More than 200ft of Romex in a branch circuit...

[EEEC]

I know that the rule of thumb is to upsize your wire gauge for every 200ft in length. I have a situation where I am wiring a space that is approximately 120 ft by 50 ft. If I run a feeder from the panel to a switch (approx 120 - 150 ft) and branch out to 12 lights, I will be using well over 200ft "worth" of wire. I've never had to take this into account before. Does the rule apply in this situation? And if so, can I run a 10-2 to the switch and then run 12-2 to each light?

Thanks in advance.

 

[electrofelon]

FWIW, I have never heard of that rule of thumb. It all depends on what the load is.

 

[kwired]

I know that the rule of thumb is to upsize your wire gauge for every 200ft in length. I have a situation where I am wiring a space that is approximately 120 ft by 50 ft. If I run a feeder from the panel to a switch (approx 120 - 150 ft) and branch out to 12 lights, I will be using well over 200ft "worth" of wire. I've never had to take this into account before. Does the rule apply in this situation? And if so, can I run a 10-2 to the switch and then run 12-2 to each light?

Thanks in advance.

First what is the load? If low enough you may not need to increase conductor at all.

Second if the lights are all in a daisy chain so to speak - you lessen the load each time you pass a light with the conductors - end of cirucit conductors aren't subjected to the same drop as the front end of the circuit is. Your circuit to the switch is going to carry all the load and sounds like a somewhat significant length though.

 

[petersonra]

there is no such rule. crunch the numbers and see what you actually have.we are not talking rocket science here.

if it is all one long chain the VD is going to be more than if you have a star configuration.

 

[EEEC]

I've downloaded the Southwire VD app and will play with the numbers. Thanks, everyone.


Is it permissible (or even logical) to feed the switch with 10-2 and feed the lights with 12-2 on a 20A circuit if need be?? I know I'm not overloading the 20A circuit with the wattage being applied...

 

[drcampbell]

... Is it permissible (or even logical) to feed the switch with 10-2 and feed the lights with 12-2 on a 20A circuit if need be? ...

The total voltage drop depends on the total circuit impedance. Upsizing only part of the circuit may reduce the total impedance to an acceptably-low level.

But don't just guess, and don't use rules of thumb. Do the Math.

 

[Ingenieur]

just rough numbers

v drop (at 3% and 16 A) = 0.03 x 120 = 2 x D x Z x I
3.6 = 2 x D x 1.7/1000 x 16
D = (1000 x 3.6)/(2 x 1.7 x 16) = 66'
max one-way D = 66'


D = (5 x % x V)/(Z x I), use 5.77 for 3 ph
% = desire V drop, ie, 3%
V = system voltage
I = load current
Z = NEC table Z per 1000'

assuming 3%
D = (15 x V)/(Z x I), use 17.3 for 3 ph
D = max one way distance for 3% drop

assume desired V drop % = 3
Z = 1.7 Ohm/1000', #12
V = 120/1
I = 16 A
D = (5 x 3 x 120)/(1.7 x 16) = 66' one way

assume desired V drop % = 3
Z = 0.16 Ohm/1000', #1
V = 480/3
I = 100 A
D = (17.3 x 480)/(0.16 x 100) = 519'

you can make an excel spreadsheet for each cond. size
assume 80% loading x ampacity for I
this will give the max one-way distance for 3% drop for 80% x ampacity
you can do it for pvc/steel/alum, although usually not much difference
it can tell you if you need to look at it more closely

you can adjust for load, just use actual load I instead of 80% x ampacity
if your table uses 80% adjust as follows
adjusted one-way max D = (0.8 x ampacity)/(act I) x D
from above, if load is only 80 A but obviously you would downsize cond. to #3)
adjusted D = 519 x (100/80) = 649'

 

[kwired]

I've downloaded the Southwire VD app and will play with the numbers. Thanks, everyone.


Is it permissible (or even logical) to feed the switch with 10-2 and feed the lights with 12-2 on a 20A circuit if need be?? I know I'm not overloading the 20A circuit with the wattage being applied...

Again, what is the load? Just because you have a 20 amp circuit doesn't mean it has to supply 20 amps of load. If you only have 3 or 4 amps of load you may not even have a voltage drop issue worth doing anything about.

 

[petersonra]

I've downloaded the Southwire VD app and will play with the numbers. Thanks, everyone.


Is it permissible (or even logical) to feed the switch with 10-2 and feed the lights with 12-2 on a 20A circuit if need be?? I know I'm not overloading the 20A circuit with the wattage being applied...

it is permissible. it might even make sense to do so if after your crunch the numbers it turns out that it solves whatever problem you think you have.

 

[EEEC]

There are twelve 100W lights, 10A of load on one circuit for the high bays. And then I have 480W on the other circuit or 4A for the strip lights. It's the distances that have me concerned.

 

[kingpb]

I've downloaded the Southwire VD app and will play with the numbers. Thanks, everyone.


Is it permissible (or even logical) to feed the switch with 10-2 and feed the lights with 12-2 on a 20A circuit if need be?? I know I'm not overloading the 20A circuit with the wattage being applied...

I don't think the SW App will help. My recollection is it does not cover romex wiring applications.

You can get the R value from Table 8 (NEC) for #12, #10, and #8 since Romex is single strand. But, if you have to run 120-150ft from panel just to get to the location; you may need to consider running #8 to the first box/j-box; then drop down in size. #8 can go approx 150' with 3% VD at a load of 16A. So the recommendation is that the distance to the farthest light cannot then exceed another 2%; for a total 5%. Don't forget the load on each light string will be less than 16A, so #12 may be sufficient.

 

[drcampbell]

What kind of lights? If they have electronic "ballasts" with a wide input voltage tolerance, they might provide good lighting even with more than 5% voltage drop.

But that's not the whole story. If the cost of electricity is high, it might be economical to upsize conductors and minimize voltage drop.

 

[petersonra]

There are twelve 100W lights, 10A of load on one circuit for the high bays. And then I have 480W on the other circuit or 4A for the strip lights. It's the distances that have me concerned.

you are going to have to run the numbers and see what they say.

in the time you have spent typing about it you could have done the figuring and know the actual answer.

 

[Marshmo]

There are twelve 100W lights, 10A of load on one circuit for the high bays. And then I have 480W on the other circuit or 4A for the strip lights. It's the distances that have me concerned.

The others are telling you that your concern for the distance should be proportional to the load on the circuit.

 

[Ingenieur]

There are twelve 100W lights, 10A of load on one circuit for the high bays. And then I have 480W on the other circuit or 4A for the strip lights. It's the distances that have me concerned.

the 10 A ckt using #10
D = (15 x 120)/(1.1 x 10) = 163' max one-way for 3% drop

the 4 A ckt using #12
D = (15 x 120)/(1.7 x 4) = 265' max one-way for 3% drop

or correcting from 16 A to 4 A
from my above post of 66' for 16 A
corrected D = 16/4 x 66 = 264', same as above

 

[sameguy]

Run a sub panel then feed off of that, no other loads in that area?

 

[Cow]

Sometimes it's simpler and cheaper to install a RIB or small contactor, then the length of your travelers aren't going to matter if they're only firing a coil.

 

[JFletcher]

Yes, you can run 10 2 to the switch box. The hundred foot rule isn't a rule so much as a rough guide on up sizing conductors based on voltage drop. If you have a 10 amp load on a 120V circuit, I think that you will find if you do voltage drop calculations that 14 gauge is good for a hundred feet 12 for 200 feet 10 for 300 Etc.

Also some breaker manufacturers recommend the maximum circuit length of 250 ft if GFCI or afci Breakers are used.

 

[ramsy]

..will be using well over 200ft "worth" of wire..

If your High-Bay lighting is selected for line voltage @ 3% VD, Pf=0.85 per NEC Tbl-9,
10A on 12-3 Romex is good for
237ft @ 208vac,
274ft @ 240vac,
547ft @ 277vac.

Even at continuous use, cable termination temperature is below 40°C.
You could bundle four 12-3 cables, with 12ccc's and still keep termination temp below 60°C.

 

[ramsy]

Sometimes it's simpler and cheaper to install a RIB or small contactor, then the length of your travelers aren't going to matter if they're only firing a coil.

Wow - Just when I thought I understood 3 ways.

Do they make a wall switch relay, or does it go in a J-box above the ceiling?