# Wiring question - length of runs

• posted on May 12, 2004, 5:44 pm
I'm doing the wiring on an addition to my house, and I'm trying to figure out the acceptable length of runs (or to put it a different way, the proper wire size for the runs I need to make).
I've got a book that seems to be saying that the longest run I can make with 12 ga. wire on a 20 amp circuit is 35 feet. If I'm reading that right it would mean that at least some of my wiring needs to be done with something larger than 12 ga. since it's about that far (35') just to get to the addition from the breaker box.
Does that sound correct, or does it sound like I'm reading the table wrong? I was planning on using 12 ga wire all the way through, but now I'm not sure....
Kemmotar
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• posted on May 12, 2004, 5:54 pm
My handy Home Depot wiring book says 50' on a 20 amp with 12 gauge. 10 gauge over 50' up to 100'. I'm in the process of wiring a workshop that is going to have a total of about 70' of wiring. It was 40' to get to the workshop. I put in all 12 gauge since I will never have a load large enough to fry anything.

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• posted on May 12, 2004, 5:56 pm
You could always run 10 gauge to a junction box in the addition then run 12 from there on. Looking back I guess that would have been the best way to go for my setup but the price on the burial cable in 12 gauge was so low at Wal Mart I could pass it up, lol.

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• posted on May 13, 2004, 2:29 am

Not all that up on codes but that suggestion seems bad to me. Works fine until someone sees 10 ga in the circuit box and decides it will handla a 30 amp breaker. Others have point out what seems to be the right way, heavy gauge to a subpanel and then lower gauge from there.
Harry K
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• posted on May 13, 2004, 11:22 am
In every house I've ever worked on or built, the electricians use 12g for almost all of the wiring (14 for light circuits). Most foundations around here are about 40-50 feet on thelong axis, and the power always comes in to a corner. So, running a wire from the box, to the other side of the house is at least 40'. Add 25' to get up to the attic and you've got at 65'.....
I don't know what the code is, or what the "books" recomend, but lots of practical experience says for normal loads, 12 is fine.......
--JD
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<%-name%>
• posted on May 13, 2004, 2:33 pm
j.duprie wrote:

The Code says you should use large enough wire to keep voltage drop less than 5%. It's better to keep it under 3%, or even 2%.
Twelve gauge copper wire has about 1.7 ohms per 1000 feet. The actual number is 1.62 ohms/1000' @ 25 degrees C, but I am derating the wire a bit in case it runs warm. Let's say the original poster was designing the circuit for 18 amp load (90% of the circuit breaker rating) and underestimated the length of the wire and it's really 50 feet.
Resistance of powerline = (2 x 50') x (1.7 ohms / 1000') = .17 ohms Voltage drop = 18 amps x .17 ohms = 3 volts
Percent voltage drop = 3 volts / 120 volts = 2.5%
#12 wire should be fine; it is unlikely that the expected load will be anywhere near 18 amps. All this talk of subpanels is foolishness, unless he wants a subpanel for convenience.
Best regards, Bob
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<%-name%>
• posted on May 13, 2004, 7:03 pm
zxcvbob wrote:

I think my original post wasn't totally clear perhaps... I said it was 35' from the panel to the addition, but I didn't mention that it's another 10' to get up to the second floor (where the first outlet on the longest circuit is) and another 35' to the final outlet. I just went and measured, and the total lenth is 80'. I calculated this up using the same method you gave, and got:
Resistance of powerline = 2(80')(1.7 ohms/1000') = .272 ohms Voltage drop = (18 amps)(.272 ohms) = 4.9 Volts
% voltage drop = 4.9 volts/120 volts = 4.1%
Which is under 5% but is closer than I'd like, given I didn't measure every little wiggle in the wire. So I calculated it again to see if doing the first 45 feet with 10 ga. would make a significant difference.
That gives me:
Resistance = 2(45')(1.02 ohms/1000') + 2(35')(1.7 ohms/1000') = .0918 + .119 = .21 ohms
Voltage drop = (18 amps)(.21 ohms) = 3.79 volts
% drop = 3.79/120 = .032%
If that calculation is right, I'd say that it might be a good idea to go with 10 ga. wire between the breaker box and the first junction box, and then 12 ga after that. Which is, (I think) what you recomended originally...
If anyone sees any problems with my calculations or reasoning, let me know!
Thanks for the help
Kemmotar
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<%-name%>
• posted on May 13, 2004, 7:29 pm
Kemmotar wrote:

The only thing suspicious about the calculations is that 18A load. That's pretty high. I was using it as a near-worst-case number.
What I recommended earlier was to run 240V to the first junction box, then split into two 120V taps from there for the outlets. Calculating the voltage drop gets complicated for this arrangement, but it is never worse than running 2 separate 120V circuits the same length. Still might not be good enough for 80', I dunno.
Run all the lights on a separate circuit (where #14 wire might very well be large enough).
How many lights are you going to install? You might can put the upstairs lights on the same circuit as the downstairs outlets, and vice-versa. That way, you only need 2 circuits but don't have to worry about losing the outlets and the lights at the same time if you overload something (assuming the breaker handles are not tied.)
Assuming you don't need any special circuits (kitchen, utility room, large window-mount air conditioner, etc.), I think I like running 240V on a 2-pole breaker using #10 to the first box to supply all the outlets, and #12 or #14 on another breaker for all the lights. BTW, 3 conductor wire (4 if you count the ground) is easier to work with than 2 conductor because it is round.
Good luck, and best regards, Bob
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<%-name%>
• posted on May 13, 2004, 10:29 pm

The worst-case drop is still the same actually - big draw on one side, none on the other -> for the same wire sizes -> same % voltage drop.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
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<%-name%>
• posted on May 19, 2004, 3:26 pm
I disagree. The judicious use of subpanels makes distribution easier and greatly reduces load driven voltage variations. I have 5 subpanels and am about to add another in my boat house.
RB
zxcvbob wrote:

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<%-name%>
• posted on May 12, 2004, 7:46 pm
Try this site. When U vist scroll down to Electric/Electronic heading
http://home.comcast.net/~entreken.fixit.htm
Look for "Electric Codes

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<%-name%>
• posted on May 12, 2004, 9:43 pm
Kemmotar wrote:

If you're running more than 2 circuits that far, you really should consider running one big wire (8-3 w/g or 6/3 w/g) to a subpanel located somewhere in the new addition. And please don't take this as an insult, but being you might not have known that going in, you might not have the experience to tackle that job yourself.
--
TP / Network Man __________________________________
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<%-name%>
• posted on May 12, 2004, 9:57 pm
I-zheet M'drurz wrote:

I'd considered that, but I'm only running 3 circuits... not sure that that's really enough to justify a subpanel. As far as experience, it's true I don't have lots (although I do have some) but if I don't do this myself then I'll never have the experience will I? :-) I do know enough to know the issues that need consideration, and I will make sure I do it right in the end!
Thanks for the help
Kemmotar
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<%-name%>
• posted on May 12, 2004, 10:34 pm
Kemmotar wrote:

3 circuits, you're right. Buy a length of 10-3 w/ground for the two that will be farthest away from the panel, that will give you some compensation for the long run. Hit a central point and put up a nice 4" square junction box, split your 2 separate circuits from there. If you go with the 3-waire, remember that it should be on a double pole (ganged) breaker. It might not be required by code, but if you ever need to open that first junction box, the safety of knowing both circuits are killed by flicking the same breaker is worth the extra couple of bucks.
The closest circuit to your panel, you can do with 12-2, unless it's goung to be maxed out with blow driers or toaster ovens or something like that <g>. If that's the case, then I would go to 10 guage (10-2 w/ground) there as well. Good luck.
--
TP / Network Man __________________________________
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<%-name%>
• posted on May 12, 2004, 9:56 pm
Kemmotar wrote:

What is the maximum load you are planning to have at the end of that 35' cable? How much voltage drop can it tolerate at maximum load? That's what tells you what look-up table to use. The 20A circuit breaker is just to protect in case of an overload, it has nothing to do with voltage drop. (Even at 20 amps, 35' doesn't sound right but my books are not handy to look it up.)
If you have 2 circuits to run that far, consider running an "edison circuit" to the first junction box and tee off into 2 normal circuits from there.
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<%-name%>
• posted on May 12, 2004, 11:08 pm
zxcvbob wrote:
> What is the maximum load you are planning to have at the end of that 35'

The downstairs circuit breaks into three branches with about 3-4 receptacles on each. The upstairs circuit is about 9 receptacles, but they're all on the same branch. Then there's a separate circuit for lights, which has about seven lights on it. I don't know if there's a number that's generally used to figure the max load per receptacle... (Obviously if I plugged 2 1000W hair dryers into each receptacle and turned them all on, I'd overload my circuit, but I'd guess that electricians assume that not all the receptacles will be in use at once)

The table I have is done by circuit breaker rating... it's not a really comprehensive table, and it's laid out a little bit awkwardly, which is why I wanted to check here to see if the concensus supported it or not.
Kemmotar
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<%-name%>
• posted on May 13, 2004, 4:10 am

For general purpose multiple outlet circuits like you described a 12 gauge cable will be fine. Worst case volt drop scenario would probably be (assuming 120 volts at the panel) a full 20 amp load at the end of a 100 foot run with no other outlets, which will still give you 112 volts at the load (6.6% volt drop). At 75 feet you'd get 114 volts (4.9%) and at 50 feet you'd have 116 volts (3.3%). Unless you're pulling heavy loads at the end of a long circuit, don't sweat it, use the 12 gauge. Even a vacuum cleaner that pulls 12 amps on a 100 foot circuit (using #12 wire) at the furthest outlet still has 115 volts (3.9% volt drop) at the load.