# 3 way switch and voltage drop



## Jason8161 (Feb 17, 2016)

So I got a guy wanting to install a 3 way switch from his garage to the outside lights of his work shop that is 150' from one another. Workshop has a 60 amp sub panel. To get away with having to go up on wire size due to the amount of voltage drop, can I not just install a relay in a box in the workshop and use the 3 way as the control loop for the relay?


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## 480sparky (Sep 20, 2007)

You can, but aren't you going to have the same VD issue with the length of the run to the light?


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## Jason8161 (Feb 17, 2016)

Voltage drop is only an issue when a load is applied? so it would take using two circuits one for the coil of the relay and one for the circuit of the light?. the light is only 20 feet from the sub panel in the workshop


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## splatz (May 23, 2015)

You could use a relay - it shouldn't be hard to find something that will operate even with the voltage drop. 

But there is wireless gear that will make that distance, some of it is even inexpensive, although I've never had my hands on the inexpensive stuff.


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## Janjak (Feb 2, 2016)

A Pico Wireless system receiver/transmitter would be much better for you


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## mpcxl (Dec 9, 2015)

Sorry I am just trying to understand this and see the big picture. So you want to run low voltage between the three ways that controls a relay? 

How are you getting wires to and fro three way switch?

Also, did you do the calculations for VD? How much bigger would you have to size the wire for 150' ?


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## Aegis (Mar 18, 2011)

Jason8161 said:


> Voltage drop is only an issue when a load is applied? so it would take using two circuits one for the coil of the relay and one for the circuit of the light?. the light is only 20 feet from the sub panel in the workshop


Yes we do it all the time.


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## mpcxl (Dec 9, 2015)

Isnt a relay still considered a load, I mean current is still being drawn? Looks like you will have a 12 volt drop in a 20 amp circuit using 12 awg.


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## Dark Knight (Jan 6, 2016)

mpcxl said:


> Isnt a relay still considered a load, I mean current is still being drawn? Looks like you will have a 12 volt drop in a 20 amp circuit using 12 awg.


The load on a relay is negligible. It would cause virtually no VD.


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## Aegis (Mar 18, 2011)

Even if the relay needed 3A, #12 would still hold under 3% for 300ft, and it doesn't draw that much anyway.


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## mpcxl (Dec 9, 2015)

I believe the voltage drop will be the same whether you are using a heat gun or a relay. Resistance in the wire is constant.

But now that you mention it, The voltage drop will account for both directions so it would be 300' . 

I wouldn't be concerned about the breaker tripping, more so the relay not receiving the correct voltage.


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## Dark Knight (Jan 6, 2016)

mpcxl said:


> I believe the voltage drop will be the same whether you are using a heat gun or a relay. Resistance in the wire is constant.
> 
> But now that you mention it, The voltage drop will account for both directions so it would be 300' .
> 
> I wouldn't be concerned about the breaker tripping, more so the relay not receiving the correct voltage.


Lol. I believe you are wrong. You need to check again.


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## PlugsAndLights (Jan 19, 2016)

mpcxl said:


> *I believe the voltage drop will be the same whether you are using a heat gun or a relay.* Resistance in the wire is constant.
> 
> But now that you mention it, The voltage drop will account for both directions so it would be 300' .
> 
> I wouldn't be concerned about the breaker tripping, more so the relay not receiving the correct voltage.


How would you reconcile this claim with E=IxR?


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## mpcxl (Dec 9, 2015)

PlugsAndLights said:


> How would you reconcile this claim with E=IxR?


Ahhhhh good point. Well I stand corrected.


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## backstay (Feb 3, 2011)

With a 100 watt load you could use #14

1 conductors per phase utilizing a #14 Copper conductor will limit the voltage drop to 0.56% or less when supplying 0.833 amps for 150 feet on a 120 volt system.

http://www.southwire.com/support/voltage-drop-calculator.htm


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## Aegis (Mar 18, 2011)

I did the calculation a while back for a 3way that was 600ft away in a warehouse. The coil of the relay was 1A. So 1200ft feed and traveller at 1A, I used #10s in a 1/2 conduit.


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## Dark Knight (Jan 6, 2016)

Aegis said:


> I did the calculation a while back for a 3way that was 600ft away in a warehouse. The coil of the relay was 1A. So 1200ft feed and traveller at 1A, I used #10s in a 1/2 conduit.


1 amp? Really? I would have guessed around 100mA at the most. I'll have to look that up. But you should have been fine with #14 even with a 1 amp load. According to the calculations in CEC you can go a maximum of 652 feet with a 3% VD.


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## Aegis (Mar 18, 2011)

Dark Knight said:


> 1 amp? Really? I would have guessed around 100mA at the most. I'll have to look that up. But you should have been fine with #14 even with a 1 amp load. According to the calculations in CEC you can go a maximum of 652 feet with a 3% VD.


It's also the the return from the switch so you have to double it (1200ft). Every relay type is different so it's a good idea to just look at the nameplate/sticker on the side of it.


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## Dark Knight (Jan 6, 2016)

You don't have to double it for the formula in the book. It's factoring for a 2-wire circuit already.


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## Aegis (Mar 18, 2011)

Dark Knight said:


> You don't have to double it for the formula in the book. It's factoring for a 2-wire circuit already.


This is interesting.

I always believed that the code is factoring in already for the neutral. So if you have a receptacle 50ft away, you calculate for 50ft and not 100ft.

The neutral in this scenario is probably 4ft long if the JB with the contactor is right above the panel. The neutral goes from the panel to directly on to the coil of the contactor.

Assuming the 1st 3way is near the panel, the hot wire is going out 600ft to the 2nd 3way, then it's coming all the way back on a traveller - 1200ft total. No neutral is pulled out to that far 3way since there isn't a need for one.


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## Dark Knight (Jan 6, 2016)

Aegis said:


> This is interesting.
> 
> I always believed that the code is factoring in already for the neutral. So if you have a receptacle 50ft away, you calculate for 50ft and not 100ft.
> 
> ...


The current travels the same distance in both scenarios. Whether it's down a hot to the load and back down the neutral to ground or down a traveler and back and a short distance to ground from there it's basically the same. That's the way I understand it anyway.


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## splatz (May 23, 2015)

The voltage drop ahead of the load would I think be the distance to the first switch, plus twice the distance between switches - the trip out on one of the travelers and the trip back on the common - plus the distance to the load. 

Look at the specs on this: 

https://www.idec.com/language/english/catalog/Relays/RSSSeries.pdf

would work, the input voltage range will tolerate the voltage drop and it only draws 10ma. 

Use the 3-way to the inputs on the relay, and for $30 or so you're done.


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## Jason8161 (Feb 17, 2016)

The relay chart I seen said it was like 158ohms for the coil of a 120v relay. With that being said the coil will only draw .8 amps . The way I was taught to do VD calculation that tells me I can go up 900' with #12 with that triggering the relay. Once the relay is trigger then the lights is powered off the second circuit which is only 30 feet from the panel.

2*12.9*.8*900/6530=2.8%? with 3% being what the NEC recommends?


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## Dark Knight (Jan 6, 2016)

Jason8161 said:


> The relay chart I seen said it was like 158ohms for the coil of a 120v relay. With that being said the coil will only draw .8 amps . The way I was taught to do VD calculation that tells me I can go up 900' with #12 with that triggering the relay. Once the relay is trigger then the lights is powered off the second circuit which is only 30 feet from the panel.
> 
> 2*12.9*.8*900/6530=2.8%? with 3% being what the NEC recommends?


That is correct, yes.


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## Achilles (Sep 11, 2014)

I know this is a bit of topic from the OP inquiry.

Would it not be cheaper to simply pull #10 then to bother installing a LV relay system?

the difference in cost of 500' of #10 vs #12 THHN is about $20

Also depending on the lighting source 5%+ VD should not have a significant impact on functionality


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## hardworkingstiff (Jan 22, 2007)

Achilles said:


> Also depending on the lighting source 5%+ VD should not have a significant impact on functionality


Depending on the type of light, it may work just fine with a 10% VD.


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## splatz (May 23, 2015)

Achilles said:


> I know this is a bit of topic from the OP inquiry.
> 
> Would it not be cheaper to simply pull #10 then to bother installing a LV relay system?
> 
> ...


No that's a good point. Of course you'd only need #14 (or even smaller) to run a relay with a line voltage coil / primary side, so it's a little more savings, and if it's going in a conduit, you can run them all together. With a relay with low voltage contacts, you can run itty bitty wire, you could run phone wire and it will work. But that might not mean much if you can't put it in the same conduit. 

But really I'd do it wireless.


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## MadDawg (Jun 12, 2012)

We have been looking at using low voltage controllers like these:

http://www.wattstopper.com/~/media/...0-series-digital-wall-switches-cut-sheet.ashx

Run a Cat5 ethernet cable to all switches then to the module at the load or loads.

Done.


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