Voltage Drop 3

[avvo0523]

Hello all

I am trying to determine which wire size to use to energize a control relay, below is the parameters:

Voltage Source: 24VDC
Length in 1 way: 500FT

What is the allowable voltage drop for 24VDC? and also what size of wire can I use for that length of a distance.

Thanks for your help.

 

[dpc]

This is all a function of the relay you select. The relay data will include coil voltage and current requirements and probably also the minimum voltage required for the relay to pickup and hold in.

Once you know that, you can calculate the voltage drop in the circuit and pick a wire size that will work for this application.

 

[avvo0523]

DPC

Thanks so much for the prompt response. I am using a Potter Brumfield relay with coil voltage 24VDC and current requirements is 1A. What is the allowable voltage drop for a control circuit? I know typically we use 3% - 5% as acceptable.

 

[dpc]

You need to look at the relay data sheet and find the minimum voltage required for pickup.

3 to 5% would probably be OK, but why guess when you can know for sure?

From the rated coil current and voltage, you can determine the coil resistance, or it may be given to you in the data sheet. With that, you can calculate the voltage drop in the circuit. Just make sure the voltage at the coil is higher than the required pickup voltage.

 

[BJC]

Good source for this. I think I saw it on a thread somewhere here.

http://ecatalog.squared.com/pubs/Mo.../Reversing, Disconnect Switch Type/M-379F.pdf

 

[itsmoked]

avvo0523; You are missing the point.

The long control wire will drop voltage. You get that.

What you're missing is that the voltage drop is a function of the current. The current running the relay's coil.

You need to look up the relay's COIL CURRENT. With that value and a wire resistance table you can calculate the round trip resistance for any suggested wire size. Using the COIL CURRENT you can then calculate the voltage drop any particular wire size will incur. Subtract that drop from the power supply voltage that will be running the relay's coil. Take that resulting voltage and make sure it is MORE than the relay's "minimum pull in voltage". If it is, then it will work.

Be forewarned that if you are going to be running other stuff on either of those two wires you must take the added current it would cause into the above calculations.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[CJCPE]

avvo0523:

Double check the coil current. Are you sure it is not 0.1 amp rather than 1 amp?

 

[itsmoked]

Yes. 1A is certainly not correct for a relay coil.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[cherry2000]

You can get hold of the Relay coil catalogue from

http://relays.tycoelectronics.com/pnb.asp

and get the relay coil data as correctly mentioned by itsmoked. Then use the traditinal voltage drop formula for a DC circuit and see if the Holding & Pickup voltages requirements are satisfied. If not, use one higher size control cable & retry.