3 Way Switches and Voltage Drop

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jap

Senior Member
Occupation
Electrician
We have a building that is 60 wide and 100' long running east and west with (5) 20' bays and garage doors.
The owner want to install (4) 2 lamp t5 Fixtures 120v fixtures in each bay for a total of 20 Fixures and switch them 10 and 10 North and south.with (2) 3 way switches at each end of the 100' run.

The fixtures draw 1 amp a piece so we figured on doing this with (2) 20 Amp 120v circuits 10 amps per circuit.
When we started figuring the voltage drop we came up with needing #10 for the 2 circuits.

Problem is with the 2 3 ways at each end of the run, you end up with 2 hots down,4 travelers back, 2 switchlegs back down, a neutral and a ground. 10 #10's which is more than what we want to deal with in a 3/4" conduit.

I was thinking on putting in 2 seperate contactors above the panel and just running the 2 circuits through them to the fixtures running #10 in a 3/4" conduit east and west which would be 2 circuits a neutral and a ground along with a circuit and a neutral for the Emergency lights we have to pick up . and runniing the branch conduit North and South in #12 to each bay of lights.

Then run the 2 3way switches through a seperate 1/2" pipe out of the contactor enclosure to the (2) sets of 3 ways with #12's since we would only be dealing with the coil load.

My partner questioned 2 things.
#1 he mentioned if we reduced the wire size from the #10 in the trunk line to #12 on the branch to the fixtures, someone could come along later, see the #10 in the panel, upsize the breaker to a 30 Amp, then the #12 would not be protected an
#2 he questioned the control for the 2 contactors being in a seperate conduit from the circuits they were controlling.

Does #1 hold any merit? I cant see having to run #10's all the way to the light fixtures. this is a large increase in material and wire cost.
also #2 is there any violation of running the control for the lighting contactors seperate from the conduit that has the lighting circutis?
I dont know of any.

Just trying to keep the runs in 3/4" with least amount of wires possible, but still do it correctly.

Thanks in advance for any suggestions.

JAP>
 

kwired

Electron manager
Location
NE Nebraska
After removing the extra circuit length involved in the three way circuit do you still have long enough run from contactor to load to warrant upsized conductors for voltage drop?

Also if using multiwire branch circuits and fairly balanced load across neutral your voltage drop should be calculated on 208, 240 or even 480 volts instead of 120 or 277, especially if running from a contactor that will close all ungrounded conductors simultaneously.

I have even used class 2 24 volt control circuit and 18 AWG conductors for the remote switches in a similar application, which of course will require separate raceway or cable to contain the class 2 circuit, but the raceway was going to the switch(es) only either way so not a big deal.
 

iwire

Moderator
Staff member
Location
Massachusetts
#1 he mentioned if we reduced the wire size from the #10 in the trunk line to #12 on the branch to the fixtures, someone could come along later, see the #10 in the panel, upsize the breaker to a 30 Amp, then the #12 would not be protected an

And when we install fusible disconnects someone can put to big a fuse in. You cannot stop people from doing the wrong things to your work in the future.

I do a lot of parking lot lighting and I may have 8 or 6, or 4 or even sometimes 2 AWG running into the panel from outside. These will be on 20 or 30 amp breakers as in the poles outside the wire size is reduced to 10 or 12 AWG.


#2 he questioned the control for the 2 contactors being in a seperate conduit from the circuits they were controlling.

That is not a problem at all as long as you do follow the rules found in 300.3(B).

Again a common practice to run control circuits separately from the circuits they control.


Now all that said, my personal opinion is this is not an installation to complicate with contactors and control wiring. I would run the 10 AWGs as needed. .... Or I might just use 12 AWG and say the heck with worrying about a little voltage drop for what are likely electronic ballasts that can handle a range of input voltages
 

jap

Senior Member
Occupation
Electrician
And when we install fusible disconnects someone can put to big a fuse in. You cannot stop people from doing the wrong things to your work in the future.

I do a lot of parking lot lighting and I may have 8 or 6, or 4 or even sometimes 2 AWG running into the panel from outside. These will be on 20 or 30 amp breakers as in the poles outside the wire size is reduced to 10 or 12 AWG.




That is not a problem at all as long as you do follow the rules found in 300.3(B).

Again a common practice to run control circuits separately from the circuits they control.


Now all that said, my personal opinion is this is not an installation to complicate with contactors and control wiring. I would run the 10 AWGs as needed. .... Or I might just use 12 AWG and say the heck with worrying about a little voltage drop for what are likely electronic ballasts that can handle a range of input voltages

I'd originally planned on runniing 10's at least from the panel to the furthest set of 3 ways then drop to 12's for the Travelers and switch legs back.
I'd like to run 12's the whole way but 100' east from the panel to the farthest set of 3 ways, 100' back west for the travelers then 120' back east to the furthest light,,, the footage seems to add up quickly.
 

James L

Senior Member
Location
Kansas Cty, Mo, USA
Occupation
Electrician
One thing I never see considered in voltage drop calculations is ACTUAL voltage people are starting with.

Everybody does calculations based on 120/208 or 120/240 (or other) when those are never the actual voltages.

Assuming that you've got a 120/208 system, I would figure that half the lights might be on and the other half off, thus 120 volts, not 208.

If you're shooting for 3% drop at 120 volts, then you have a target minimum voltage of 116.8

But what actual voltage are you starting with? Most times it's in the 123-124 volt range. You shouldn't care so much about % drop, but instead shoot for a minimum of 116.8 volts. If you start with 123.7 volts, you can have more than 3% drop before you get to your target

I wired some parking lot lights with about the same load and footage you've got. 11 amp pole sign on one circuit, and 6 amps worth of pole/enter/exit lights on the other, 280 feet. Calculating from 120 volts, I would have needed a #8 conductor.

But I was starting with 124.3/215 volts. That's 6% drop before I reached my target 116/201 volts, and I was able to use #10 conductors

Considering that I was using an existing 3/4" conduit underground, those #10s were a lot easier and less expensive
 

Smart $

Esteemed Member
Location
Ohio
I'd originally planned on runniing 10's at least from the panel to the furthest set of 3 ways then drop to 12's for the Travelers and switch legs back.
I'd like to run 12's the whole way but 100' east from the panel to the farthest set of 3 ways, 100' back west for the travelers then 120' back east to the furthest light,,, the footage seems to add up quickly.
Why are you running the hots all the way to the opposite end first???

Run hots and neutral to nearest set of 3-ways...

...travelers and neutrals to set of 3-ways at other end, in the same runs that the whips drop out to fixtures...

...switch legs back.

As it works out, the voltage drop is just 100' one-way distance (maybe a little more for offsets north and/or south to fixtures, switches to runs, panel to switches, ups and downs), because you tap the neutral as it runs to the other end. The loop is panel (hot) to switch, travelers to other switch, switch leg to fixture, neutral back to panel... 100' (+).

Let me know if you need a diagram... :p
 
Last edited:

jap

Senior Member
Occupation
Electrician
Why are you running the hots all the way to the opposite end first???

Run hots and neutral to nearest set of 3-ways...

...travelers and neutrals to set of 3-ways at other end, in the same runs that the whips drop out to fixtures...

...switch legs back.

As it works out, the voltage drop is just 100' one-way distance (maybe a little more for offsets north and/or south to fixtures, switches to runs, panel to switches, ups and downs), because you tap the neutral as it runs to the other end. The loop is panel (hot) to switch, travelers to other switch, switch leg to fixture, neutral back to panel... 100' (+).

Let me know if you need a diagram... :p

Dont need a diagram. was going to run to the far end with 10's (just on the Hots) but after thinking about it, you dont really gain anything by doing so.Are you saying there's no need for the 10's at all and all this can be done with #12's?
 

jap

Senior Member
Occupation
Electrician
After removing the extra circuit length involved in the three way circuit do you still have long enough run from contactor to load to warrant upsized conductors for voltage drop?

Also if using multiwire branch circuits and fairly balanced load across neutral your voltage drop should be calculated on 208, 240 or even 480 volts instead of 120 or 277, especially if running from a contactor that will close all ungrounded conductors simultaneously.

I have even used class 2 24 volt control circuit and 18 AWG conductors for the remote switches in a similar application, which of course will require separate raceway or cable to contain the class 2 circuit, but the raceway was going to the switch(es) only either way so not a big deal.

If I took the 3 ways out of the equation, and took a 120v circuit straight from the breaker to the furthest fixture (one in a group of 10)f it would be 170 feet.
 

jap

Senior Member
Occupation
Electrician
Why are you running the hots all the way to the opposite end first???

Run hots and neutral to nearest set of 3-ways...

...travelers and neutrals to set of 3-ways at other end, in the same runs that the whips drop out to fixtures...

...switch legs back.

As it works out, the voltage drop is just 100' one-way distance (maybe a little more for offsets north and/or south to fixtures, switches to runs, panel to switches, ups and downs), because you tap the neutral as it runs to the other end. The loop is panel (hot) to switch, travelers to other switch, switch leg to fixture, neutral back to panel... 100' (+).

Let me know if you need a diagram... :p

I have to bring the circuits to the far end to pick up the emergency lights that are in each group.(Non Switched)
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
One thing I never see considered in voltage drop calculations is ACTUAL voltage people are starting with...
I agree with this. Along with it I see a lot of hand wringing about voltage drop and percentage of voltage drop when it is not a real issue.

Right now we are talking about some T-5 light fixtures. I don't know what the literature for them says but my gut tells me that they would operate just fine with an actual voltage below 110V, maybe even as low as 90V.
 

Smart $

Esteemed Member
Location
Ohio
Dont need a diagram. was going to run to the far end with 10's (just on the Hots) but after thinking about it, you dont really gain anything by doing so.Are you saying there's no need for the 10's at all and all this can be done with #12's?
I'm inclined to believe #12's are sufficient.
I have to bring the circuits to the far end to pick up the emergency lights that are in each group.(Non Switched)
Still not much different. You simply splice in the nearest 3-way and run hots with the travelers to the last emergency light (EL) served. Don't forget supplying the EL's on separate wires reduce the amps on the travelers and switch legs, thereby reducing their Vd.

If you have 2 EL's per circuit, you only have 2A max' on the hots and 8A max' on the travelers/switch legs, 10A max' on neutrals... and the max' occurs only in the 'panel' portions of their routing. The amps get swapped from hot/SL to neutral in the routing as loads are tapped in, so it is not over the entire round-trip distance. You'll have 8A on traveler/SL to farthest 2 fixtures tap, but only 6A on switch leg and 2A on neutral to next closest-to-panel tap. If one of the next 2 closest-to-panel is an EL, then you'd have 1A on the hot, 5A on the switch leg, and 4A on the neutral to the next closest-to-panel tap.
 

Smart $

Esteemed Member
Location
Ohio
I was bored, so here's a diagram with amps at various points in the circuit of one set of 10 fixtures...

lightingVd.gif


Since voltage drop is directly related to the current on the conductor (more current, higher VD), you can see how the VD is less than figuring the full current for the entire round-trip distance.


(Sorry about the light green hue to the image. Photobucket did that. My original image has an all white background.)
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
With the wiring done that way (hot and neutral feeding from opposite ends) the sum of the hot wire VD and the neutral wire VD will be the same for all luminaires, except for the contribution of the emergency lights.
 

kwired

Electron manager
Location
NE Nebraska
I was bored, so here's a diagram with amps at various points in the circuit of one set of 10 fixtures...

lightingVd.gif


Since voltage drop is directly related to the current on the conductor (more current, higher VD), you can see how the VD is less than figuring the full current for the entire round-trip distance.


(Sorry about the light green hue to the image. Photobucket did that. My original image has an all white background.)
Other then the emergency lights throwing balance off a little, each load has same voltage across it. The closest load to the source has more drop on the ungrounded conductor but less drop on the grounded conductor. The furthest load from the source has less drop on the ungrounded but more on the grounded - but with a net very similar as for the closest load.
 

jap

Senior Member
Occupation
Electrician
Im probably just overthinking this.

Thanks for the responses.

JAP>
 

Smart $

Esteemed Member
Location
Ohio
Other then the emergency lights throwing balance off a little, each load has same voltage across it. The closest load to the source has more drop on the ungrounded conductor but less drop on the grounded conductor. The furthest load from the source has less drop on the ungrounded but more on the grounded - but with a net very similar as for the closest load.
I was curious how the voltage drop is affected by running the hot to the far end, then turning it around to make load taps on its way back. Figured it up in Excel...

lightingVd2.gif
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
There is an old puzzle about using 2 three way switches and 4 circuit conductors (Continuous hot, switched hot, neutral, and _one_ traveler) to control lights and provide continuous power for a circuit.

In one of the switched positions it has higher voltage drop than the other.

It is probably code legal (there is a possible interpretation that makes if violate the parallel conductors requirements).
 

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