V DC signal is used to drive a remote relay in an MCC approx 1/2 a mile away (no kidding - I didn't design this PoS installation!) and we have had some real success with a solid state relay at the remote end. The SSR accepts anything from 3 - 32V input, and we have loaded the relay end of the cable with 1k[Ω] to help prevent any stray pickup causing false triggering. It is working beautifully. Prior to this, the maintenance techs had tried various lash-ups using paralleled spare conductors to get the loop resistance down without success.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Do tell.

So you say the 24VDC didn't make the 1/2 mile? Why not? What was the remote relay current requirement?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com