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# Voltage Drop Calculations for Feeder and Branch Circuit Combination

## Voltage Drop Calculations for Feeder and Branch Circuit Combination

#1
01-13-14, 02:20 PM
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Voltage Drop Calculations for Feeder and Branch Circuit Combination

Hi,

I am interested in seeing what I would have to do to comply with the NEC recommendation to have no more than 3% voltage drop for the farthest outlet on a branch circuit and no more than 5% drop for the combined feeder and branch circuit.

The installation would be a 50A sub panel located at ~75-85' (worst case scenario of 85') from the main panel in an attic. I was hoping that 6/3 copper NM-B would suffice to connect the main to the sub panel.

The question is whether I should use 120V or 240V in the calculator to estimate the drop from the main to the sub? If I were running 240V circuits in the branch circuit, it makes perfect sense to use 240V when estimating the feeder and branch drop.

If I were running 120V circuits in the branch circuit, it seems to make more sense to use 120V when estimating the branch AND feeder drops, as 120V is using only one of the legs at a time. Am I thinking about this correctly? If you have a good reference to explain this to me, I would appreciate it.

#2
01-13-14, 03:29 PM
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#3
01-13-14, 03:39 PM
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Does this help: Voltage Drop Calculator

#4
01-13-14, 04:46 PM
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Hi guys, thanks for the posts. mossman, i have been using that calculator, but i still don't know whether to use 120 or 240 for the main to sub panel. I am starting to think it is an issue of how balanced the sub panel legs are. The unbalanced component is contributing to 120V voltage drop I think, and the balanced component is contributing to 240V voltage drop, so I should maybe just design for worst case scenario unbalanced drop.

Pulpo, that's a very nice page.

#5
01-13-14, 05:21 PM
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The worst case 120 voltage drop occurs with all of the load on one leg and no load on the other leg. Use the 120 vlt calculations.

If you are running a 120/240 volt feed, then after you have assigned the various 120 volt loads to one leg or the other, you might find that the total number of amperes on each leg is smaller than you expected and the voltage drop will be correspondingly less.

If you are willing to stay within 5% voltage drop most of the time as opposed to allof the time, then use 6% as the worst case voltage drop nd run the calculations that way.

Last edited by AllanJ; 01-13-14 at 05:36 PM.
#6
01-13-14, 05:43 PM
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If you're installing a 50A subpanel and only connecting 120V circuits, the calculator says to use #4 aluminum, which has an ampacity of 55A and resistance of 0.4848 ohms per 1,000 ft. At 85 feet and full load of 50A, that is only 0.082 ohms total (round trip). This equals a 4.10V drop for a purely resistive load, which is equivalent to 3.4% (of 120V). The voltage drop on the cable is independent of the source voltage: Vdrop = I (current on wire) x 2R (total resistance of wire--round trip). Regardless if you use 120V or 240V for your calculation, as long as the highest Vdrop/Vsource is less than 5%, you're good to go. The calculator I linked you too considers reactive loads as well with a power factor of 0.9, so the Vdrop numbers will be a bit lower than the purely resistive ones I calculated. Power factor of 1 (purely resistive) gives you worst case.

Last edited by mossman; 01-13-14 at 07:55 PM.
#7
01-13-14, 06:18 PM
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AllanJ, I just thought about the major users on my subpanel, and I think the maximum sustained unbalanced amperage would be closer to 22-30 amps. I used the calculator at
Wire Size Calculator
to find the maximum unbalanced amperage allowed for 6 gauge wire at 85' to get a 2% drop on the feeder (120V source) and it is 33 amps. that allows me a full 3% drop on the branch circuit, which will be fine. 33 amps is a lot of wattage for my situation. so, i think i will be good with 6 gauge copper wire for the 85' feeder. I also agree that "most of the time" may be key. for example, if a hair dryer is used for a few minutes while a baseboard heater is on for one leg, it will go over, but the hair dryer only runs for a few minutes, so who cares!

#8
01-13-14, 06:22 PM
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mossman, i don't think the voltage drop on cable is independent of source voltage. if you are using 240V, the drop should be half. the aluminum calc you did appears to be correct for what i am seeing as well. i'd prefer 2% drop to the subpanel though, so i can count on 3% drop on the branch circuits. i don't think using the full 50 amps for the calc makes sense in practice for me though. i'm not going to have a full 50 amps on one leg except in rare cases for a very short time (a few minutes)

#9
01-13-14, 06:56 PM
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mossman, i don't think the voltage drop on cable is independent of source voltage. if you are using 240V, the drop should be half.
Sure it is. The resistance of a given wire does not change based on the voltage applied. It is (essentially) a constant. #3 aluminum is 0.3897 per 1,000 ft and #4 aluminum is 0.4848 per 1,000 ft. The amount of voltage drop depends on how much current is flowing through the wire. No current = no voltage drop, maximum current (50A in your case) = maximum voltage drop. Now if you're talking about supplying a fixed load with a given amount of power using 120V vs. 240V, the current will be half (and Vdrop would be half) when using 240V, but that doesn't apply to what you are doing. You are trying to ensure that you use the proper size wire to feed your 50A subpanel without exceeding a 5% voltage drop. #4 aluminum supplying a 50A panel with 120V will accomplish that.

i don't think using the full 50 amps for the calc makes sense in practice for me though.
I don't know what the NEC says, but I'm assuming you should be taking the highest possible current draw when making your calculations. For instance, I'm sure the cable feeding your main panel is rated to handle the full 100A or 200A (whatever your main breaker is). Anything less would violate fundamental rules and would be a fire hazard.

A different way of looking at it (given your 2% requirement):

2% of 120 = 2.4V max drop permitted
A 2.4V drop at 50A would give you a maximum allowable resistance of 0.048 ohms over 170 feet (85 x 2). In this case, you would need to use a cable that has under 0.282 ohms per 1,000 feet.

Last edited by mossman; 01-13-14 at 07:32 PM.
#10
01-13-14, 07:40 PM
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so who cares!
The inspector cares.

If you are installing a 50A subpanel with a 50A main breaker in it, you need a cable that is of sufficient size to handle that amount of current (regardless if you will be using all of it) while keeping the voltage drop under 2%. I don't believe #6 copper is going to get you there. Maybe I'm missing something. I'll let someone else chime in that is more experienced with this sort of thing.

#11
01-13-14, 08:01 PM
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Trust me, the inspector isn't going to care. They don't even consider voltage drop around here. I've had another job inspected and a friend just had something inspected and they didn't even consider it. However, I want to do it the right way, regardless of what the inspector says. i can see your points. the safest is to engineer it to assume a 120V load for 85'.

Also, the code doesn't require the voltage drop percentages, so it's probably up to the inspector to decide that. I'm sure they'd just look to see if the cable is rated to carry 50 amps, and 6 gauge is rated to carry 55 amps (for NM-B at least) in the table.

I guess the bottom line is that if I want only 2.4 V dropped (2% of 120V) from the main to the sub panel, I need 4 gauge copper or 2 gauge aluminum.

#12
02-03-14, 05:31 PM
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I just wanted to post an update for people reading this in the future. I decided to go up another route, cutting my run down to 40-45'. I tried a test piece of SER, Al, 4-gague. It tore on the side or an existing tear (a friend gave it to me to try after using part of it) tore wider by rubbing on some wood or a nail in the wall or something. The insulation is really weak on SER. I had 1.5" holes in the wall, and it feeds fine, but it did tear. I think I am going to use Al Flexible Metal Conduit, as it is VERY flexible and will offer much better protection for the wires.

Just some info. for thought. Oh, I also found out that you can by a tool that allows drilling horizontally in existing walls, so you don't have to have super long cable runs in the walls when you can't go from directly below the wall. You can use a DeWalt right angle drill attachment device OR you can use a flexible bit extension, OR you can use a flexible auger bit. Any of them will allow you to drill through an existing wall horizontally, BUT you WILL have to put in a receptacle b/w every other stud gap using this method.

#13
02-03-14, 05:54 PM
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Thanks for the update.

BUT you WILL have to put in a receptacle b/w every other stud gap using this method.
I don't understand. The flexible bits we use are long enough to get us through 3 studs or joists if they're 16 " o.c., or 2 if they're 24" o.c. - and we only set outlets for receptacles or other devices or fixtures where they're called for. Any other openings get patched.

#14
02-03-14, 06:10 PM
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outlets vs patch

Of course, if you don't mind patching or buying a long auger bit, you can make it work I'm sure. I'd rather just put in more receptacles so I don't have to patch the plaster, and I don't want to buy an expensive auger bit. My right angle drill attachment should do fine! I guess it comes down to personal preference. My walls are short enough that putting receptacles every other stud is fine. I have to go up and over to get to the perpendicular wall anyway, so I'm not gaining by going longer.

#15
02-03-14, 06:32 PM
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OR you can use a flexible auger bit
if you don't mind patching or buying a long auger bit, you can make it work
Did you not mean to refer to a flexible auger bit? That's what I was referring to.

They make them longer, but mine are 54" long. I buy the ones from Milwaukee because they're the only ones I've found that have a wire-pulling hole in each end.

#16
02-03-14, 07:25 PM
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I just haven't used one, so I don't know what you can really do with it, actually. the angle drill attachment and flexible bit extender worked pretty well, but they are short of course. I might try the auger bit and see how easy it is to use.

#17
02-03-14, 07:48 PM
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Nashkat1, how long do those milwaukee auger bits last? do they break very easily or go dull, i'd think not for doing one house worth of holes. i can get one for just over \$50 online. that might be really handy. Can I use it by myself, or do I need one person to hold it on track while I drill?

Can i ask you about conduit as well? I was thinking of running about 40' of 1" flexible aluminum conduit up to my subpanel in my attic. Could I pull 3 4-gauge THHN wires and 1 8-gauge THHN wire through it without too much pain? I'd need to go up around 20' from my main panel in an exterior wall and then over 20' at 90 degrees across my attic. I can then just bend the conduit into place to my subpanel if I can get it that far. The vertical wall has a few jogs to make as I weave it through 3 holes that are slightly offset.

#18
02-03-14, 08:01 PM
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I was thinking of running about 40' of 1" flexible aluminum conduit up to my subpanel in my attic
I'd use 1" steel flex rather than aluminum. You might pull the aluminum flex apart, it's pretty fragile.

#19
02-03-14, 08:15 PM
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ok, i'll try that. i was drawn to the very light, flexible nature of the stuff I saw at Lowes, but it did seem a little wimpy. The liquid tight NM flex seemed pretty durable, but it was really coiled up and seemed like it would be harder to pull.

#20
02-03-14, 10:05 PM
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Nashkat1, how long do those milwaukee auger bits last?
I've been using the two I have now for maybe 10 years.

do they break very easily or go dull,
I've touched up the cutting faces with a file from time to time, when I hit a nail or something. I have no idea how to break a bit that can practically be tied in a knot. I never thought of trying to do that.

i can get one for just over \$50 online.
You can pay that much if you want to. I can get one for less than \$40 online from HD.

Can I use it by myself, or do I need one person to hold it on track while I drill?
The person holding the drill has to use one hand to guide the bit. There's no way another person could sense and control the bit's position and movement to place it and guide it where you want it to go. Remember, you still have to drill the holes through the the center of each framing member.

I was thinking of running about 40' of 1" flexible aluminum conduit up to my subpanel in my attic.
Use steel FMC, as Joe suggested.

Could I pull 3 4-gauge THHN wires and 1 8-gauge THHN wire through it without too much pain?
They'll all fit. How hard it is in part depends on how experienced you are at pulling wire. You can feed the conduit down and pull the wire up or pull the conduit up and feed the wire down. Think about which way will be easier before you start.

I'd need to go up around 20' from my main panel in an exterior wall and then over 20' at 90 degrees across my attic. I can then just bend the conduit into place to my subpanel if I can get it that far. The vertical wall has a few jogs to make as I weave it through 3 holes that are slightly offset.
No more than 360[SUP]o[/SUP] of bend between boxes, including jogs. Use a bond bushing on each end.