voltage drop

normbac

Senior Member
wanted to get some info on how to calculate run for volt drop. It would go from panel 600 ft then branch off two directions 800 ft. each direction this is for lighting 22 signs 13 watt fluorescent fixture per sign also they want a receptacle at each sign for holiday rope lighting and they do not have a total wattage for them. I am not sure how to calculate this since I assume after a certain distance I can lower wire size. Not sure where along run I can step down wire size or how to figure this out. I would have to let customer know load to be total 20 amps. Any help would be appreciated TIA
 

JDBrown

Senior Member
Location
California
Let me try and clarify a few things first.
  1. What are your voltage and phase? If it's all 120V, single-phase (I'm assuming this based on the mention of a receptacle and rope lighting), you're going to need some BIG wire to travel that distance.
  2. What's your acceptable level of voltage drop? Are you going by the "standard" 5% from the POCO meter to the last outlet, or using some other standard?
  3. Are there 22 signs at the end of each 800' run, or are there 11 signs at the end of each 800' run for a total of 22 signs?
  4. How many circuits do you plan on using?
  5. I don't have any experience with holiday rope lighting, but my experience with the "regular" strings of Christmas lights is that residential grade is usually fused at 3 amps and commercial grade is usually fused at 5 amps. Actual current will, of course, depend on how many they string together.
...
I assume after a certain distance I can lower wire size. Not sure where along run I can step down wire size or how to figure this out.
That's a design decision that will depend on your specific layout. However, I have found that in these types of installations it's often best to just use your largest wire size from the panelboard all the way to the last outlet and pigtail off of it with smaller wire to any devices that have terminations too small for the large wire.
I would have to let customer know load to be total 20 amps. Any help would be appreciated TIA
I don't understand what you're saying here. Do you know the load is 20 amps? Or are you saying you're going to use 20 amp circuits?
 

normbac

Senior Member
120 single phase signs are evenly spaced along each 800 ft run as far as telling them 20 amp max its to cover myself so they understand the amount of holiday lights they can use since they are unsure of what they want. The signs are back to back so basically five locations one direction and six the other evenly spaced. The sign load is 22x13 watt but since they say they just want to plug in some rope light without knowing watt totals I have to give them a minimum amp unles they want to pay more.
 

GoldDigger

Moderator
Staff member
120 single phase signs are evenly spaced along each 800 ft run as far as telling them 20 amp max its to cover myself so they understand the amount of holiday lights they can use since they are unsure of what they want. The signs are back to back so basically five locations one direction and six the other evenly spaced. The sign load is 22x13 watt but since they say they just want to plug in some rope light without knowing watt totals I have to give them a minimum amp unles they want to pay more.
You have to protect all of the wire by the circuit breaker or other OCPD, so even at the far end you will have to use wire suitable for 20A.
But if you want to get the best effect from oversizing some of the wiring to decrease the voltage drop you will be best off to upsize the wires closest to the source rather than equally all the way down the line.
If it were not for the decreasing load over distance, upsizing one section would not be more valuable than upsizing another since the current to the farthest load will pass through all of the wires.
But in your situation, the fact that the central wires will be carrying more current makes them more important for VD reduction.

To actually calculate the results, do the VD calculation for each segment separately, with your choice of wire size and using the total current for that segment. Then add up all of the drops out to a particular load to find out what the voltage at that point will be.
 

JDBrown

Senior Member
Location
California
So, is the sketch below accurate (more or less)? How you figure your voltage drop really depends on how you decide to circuit the system. And that will depend on how you decide to figure your loads.
signs.jpg
The bare minimum Code requirement is going to be 13W per sign plus 180VA per receptacle. You'll need to verify the input power of your signs, as it's possible (even likely) that they use 13W lamps, but are rated for a bit higher input power due to ballast losses, power factor, etc. What you really need is the input amps or input VA of your signs (having one will allow you to calculate the other).

For the moment, though, let's assume 13W per sign at 100% power factor. You have 22 signs, and 22 duplex receptacles (1 at each sign location). (22 x 13) + (22 x 180) = 4246 VA total for your system. This is the minimum Code requirement, and probably about the load you can expect to have if they only plug in one rope light at each receptacle.

I don't think it would be a stretch to say this is going to be a continuous load (on for more than 3 hours at a time), so the maximum load you can put on your 20 amp circuit would be 16 amps, or 1920 VA. Knowing this, you will need a minimum of three 20 amp circuits. However, if you install all this on three 20 amp circuits, your customer is likely to end up tripping breakers if they plug in a lot of rope lights. I'm going to guess that they will want to plug a rope light into each receptacle, and they will want to daisy-chain together as many rope lights as they can. Based on my experience with regular ol' Christmas lights, that would mean 5 amps for each string or 10 amps at each duplex receptacle.

Unfortunately, since it's a continuous load, you would not be able to put two receptacles @10 amps each on a 20 amp circuit, so you would need a separate circuit for each receptacle. That seems a bit of overkill to me, so maybe you could assume they won't max out every string of rope lights and you'll be safe putting two receptacles to a circuit. Now you've got 11 circuits for receptacles plus one more circuit for the signs. The sign load is small enough that you'd probably be fine using one circuit for each location (2 duplex receptacles and 2 signs on one 20 amp circuit), cutting the total number of circuits to 10.

As you can see, you've still got some design decisions to make before it's time to start worrying about things like voltage drop and conductor sizes.
 

normbac

Senior Member
When I calculate voltage drop at last sign I get wire sizes as large as 4/0 al is this correct?
[
QUOTE=JDBrown;1512722]So, is the sketch below accurate (more or less)? How you figure your voltage drop really depends on how you decide to circuit the system. And that will depend on how you decide to figure your loads.
View attachment 9100
The bare minimum Code requirement is going to be 13W per sign plus 180VA per receptacle. You'll need to verify the input power of your signs, as it's possible (even likely) that they use 13W lamps, but are rated for a bit higher input power due to ballast losses, power factor, etc. What you really need is the input amps or input VA of your signs (having one will allow you to calculate the other).

For the moment, though, let's assume 13W per sign at 100% power factor. You have 22 signs, and 22 duplex receptacles (1 at each sign location). (22 x 13) + (22 x 180) = 4246 VA total for your system. This is the minimum Code requirement, and probably about the load you can expect to have if they only plug in one rope light at each receptacle.

I don't think it would be a stretch to say this is going to be a continuous load (on for more than 3 hours at a time), so the maximum load you can put on your 20 amp circuit would be 16 amps, or 1920 VA. Knowing this, you will need a minimum of three 20 amp circuits. However, if you install all this on three 20 amp circuits, your customer is likely to end up tripping breakers if they plug in a lot of rope lights. I'm going to guess that they will want to plug a rope light into each receptacle, and they will want to daisy-chain together as many rope lights as they can. Based on my experience with regular ol' Christmas lights, that would mean 5 amps for each string or 10 amps at each duplex receptacle.

Unfortunately, since it's a continuous load, you would not be able to put two receptacles @10 amps each on a 20 amp circuit, so you would need a separate circuit for each receptacle. That seems a bit of overkill to me, so maybe you could assume they won't max out every string of rope lights and you'll be safe putting two receptacles to a circuit. Now you've got 11 circuits for receptacles plus one more circuit for the signs. The sign load is small enough that you'd probably be fine using one circuit for each location (2 duplex receptacles and 2 signs on one 20 amp circuit), cutting the total number of circuits to 10.

As you can see, you've still got some design decisions to make before it's time to start worrying about things like voltage drop and conductor sizes.[/QUOTE]
 

JDBrown

Senior Member
Location
California
When I calculate voltage drop at last sign I get wire sizes as large as 4/0 al is this correct?
I haven't done the calculations, but I wouldn't be surprised at all. 1400 feet is a LONG way, especially at 120 volts. It's a long distance at 480 volts, 3 phase.

I don't know what your situation is with your customer, but now might be the time to explain to the customer that the capability for rope lighting is going to add a LOT of cost to the project. Or, if they're going to just pay whatever you say it's going to cost, give them a full capacity system and cash the check. :D
 

normbac

Senior Member
I agree
I just wanted to make sure I was looking at this correctly. Thanks for all replies:D:D

I haven't done the calculations, but I wouldn't be surprised at all. 1400 feet is a LONG way, especially at 120 volts. It's a long distance at 480 volts, 3 phase.

I don't know what your situation is with your customer, but now might be the time to explain to the customer that the capability for rope lighting is going to add a LOT of cost to the project. Or, if they're going to just pay whatever you say it's going to cost, give them a full capacity system and cash the check. :D
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
This seems like a very odd situation. Can you elaborate more on what is actually going on here?

If you only have 120V available get some 120:600V xfmrs for the feed and put a 600:120V xfmr at each drop maybe.

That would get the current down by a factor of 5, which would go a long way toward reducing the size of the conductors.

How were you planing to run the feeders? Conduit?
 

kingpb

Senior Member
not available can get 208 3 phase but its 1000 ft away
Seems to me you need 4 x 20A 120V ckts; or 2 ckts for each 800' direction. One for the signs, and one for the receptacles.

Hopefully, if there is 120V, it would be possible to get a 2-pole to give you 240V.

If so, set a 240-480V, 5KVA transformer at the panel, then run 480V circuit out to the 600ft location - you will only need (2) #10AWG and a ground.

At the 600ft mark, place a 5KVA, 408-120/240V mini power center. This will give you the 4 x 120V ckts you will need. By doing this you now can utilize the taps on the transformer to adjust your voltage. Thus you can actually take a 6% drop on the LV side because you can start with a voltage that is at least 2.5% above nominal. (Utilize the full 5% tap) This reduces the cable on the branch circuits.

For the receptacle circuit it will require a #4 AWG, and for the signs it will require a #10. Set a tap box/handhole at each location and use #12 to tap to each receptacle and sign.

This setup will allow approx. (on average) 400ft of 120V, LED rope lights at each outlet, without overloading the 20A circuit.
 

normbac

Senior Member
nursery has signs along .3 mile highway
This seems like a very odd situation. Can you elaborate more on what is actually going on here?

If you only have 120V available get some 120:600V xfmrs for the feed and put a 600:120V xfmr at each drop maybe.

That would get the current down by a factor of 5, which would go a long way toward reducing the size of the conductors.

How were you planing to run the feeders? Conduit?
yes
 
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