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Wire Size Substitution Question
By RCpilot, Section Homebrewed Electricity Posted on Sat Aug 25, 2007 at 05:16:26 AM MST
Where can I find a chart or info about substituting several smaller wires for one larger one?

I just joined this board around a week or two ago and have been searching for an answer to my question but haven't been able to come up with anything. I am just about to run wires between my solar panels and the charge control. The run is about 75' Since I plan to add more panels down the road I want to run the proper wire now. I am starting with 4 100 watt panels and plan to wire in series. In this configuration I should be able to get by with #10 stranded wire from what I have read. I have access to pretty much free wire in that size, like the insulated wire on the spools that you run in EMT conduit. My question is: what is two #10 wires equal to if I double them? Triple, etc. I have 3/4 conduit and should be able to get a lot of #10 wires in it. So, If I run a bunch of #10 wires I haven't been able to find what more than one wire of a particular size is equal to when you start adding them together. How many #10 equal a #6 etc. Can anyone direct me to a chart or know the answer?
Thanks, Kelly

Wire Size Substitution Question | 9 comments (9 topical)

Re: Wire Size Substitution Question (3.00 / 0) (#1)
by luckeydog on Sat Aug 25, 2007 at 12:27:36 AM MST

here is a chart

http://www.coloradowindpower.com/page.php?11

Luckeydog
wind gens are much funner to watch than solar panels. Broomfield,Colorado

Re: Wire Size Substitution Question (3.00 / 0) (#2)
by luv2weld on Sat Aug 25, 2007 at 07:12:19 AM MST

RCpilot,
here is a cut and paste
"every 3 gauge decrease doubles the wire cross sectional area."

It came from this site-----http://www.powerstream.com/Wire_Size.htm

If I remember correctly, that means that when you double the wire, you go
3 sizes larger. I hope someone will pull me out of the fire here if I have
screwed it up completely!!!

I know that DanB had something similar in one of his posts, but I can't find it
right now. Maybe someone else knows the link.

Use this and the chart luckydog referenced and you should be set.

Ralph
"The best way to kill time is to work it to death!!"

Re: Wire Size Substitution Question (3.00 / 0) (#3)
by ghurd on Sat Aug 25, 2007 at 07:22:44 AM MST

Look at the circular mills in the linked chart.
Two #10 (10384 x 2) =~ one #7 (20822).
Three #10 (10384 x 3) =~ one #5.5?
Notice the ohms behave similarly.

That's a good price on that wire. ;-)
G-
Ghurd.info

Re: Wire Size Substitution Question (3.00 / 0) (#4)
by stephent on Sat Aug 25, 2007 at 07:28:53 AM MST

Hopefully you won't have an electrical inspection on that paralleling of #10's.

You can run about 22-24 amps through a #10 with just a bit of heating--putting several of them paralleled together inside a conduit needs a bit more of derate for that heating.
Even installations of Solar are covered by the NEC, so if you live where there's a "permit" or inspection required...don't parallel #10's.

Re: Wire Size Substitution Question (3.00 / 0) (#5)
by RCpilot on Sat Aug 25, 2007 at 07:54:04 AM MST

Everyone, thanks for the links and explanations, this is just what I needed. I am friends with a scrap yard, trading the owner computer knowledge and help for pretty much anything I need. He has mountains of wire and all I have to do is pick out what I want and will give it to me. Last month, I installed a small 4 camera security system for him (he paid for the system) I installed it and taught him how to use it and he gave me breaker boxes, disconnects, Unistrut, 4.5 inch metal pole 9 feet long for the tracking mount and some other odds and ends. The only problem going to the yard is I always find too much stuff to drag home :(

I live outside of city limits and outside of any inspections, codes, rules and can pretty much do whatever I want on my property. So I'm not worried about following code but being an electrician for 5 years once, I learned how to properly run electric in the AC flavor. I don't plan to fill the conduit any more than a Fill Chart recommends.

This board ROCKS! As soon as I get some solar electric flowing, it's off to one of those 10' or 12' wind generators with the homemade coils, etc. using the car disc brake rotor. Walking around the salvage yard I have already seen everything a guy could need to build one including a 50' pole to mount it on :)

Thanks! Kelly

Re: Wire Size Substitution Question (3.00 / 0) (#6)
by Phil Timmons on Sat Aug 25, 2007 at 11:40:45 AM MST

Hey Kelly,

Welcome to the board. Pretty good folks around here.

For a real specific code book type answer to your question:

What you are seeking to do is called parallel feeds -- 2 or more smaller wires running from the same source, with the same breakers or fuse and disconnect, to the same load. As mentioned above, #10 is considered too small to need to be paralleled, and parallel feeds are generally only approved for 1/0 (pronounced "one ought.") or larger.

By the book (NEC, table 310-16) a single #10 is considered good for 30 amps. Parallel feeds add by simple math -- so (2) #10 can carry 60 amps, 3 give you 90 amps. As was stated above, some discounts (called de-rating) may be in order for long runs due to voltage drops and heating. But again, this not considered a proper "by the book" installation to do this, and is a little hackerish, or a kludge.

Since you have the resources to trade / swap, you may also want to consider swapping the "junk" you get for other things -- such as larger wire, parts, etc., with folks in your area who may have the exact parts you need, so that you are not forced into kludges.

Some of the stuff you get, I tend to use (and buy and trade as surplus) in the real world. Probably other folks around this site, also. Where do you operate from? (asking for if you would like to work on swaps, trades, or buys). I generally work and deal out of Texas, but travel the US and beyond on projects and deals.

[ Parent ]

Re: Wire Size Substitution Question (3.00 / 0) (#8)
by Ungrounded Lightning Rod on Sun Aug 26, 2007 at 11:40:09 PM MST

The problem with parallel feeds in normal power distribution is that if one of the two wires comes lose the current will all go through the other one - doubling the current and quadruping the heating (due to the square law) in that wire. Oops! So it's only allowed with the big stranded lines where a) they won't be coming lose separately due to the connector design and b) you pretty much have to do it since you often can't get the right wire.

That's not such an issue for a solar panel: The current is low enough (even under short-circuit conditions) that a single run of the wire size you're contemplating would not have heating issues. In fact it could safely handle three sets of your panels in parallel. The extra runs are just to reduce the voltage drop for lower power loss.

Which means it's safe - but that doesn't mean the code allows it. You'll have to check with the code guys. (You may be able to convince them to allow it by using the argument above.)

Also: If you put in multiple runs and later add so many more panels that a single run would be beyond its safe rating if it ended up carrying all the current, you might want to split the runs at the panel end and only combine them at the battery/inverter end. If you have to split them more than two ways, fuse them just before the combine: Otherwise a short at a panel could pull current from all the rest of the runs and overtax its own wiring.

Note that the code is what your local inspectors say it is. In the US the NEC is usually used as a model and passed into law - unaltered or with local modifications - by the county or city government. But the inspectors are the ones that apply the local law, so their interpretation is what rules.

You want your instalation to be inspected and certified by the local code authority because your fire insurance may refuse to pay off if you have a fire that is started or accellerated by an out-of-code electrical instalation.

[ Parent ]

Re: Wire Size Substitution Question (3.00 / 0) (#9)
by RCpilot on Mon Aug 27, 2007 at 08:31:57 PM MST

Thanks for the welcome, lots of awesome information here. I ended up trading for a roll of #8 that is very fine stranded wire. Very flexible. Anyway, planning for future use, I ran a 1" conduit for 80' and ran four of the #8's out to the Satellite mount. Since I am going to series the four 100 watt panels there shouldn't be little if any loss at all. Then I will have two more #8's for down the road when I add four more panels and will be able to feed another charge control just like the one I bought which was a BZ MPPT 500. I am hoping within another year to have four more panels, giving 800 watts of Solar, cash permitting :) Here is a shot of one of the panels.

Kelly

[ Parent ]

Re: Wire Size Substitution Question (3.00 / 0) (#7)
by ghurd on Sat Aug 25, 2007 at 04:07:08 PM MST

I think everyone needs to remember a 100W PV puts out less than 6A, and he is wiring 4 in series for still less than 6A.

Paralleling the 2 runs of #10 will be less than 3A per conductor. Sounds over 90% de-rated to me!
G-
Ghurd.info

Wire Size Substitution Question | 9 comments (9 topical)

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