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12222016, 06:22 PM #1
wire size to compensate for voltage drop
I'm planning on running a conduit from main panel at cabin to a workshop shed about 200 feet away. I'm going to put maybe two 4' lights and maybe two outlets. Not a huge load. My thought was to run #10 wire for a single 20 amp circuit.
Opinions on if this wire size will work for anticipated load, over this distance?
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12222016, 06:35 PM #2
Re: wire size to compensate for voltage drop
I'm not going to look up how to derate the wire for that distance because I'm sure someone who can do it much faster than I will answer. #10 200feet for 20A, not sure though.
From a practical standpoint I would think about more probable load. Give a guy a workshop space and he'll find something to plug in besides just a couple lights.

12222016, 06:48 PM #3
Re: wire size to compensate for voltage drop
This is the simplest way: Voltage Drop Calculator
I plugged in single phase, copper, conduit, 120 volts, 5% max voltage drop, 200 feet, 20 amps, and the answer is:
(I know, that voltage drop calculator is 'cheating', but who better to use as 'cheater' information than Southwire?)
1 conductors per phase utilizing a #8 Copper conductor will limit the voltage drop to 4.61% or less when supplying 20.0 amps for 200 feet on a 120 volt system.
For Engineering Information Only:
40.0 Amps Rated ampacity of selected conductor
0.7421 Ohms Resistance (Ohms per 1000 feet)
0.052 Ohms Reactance (Ohms per 1000 feet)
6.0 volts maximum allowable voltage drop at 5%
5.524. Actual voltage drop loss at 4.61% for the circuit
0.9 Power Factor
**Note to User:All ampacity values are taken from the Section of 31015 of the NEC. The conductor characteristics are taken from Table 9 of the NEC. The calculations used to determine the recommended conductor sizes for branch circuits are based on 60°C ampacity ratings for circuits rated 100 amps or less or marked for use with #14 AWG  #1 AWG. Circuits rated over 100 amps or marked for conductors larger than #1 AWG are determined using 75°C ampacity ratings. Calculations to determine service and feeder conductor sizes are based on overcurrent device ratings rather than actual expected loads which are conservative and may yield oversized conductors. No calculations take into account temperature correction factors or conductor derating.
This voltage drop calculator is applicable only to NEC applications. It does not optimize conductor sizes for several different loads at various points in a circuit. The total combined load and length of the circuit must be used. Consult with an engineer if your application requires more complex engineering calculations.
Added with edit:
If you set the maximum voltage drop to 3% (where it should be set to), then this is the answer:
1 conductors per phase utilizing a #6 Copper conductor will limit the voltage drop to 2.95% or less when supplying 20.0 amps for 200 feet on a 120 volt system.
Going from a #8 to a #6 is a big difference for 2% less voltage drop, which shows how important voltage drop is.
Last edited by Jerry Peck; 12222016 at 07:35 PM. Reason: see added with edit part
Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
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12232016, 05:53 AM #4
Re: wire size to compensate for voltage drop
Thanks Jerry! I'm not sure I like the idea of the price tag for #6. This workshop shed is pretty small, so I'm not going to use any major tools. Heavy work gets done in my 1600SF basement shop at home. Use might be small bench grinder for sharpening tools, or an occasional router work. 20 amps would carry the anticipated load fine.

12232016, 08:33 AM #5
Re: wire size to compensate for voltage drop
Jack,
One way to look at it may be that you are running future power for other uses  instead of 2  20 amp 120 volt circuits or a multiwire branch circuit (which saves one conductor), use it as a feeder to run 120/240 out there.
Once out there set a small panel and you now have 120 and 240 available for future uses (still limiting it to 20 amps each leg, but without worrying about voltage drop or overheating anything.
At a 5% VD, the voltage will be 114 volts or so (depending on what the voltage is at the source).
The cost is something to consider, but so is the 200 feet you are running it.
Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
www.AskCodeMan.com

12232016, 04:54 PM #6
Re: wire size to compensate for voltage drop
I don't think the voltage drop for 2 lights and 2 outlets is a big concern. I powered the wife's greenhouse with 150 feet of 12/2 @ 15 amps. She ran lights, fan, radio maybe.
We use the black stuff in my country, NMW, which can be buried.
John Kogel, RHI, BC HI Lic #47455
www.allsafehome.ca

12232016, 05:56 PM #7
Re: wire size to compensate for voltage drop
It's not the two lights and the two outlets ... it's what may be plugged into those two outlets.
Depending on how small the bench grinder is, it may draw 15 amps or so by itself (I have a bench grinder mounted on a floor stand, it is plugged into an outlet strip which has a 15 trip, it is plugged into a 20 receptacle outlet circuit, the breaker has never tripped, but the outlet strip breaker trips if I run the grinder too long, which tells me it is likely pulling right at about 15 amps and thus takes time to trip the 15 breaker (I could use my voltage drop tester setup and measure the amp draw it has, just have never done it).
If his bench grinder is pulling 15 amps, that does not leave much for anything else. I would put a larger breaker at the source (sized to protect the wire size) and then a smaller breaker at the cabin/shed so if it trips Jack would not need to make the 200 foot trip through snow or rain just to reset the breaker, much easier to have the properly sized smaller breaker at the load end ... but this is coming from someone who has a GFCI on each bathroom receptacle even though they are only 6 feet about, and a GFCI on each kitchen receptacle even though they are less than 4 feet about ... why walk/reach farther than necessary?
Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
www.AskCodeMan.com

12232016, 08:07 PM #8
Re: wire size to compensate for voltage drop
JP, I bow to thy superiority.
Happy holidays, everyone!
John Kogel, RHI, BC HI Lic #47455
www.allsafehome.ca

12232016, 08:11 PM #9
Re: wire size to compensate for voltage drop
Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
www.AskCodeMan.com

01252017, 12:18 PM #10
Re: wire size to compensate for voltage drop
Great app from Southwire Jerry, thanks. I always look at worst case scenarios when designing stuff like this. In the U.K. where I plied my trade, we were supposed to get 240v, and frequently, especially in rural areas saw 220210v delivered. I have seen similar situations here in Canada where we are supposed to get 120v but frequently see only 110v delivered.
When you plug these figures into the app you get the following:
1 conductors per phase utilizing a #4 Copper conductor will limit the voltage drop to 2.09% or less when supplying 20.0 amps for 200 feet on a 110 volt system.
I found this interesting, because (and it quotes this in the engineering notes) a #4 Copper conductor is rated for 70 Amps, which to me seems highly over spec'd.
So I turned the problem on it's head. In the U.K. I was taught to use the wattage or VA of a device to calculate the needs of the wiring.
This is, for the uninitiated, because an appliance big or small will try to draw the maximum watts (or VoltAmps) it is designed to. Using just the amperage as a calculation can end up in different figures and either the oversizing (expensive) or undersizing (dangerous) of the conductors.
For example. Take a workshop. Two 100 watt lights, beer fridge (150W), 42" color TV (150W) and a grinder (1500) Watts = 1900 watts.
At 120v, that's around 15.8 Amps. Allowing for a 10% Voltage drop (to 108V) this would push the required amperage at the outbuilding to 17.6 Amps.
Plugging these figures into the Southwire Calculator, using the voltage drop calculation, and a single overhead copper conductor per phase, expecting 120 volts but assuming a 10% voltage drop to a worst case of 108v, over 200 feet and delivering 20Amps (because they don't make 17.6 Amp breakers) using the figures, we get:
1 conductors per phase utilizing a #10 Copper conductor will limit the voltage drop to 7.00% or less when supplying 20.0 amps for 200 feet on a 120 volt system.
As the question is about the delivery of power to a location 200 feet away, isn't this, for economy purposes, the best way to use the calculations?
The Southwire calculator appears, at first glance, to upsize the conductor requirements, but that is because my original view was to minimise voltage drop over the distance.
When I reexamined the original question, I realised I should have been looking at the requirements for power delivery and calculated that on the commonly expected voltage drop percentages. The #10 conductor is rated for 30Amps, so in a dire situation, where the expected delivered voltage at the source end of the wire have already dropped to below 120v, the extra ampacity rating of the conductors wll take up the slack of the increased amperage required by the VA draw of the appliance.
In fact I calculate that a 20Amp breaker protecting the distribution wiring at the workshop would only trip if the source voltage dropped below 104.5v with another 10% VD on the 200 feet, but the #10 wire would still be within 30% safe tolerance of it's ampacity rating.
Or am I missing the point?
p.s. As the wire is supposed to be protected at the source end, a #10 wire could have a 30Amp breaker at the source end. Doesn't this also mean that the workshop could have multiple circuits, providing support for up to 2600 Watts with a worst case scenario of a delivered voltage (to the workshop) of 104v, as long as the total draw in the workshop did not exceed 25Amps. (Main shutoff protection at the workshop)?
Last edited by Len Inkster; 01252017 at 12:30 PM. Reason: Typos and repetetive wording

01252017, 03:38 PM #11
Re: wire size to compensate for voltage drop
Len,
Reducing the voltage typically also reduces the wattage (VA) being drawn as the current typically drops too, in which case your calculations would need to be redone.
Now, if one reduced the voltage to a motor and kept the motor fully loaded, the current may actually go up, not down, which would affect the VA being used by the motor.
So the load matters as well as the load on the load (sounds strange to say it that way).
Reduce the voltage on an incandescent lamp and the current goes down, and the VA (wattage goes down, and it is not a linear relationship due to the filament and its resistance in the lamp.
Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
www.AskCodeMan.com

01262017, 03:51 AM #12
Re: wire size to compensate for voltage drop
My Bad, I should have differentiated between resistive loads and inductive loads in my calculations which made the assumption the total load in the workshop was inductive.
Of course, incandescent bulbs are resistive, and the modern TV would likely have a switch mode power supply.
I probably should have also incorporated comments regarding voltage drop and voltage sag too as it is unlikely that the decrease in Voltage would be permanent on the wire, and could be caused by the actual load applied to it......DoH! (on me)
Thanks for reminding me Jerry, it's been a while.

01262017, 04:58 AM #13
Re: wire size to compensate for voltage drop
Jack, what about 6/2 UF W/GR under ground direct burial wire.
Some saving will be gained. No conduit, less labor. ><$300.00 for 200'.
Anticipated load and workshops. Plans for a benchtop? Tools need amps.
Good safe lighting. Accessible outlets. The ability to use a variety motors.
As well, think of anticipated supply if the workshop becomes a man cave.
Keep us posted. Love to see your cabin!
Robert Young's Montreal Home Inspection Services Inc.
Call (514) 4891887 or (514) 4413732
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