# Sizing lighting branch circuit



## nitro71 (Sep 17, 2009)

I've got some 1000w metal halide pole lights and I'm working on sizing the branch circuit conductors. 210.23(B) says I can use a 30amp circuit if the lamp holder is heavy duty. I'm considering options and was thinking about 30a 240v circuits.

Cooper catalog number is: MHNK-S76-1000-MT

I can't find much on the lights to figure out if they have a heavy duty lamp holder.


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## Bob Badger (Apr 19, 2009)

1000 watt fixtures have heavy duty lamp holders.

Just remember that voltage drop becomes more of an issue as you max the circuits out.


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## MDShunk (Jan 7, 2007)

If you're using very many 1000 watt-ers, you might want to seriously think about getting an appropriately sized 480 secondary transformer and a small 480 panel. It might not be a cost savings over the large copper you'd otherwise need, but one thing it would also do is guarantee that this would be your installation to maintain, since a handyman is not likely to fuss with 480.


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## nitro71 (Sep 17, 2009)

Thanks Bob! I'm doing some pro-bono work for non-profit so using some lights they have. Here's the layout, 6 poles, 8 lights total, 1000w per light. Distance to last light is about 350'. At 240 volts the lights draw 4.7 amps. If I run 30 amp #10 circuits I should be able to put four of those lights on one circuit at 18.8 amps. Max on my 30 amp circuit would be 22.5 amps at continuous load. Southwires voltage drop calculator wants load at the very end of the run. Do you figure load at the end to be 4.7 amps then at the next pole add another 4.7 and so on?


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## nitro71 (Sep 17, 2009)

MD, the service size is a 200 amp 120/240 single phase service. Looking at ten 1000w lights total load. Eight on the run around the field.


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## MDShunk (Jan 7, 2007)

A voltage drop calculation for loads at various distance, that drop off as the circuit passes by is a bit trickier.

For instance, if you have 4 poles, one at 100 feet, one at 200 feet, one at 300 feet, and one at 400 feet, you need to do 4 calculations.

You do a voltage drop calc for the load of all 4 poles at 100 feet, with a starting voltage of 240 and get the resulting "remaining voltage". 

You do another calculation for three of the poles at 200 feet, using that "remaining voltage" from your previous calculation as the beginning voltage in your calculation, and get your "remaining voltage".

Etc, etc. 

That will give you the actual voltage observed by the last pole. 

It would be pretty normal to have the pretty heavy conductors to the first pole, then have conductors drop in size as the loads drop off.


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## nitro71 (Sep 17, 2009)

I just did a quick rework on my voltage drop with Southwires tool. 4 lights, 4.7 amps each at 240v. 

185' 18.8 amps 3.1% drop with #10
235' 14.1 amps 4.8% drop with #12
260' 9.4 amps 3.5% drop with #12
320' 4.7 amps 3.3% drop with #14

Am I correct in thinking I can take #10 to my first light then #12 to the rest down the string? Appreciate the help!


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## BuzzKill (Oct 27, 2008)

nitro71 said:


> I just did a quick rework on my voltage drop with Southwires tool. 4 lights, 4.7 amps each at 240v.
> 
> 185' 18.8 amps 3.1% drop with #10
> 235' 14.1 amps 4.8% drop with #12
> ...


that second one, I'd run #10 also, that's darn near the 5% limit; actually, you might contact manufacturer to see anout how they handle those VD's in the 3% range.


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## MDShunk (Jan 7, 2007)

nitro71 said:


> I just did a quick rework on my voltage drop with Southwires tool. 4 lights, 4.7 amps each at 240v.
> 
> 185' 18.8 amps 3.1% drop with #10
> 235' 14.1 amps 4.8% drop with #12
> ...


No. Your conductors to the first light at 185 feet is going to have the load of all 8 fixtures on them at that point. You're going to need at least #6 to that first light.


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## nitro71 (Sep 17, 2009)

And I got caught up with the tool. If I'm running a 30 amp circuit I would need to take #10 all the way anyways.


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## nitro71 (Sep 17, 2009)

MDShunk said:


> No. Your conductors to the first light at 185 feet is going to have the load of all 8 fixtures on them at that point. You're going to need at least #6 to that first light.


I was breaking it out to just the four fixtures on the backside of the field for that set of calcs.


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## Bob Badger (Apr 19, 2009)

nitro71 said:


> 185' 18.8 amps 3.1% drop with #10
> 235' 14.1 amps 4.8% drop with #12
> 260' 9.4 amps 3.5% drop with #12
> 320' 4.7 amps 3.3% drop with #14


You want almost 15% drop at the last light?

IMO you should keep each section under 1.25% drop so at the end you would be at 5% drop.


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## nitro71 (Sep 17, 2009)

I think I'll give Cooper a ring on Monday to see what the max voltage drop on those lights would be.


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## MDShunk (Jan 7, 2007)

If you want to stay at 3% or under at the last light (at 320 feet), you need:

1.7% drop to the first light at 185 feet
2.2% drop by the time you hit the second pole at 235 feet
2.4% drop by the time you hit the third pole at 260 feet
3% by the time you hit the last pole at 320 feet

Size your conductors accordingly.


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## MDShunk (Jan 7, 2007)

Here, I did the calc for you:

1.7% drop to the first light at 185 feet, 37.6 amp load, use #1 copper from the panel to the first pole, giving an ending voltage of 236 volts.

2.2% drop by the time you hit the second pole at 235 feet, an added distance of 50 feet, and a load of 18.8 amps remaining on that conductor, use #12 copper between the first and second pole, giving an ending voltage of 232 volts. This is below the NEC minimum for a circuit with overcurrent protection sized to protect a 37.6 amp load.

2.4% drop by the time you hit the third pole at 260 feet, an added distance of 25 feet, and a load of 14.1 amps remaining on those conductors, use #14 copper between the second and third poles, giving an ending voltage of 230 volts.This is below the NEC minimum for a circuit with overcurrent protection sized to protect a 37.6 amp load.

3% by the time you hit the last pole at 320 feet, an added distance of 60 feet, and a load of 4.7 amps remaining on those conductors, use #14 conductors between the third pole and the final pole, giving an ending voltage of 228 volts. This is below the NEC minimum for a circuit with overcurrent protection sized to protect a 37.6 amp load.


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## nitro71 (Sep 17, 2009)

I think I see now. You need to drop the voltage as you go based on how much it dropped on the last pole. Thanks for the help on this!


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## MDShunk (Jan 7, 2007)

nitro71 said:


> I think I see now. You need to drop the voltage as you go based on how much it dropped on the last pole. Thanks for the help on this!


You still have an issue, in that you have nearly 40 amps of load, and you plan on running a 30 amp circuit.


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## nitro71 (Sep 17, 2009)

I ran numbers again at Southwire. One circuit, four lights. 240volt. Here's what I got:

185' 18.8 amps 1.25% drop #4
235' 14.1 amps 1.25% drop #8
260' 9.4 amps 1.25% drop #12
320' 4.7 amps 1.25% drop #14

I wouldn't use #12 or #14 on the last poles but was showing what Southwire calculated with those numbers.

MD did you do your calc based on eight lights or on four?


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## MDShunk (Jan 7, 2007)

nitro71 said:


> MD did you do your calc based on eight lights or on four?


I guess 8. I wasn't sure how you were dividing your circuits up, since it looks like you're running two circuits, and I didn't realize it. 

You've got the technique down pat now, so it seems. Voltage drop calcs for streetlights and parking lot lights are tricky and time consuming.


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## nitro71 (Sep 17, 2009)

Glad I didn't bid this : ) I'm learning a lot on this pro-bono project. They won't be happy to hear this but can't have champagne tastes on a beer budget.


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## Bob Badger (Apr 19, 2009)

MDShunk said:


> Voltage drop calcs for streetlights and parking lot lights are tricky and time consuming.


But can be well worth the time invested when running copper. I know your figures where based on 3% which of course is what is suggested I might let the last pole or two slide back to 5% VD maybe a bit more. 

*nitro71* do not overlook 250.122(B), that will require that your EGC remains the same size as the circuit condutors.

If you use 6 AWG from the panel to the fist pole the EGC will have to be 6 AWG as well.


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## MDShunk (Jan 7, 2007)

nitro71 said:


> Glad I didn't bid this : ) I'm learning a lot on this pro-bono project. They won't be happy to hear this but can't have champagne tastes on a beer budget.


Hey, you can run #10 the whole way, but no one will be too happy with the result.


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## nitro71 (Sep 17, 2009)

In general is it more cost effective to run the larger conductors and load up the circuits, say 50 amp circuits or to lower the load and run more small circuits?


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## MDShunk (Jan 7, 2007)

nitro71 said:


> In general is it more cost effective to run the larger conductors and load up the circuits, say 50 amp circuits or to lower the load and run more small circuits?


You really need to run the clacs for each job, considering the distances and pole locations involved. Every job is different, in that regard. If each pole gets further and further from the panel, larger circuits make more sense. If the poles are located such that you can spur out from the panel to hit each group, then several smaller circuits might make more sense. Using the highest voltage possible for the ballasts can often make even more sense.


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## nitro71 (Sep 17, 2009)

MDShunk said:


> Hey, you can run #10 the whole way, but no one will be too happy with the result.


I can see that the lights plain out won't work if that happens! LOL!



Bob Badger said:


> But can be well worth the time invested when running copper. I know your figures where based on 3% which of course is what is suggested I might let the last pole or two slide back to 5% VD maybe a bit more.
> 
> nitro71 do not overlook 250.122(B), that will require that your EGC remains the same size as the circuit condutors.
> 
> If you use 6 AWG from the panel to the fist pole the EGC will have to be 6 AWG as well.


Thanks for the heads up on the EGC. I learned a lot today about voltage drop. Thanks for all the help!


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## Bob Badger (Apr 19, 2009)

I agree with Marcs points for sure.

My own personal feeling is to break the circuits up and if I could it would be in an alternating fashion. This way if there is a short ..... and at some point there will be ..... you will still have some working lights.

Putting a public parking lot into total darkness is bad situation.


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## nitro71 (Sep 17, 2009)

Bob Badger said:


> I agree with Marcs points for sure.
> 
> My own personal feeling is to break the circuits up and if I could it would be in an alternating fashion. This way if there is a short ..... and at some point there will be ..... you will still have some working lights.
> 
> Putting a public parking lot into total darkness is bad situation.


Luckily this is a radio control car clubs field lighting. Of course I don't think they would be happy if they were having a race and the lights went out. They have some big race coming up next month. 250 racers or something like that. I think their schedule is a little tight but going to help them as much as possible.


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## MDShunk (Jan 7, 2007)

nitro71 said:


> Luckily this is a radio control car clubs field lighting. Of course I don't think they would be happy if they were having a race and the lights went out. They have some big race coming up next month. 250 racers or something like that. I think their schedule is a little tight but going to help them as much as possible.


They might want to just think about renting lightall's for special night events and forgo the entire lighting project.


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## nitro71 (Sep 17, 2009)

Wonder what the cost of renting lights is? They have free bases(with broken conduits, can be fixed), have the poles that came off them, heavy duty steel ones, and 16-20 of those 1000w lights. I think they are planning on renting lights if these don't get finished. 

They're also building a drivers stand. It's a large affair. I'm not working at the moment so figured I'd help them out. They are the permit holder.


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## MDShunk (Jan 7, 2007)

nitro71 said:


> Wonder what the cost of renting lights is? They have free bases(with broken conduits, can be fixed), have the poles that came off them, heavy duty steel ones, and 16-20 of those 1000w lights. I think they are planning on renting lights if these don't get finished.
> 
> They're also building a drivers stand. It's a large affair. I'm not working at the moment so figured I'd help them out. They are the permit holder.


I think I pay around 100 bucks a day for a light tower. Been a good while since I had to rent one.


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## nitro71 (Sep 17, 2009)

I think it makes financial sense for them to install the lights they have. Say they rented six of those lights for one event and they run that event every year. Probably won't take them long to recoup the cost of the materials.


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