What gauge of wire to run off a20 amp breaker?

12

12 is pretty brief! Correct, at least somewhat. Def. NOT #14. BUT better is to ask what the typical load is and HOW FAR of a run. If a long run... like over 50'... and the typical load was like 15 amp, then I'd go to #10. If a VERY long run, #8.

Jalu, the answer given is valid for the question posed.

I need to disagree with your expanded answer.

Can you show the voltage drop calculations that would illustrate the reason that a 15 amp load at 50 feet would need #10 wire?

If what you say is true, then there are a few million buildings that need to be re-wired.

Well, TK, It depends on the word "SHOULD". I have both my "little black pocket ref." AND a copy of the NEC code book. The reference book has a chart: "max wire feet for 2% max voltage drop". I'm retired from a Power Company, and that "2% "2% " be the max drop" was a " be the max drop" that our "rule of thumb" electricians used too. I plugged the phrases into Google and get like 68,000 hits.

If you care: "voltage drop" "less than 2%" - Google Search (http://www.google.com/search?hl=en&q=%22voltage+drop%22+%22less+than+2%25%22)

In any case, the chart says "building service" is 47 ft. with #12 copper." Therefore, 50 foot SHOULD use #10.

If you'd like the math, first, remember a 50' circuit is 100' of wire. I drew it out with the source, 50' out, the 15a load, and then 50' back. My ref shows #12 as having 1.62 Ohm per 1000'. The 100' is easy math: 100' = 0.162 Ohm. Since E=IR, 15a through 0.162 Ohm will drop 2.43 volts IN THE WIRE. If you divide 2.43 by 120 vac, you get that the drop is over 2%.. . Just barely. I know, but IF you use the 2% "max distance @ 15a to not exceed 2% drop" be the max drop as gospel, then #12 ain't enough... use #10.

Now in the real world, I'd use #12 to MY outlets too. BUT if I go up to my family room -- the far side of the house from the entry panel -- I see a worse drop. If I plug in a heavy "multi strip" Therefore, 50 foot SHOULD use #10.

If you'd like the math, first, remember a 50' circuit is 100' of wire. I drew it out with the source, 50' out, the 15a load, and then 50' back. My ref shows #12 as having 1.62 Ohm per 1000'. The 100' is easy math: 100' = 0.162 Ohm. Since E=IR, 15a thru 0.162 Ohm will drop 2.43 volts IN THE WIRE. If you divide 2.43 by 120 vac, you get that the drop is over 2%. .... Just barely. I know, but IF you use the 2% "old days" that probably DO need to be re-wired. Heck, it's done all the time. JaLu

Now that's taking a simple question and complicating the hell out of it :)

Let's add a little more complication shall we? I just wanted to add that for any given circuit stranded wire is always better because it presents with 30% more surface area than solid wire. Since current flows along the surface of the conductor it boils down to greater efficiency and lower electrical bills since the current has an "easier" time getting to where it's going to be used. If you really want to go nuts you can even upsize the wire. None of our jobs have 14 ga. Wire. We start with a mininum of 12 ga. STRANDED. The initial costs are indeed significantly greater, but take the long view. The return on your investment will continue for years after you've recouped the initial extra cost.

JaLu-Your info is a bit dated. The current NEC recommeds a maximum 3% VD for either the branch circuit or the feeder. Using the largest wire possible is great for future upgrades, but in most cases is cost prohibitive. I have friends with new houses 3,000 + sq. ft. and the largest wire used in branch circuits is #12.

I started working this answer at 5 AM, and just did not have time to get back to it until now. Sorry, but not retired here, but very tired.

I see shader is on the right track. 2% Vd is a great goal, but a bit unrealistic.

Recommended Vd is a total of 5% for both feeder and branch circuit to the last outlet on the circuit. One can use 3% for the feeder and 2% for the branch. Or vice versa.

For a simple circuit, assuming the feeder is fine, the 3% can be used for a branch circuit. This gives a bit more tolerance.

If the circuit originates at a panelboard, then the branch circuit to one outlet 50 foot away actually can be subjected to the 5% maximum Vd.

Try this popular Vd calc:

2 x length x amp load x ohms/1000 ft / 1000 = Vd

The reference I have shows #12 wire at 1.93 ohms/1000 ft, a bit higher than your ref,and assumes a resistive load.

Assuming nominal 120 volts, 5% Vd will result in 6 volts dropped.

2*50*15*1.93/1000=2.895 Vd

Practical engineering designs specifies to increase the size wire of a circuit when the distance exceeds 100 feet.

I just wanted to be clear on this topic as I do not see any reason to recommend to DIY'ers that any 15 amp circuit over 50 feet be increased from #12 to #10 wire.

Tk-your info correct as usual. In my response I was referring to the recommended max VD per one circuit, didn't take into account aggregate. Thanks for the clarification. BTW-get some sleep!!

Now I have to measure up to see if my one outlet is 50 feet from the panel. Of course along with several others, it is wired up out of the coil of #10 I was given.

While I am not arguing that larger wire is better, the additional cost may be a waste of hard earned dollars, esp with the cost of copper today, if not necessary.

One other important item to consider is that standard 15 and 20 amp devices are not designed to handle #10 wire, and result in poor connections that can cause more damaging problems.

Just a footnote to Mr. Russell's posting above. #12 AWG is the correct wire size for a 20 Amp conductor.