Wire gauge calculations

NoEdison

Joined Feb 18, 2017
12
I’m running a couple new circuits to a shed with a wire-path about 135 feet.

Circuit 1: 240V 10AWG 30A 135’
voltage drop calculated is ~10V, 4.2%

Circuit 2: 120V 10AWG 20A 135’
voltage drop ~ 7V, 5.6%, 113V

the 240V is to run an electric heater of 5,000W, = ~21A
The 120V is for minor loads, but periodic high loads, e.g. miter saw (short bursts) plus 2 vacuums (left on).

the present circuit is has too many outlets 120v 20A 12AWG ~150’. It very rarely trips. The voltage only drops from 120 to 118 with minimal load.

The question(s) are
1) Are the wire sizes calculated above based on full amperage all the time?
Is the calculation for circuit 2 to supply close to 20A and 113 V all the time?

2) is it reasonable to expect the 240V circuit to safely draw the calculated 21A nearly full time ?

I’m trying to not use any thicker wire than necessary ($$) thank you! Reloadron Joined Jan 15, 2015 7,580 Well we can start with a 5.0 KW heating element powered by 240 VAC will draw right around 21 Amps. AWG 10 Copper wire has a resistance of about 1.0 Ohm per 1000 feet so lets leave overhead and say a 200 foot run of AWG 10 will be 0.4 Ohm round trip. So we have 21 amps and 0.4 Ohms or 0.4 * 21 = 8.4 Volts dropped. Under a full load for AWG 10 wire it would be 30 Amps * 0.4 Ohms = 12 volts. While I doubt the heater element will care much even running on 230 volts verse 240 volts I would likely run AWG 8 3 conductor direct burial cable. Will AWG 10 work? Likely just fine. Using AWG 10 3 conductor UF (Underground Feed) You are looking at about 165.40 per 100 feet. Looking at copper wire increases in cost a year or two from now make that 165 about 200. I would just run with AWG 10 3 rather than drop to AWG 8 wire. Just My Take... Ron Thread Starter NoEdison Joined Feb 18, 2017 12 Well we can start with a 5.0 KW heating element powered by 240 VAC will draw right around 21 Amps. AWG 10 Copper wire has a resistance of about 1.0 Ohm per 1000 feet so lets leave overhead and say a 200 foot run of AWG 10 will be 0.4 Ohm round trip. So we have 21 amps and 0.4 Ohms or 0.4 * 21 = 8.4 Volts dropped. Under a full load for AWG 10 wire it would be 30 Amps * 0.4 Ohms = 12 volts. While I doubt the heater element will care much even running on 230 volts verse 240 volts I would likely run AWG 8 3 conductor direct burial cable. Will AWG 10 work? Likely just fine. Using AWG 10 3 conductor UF (Underground Feed) You are looking at about 165.40 per 100 feet. Looking at copper wire increases in cost a year or two from now make that 165 about 200. I would just run with AWG 10 3 rather than drop to AWG 8 wire. Just My Take... Ron Thanks Ron! That’s good info and analysis. My concerns (in order) - overheat wire and cause a fire - overload circuit and cause breaker-tripping - over-size wire The 240V circuit seems adequate; well under the breaker size. The 120 circuit is a bit of an unknown; I suppose I could follow your logic and calculate the heat generated under maximum load. it’s still unknown to me if the online wire calculations are assuming continuous maximum load or not; that knowledge would simplify the matter. It’s too darn expensive to oversize the wiring unnecessarily. Pete nsaspook Joined Aug 27, 2009 13,571 Oversizing the wire is cheap insurance for future upgrades (generator, solar, etc...) and some things, like peace of mind, are priceless. Thread Starter NoEdison Joined Feb 18, 2017 12 Oversizing the wire is cheap insurance for future upgrades (generator, solar, etc...) and some things, like peace of mind, are priceless. True, but I want to understand the rationale behind the calculations so I size the wire appropriately, rather than throw 600 or more on unnecessary material. ThePanMan Joined Mar 13, 2020 803 Any good engineering practice is to leave yourself overhead. Typical overhead engineering aims to be 133% of anticipated load. If you expect to use 100 watts, build for 133 watts. That way you're never going to worry about being at a max level. Oversizing the wire is cheap insurance for future upgrades This is another part of good engineering practice. If you build a 2 lane road for the amount of traffic it sees daily, in a few years the road will be undersized and you'll have to upgrade the roadway; costing twice what it would have cost had it been built with 4 lanes in the first place. I'm with spook on this one - go with 8 AWG 3 conductor. Black, Red, White (and the bare copper ground). When talking about 8-3 wire they don't count the ground. Same is true for 14-3. It has the three color wires and the ground. Ground is never counted as a conductor. That's because it's there for safety and should not be used except in case of an electrical failure that might put a person at risk of electric shock. I’m trying to not use any thicker wire than necessary ($$$) Understood. It's your project. Just don't live to regret not spending the extra few bucks. Because digging up the 10 AWG wire and laying 8 AWG will certainly cost you lots more "$". Cheaper now - more expensive later. LOTS more expensive.
And
peace of mind
.

Last edited:

Joined Jan 15, 2015
7,580
Keep in mind I based AWG 10 on the fact that I can buy the UF wire for direct burial in AWG 10. I am not seeing AWG 8 in UF. Now if you run with for example AWG 8 then you are looking at TFFN & THHN Wire and the pipe to run it in and any other assorted hardware. You can find AWG 8 UF and you are looking at a continuous 150 foot length of 8/3 Copper UF-B cable cut from a master reel and hand coiled with a cost of $444.20. With 240 volt service and AWG 10 it will play out as originally covered. I have no idea if you plan above ground or underground feed? Personally I prefer buried when possible. Several years ago I ran 240 volt out to a new garage. About a 100 foot run. I used AWG 6 but only because I had several hundred feet of the stuff. I put it in PVC pipe and done. Again, because I had it. I doubt your 5 KW heating element will be running 24/7. Only you can decide what works best for you all things considered. Now if by chance you are anywhere near Cleveland, Ohio feel free to get in touch with me as I still have plenty of that AWG 6 silver plated copper wire with an armor sheath as can be seen as the entry to my garage panel below. NSA Spook May recognize that type of cable. Years ago that cable was over$15 / ft.

Normally I am the guy telling people to use the heavier wire but at todays prices you have to weigh your budget into things. Just keep in mind some of what was mentioned as to any possible future expansion.

Ron

nsaspook

Joined Aug 27, 2009
13,571
Keep in mind I based AWG 10 on the fact that I can buy the UF wire for direct burial in AWG 10. I am not seeing AWG 8 in UF. Now if you run with for example AWG 8 then you are looking at TFFN & THHN Wire and the pipe to run it in and any other assorted hardware. You can find AWG 8 UF and you are looking at a continuous 150 foot length of 8/3 Copper UF-B cable cut from a master reel and hand coiled with a cost of $444.20. With 240 volt service and AWG 10 it will play out as originally covered. I have no idea if you plan above ground or underground feed? Personally I prefer buried when possible. Several years ago I ran 240 volt out to a new garage. About a 100 foot run. I used AWG 6 but only because I had several hundred feet of the stuff. I put it in PVC pipe and done. Again, because I had it. I doubt your 5 KW heating element will be running 24/7. Only you can decide what works best for you all things considered. Now if by chance you are anywhere near Cleveland, Ohio feel free to get in touch with me as I still have plenty of that AWG 6 silver plated copper wire with an armor sheath as can be seen as the entry to my garage panel below. View attachment 315206 NSA Spook May recognize that type of cable. Years ago that cable was over$15 / ft.

Normally I am the guy telling people to use the heavier wire but at todays prices you have to weigh your budget into things. Just keep in mind some of what was mentioned as to any possible future expansion.

Ron
Fell off the back of a Navy truck during a "Survey". I'm spoiled too, working on a high end industrial site with lots of heavy wire electrical scrap.

Joined Jan 15, 2015
7,580
Fell off the back of a Navy truck during a "Survey". I'm spoiled too, working on a high end industrial site with lots of heavy wire electrical scrap.
Nothing gets past you. They were scrapping three spools of between 450 and 500 ft per spool. Scrapping as into the dumpster. I showed them how to scrap it into my truck.

Anyway if the thread starter is anywhere near me I would be happy to let dome go on the cheap like a buck a foot.

Ron

Papabravo

Joined Feb 24, 2006
21,314
Nothing gets past you. They were scrapping three spools of between 450 and 500 ft per spool. Scrapping as into the dumpster. I showed them how to scrap it into my truck.

Anyway if the thread starter is anywhere near me I would be happy to let dome go on the cheap like a buck a foot.

Ron
Ever the capitalist searching for an opportunity. Bless you.

nsaspook

Joined Aug 27, 2009
13,571
Ever the capitalist searching for an opportunity. Bless you.
That's a steal of a deal. I wrangled that stuff during ship retrofits at LBNS in the 80's. It's armoured cable. That's expensive, even from China.

NoEdison

Joined Feb 18, 2017
12
Thanks everybody, great input. I’m going on the cheap as funds are very low. If were even remotely close, I would absolutely take up the offer of free wire; wire cost out of sight.

I have 2” pvc underground from garage to shed, so I can pull more wire later. There is #10 THHN already there.

A single 240 run to a sub panel would be most efficient; I’ll look into that as well

NoEdison

Joined Feb 18, 2017
12
Just to follow up, based on lots of input, my best idea is to run 8-3 wire from a 40A 240V breaker to a sub panel, then split into 15A 120v breaker and a 25A 240V breaker for most efficient power transfer to the shed.
I will never be drawing the full 40A, both 20A @ 240V plus 15A @ 120V, but if I do, I figure that’s what the 40A source breaker is for.

MisterBill2

Joined Jan 23, 2018
19,466
For those circuits,#10 copper wire is allowed for up to 30 amps constant duty, properly installed with the suitable insulation. Any heating issues will be at the connections if they are not adequate. So the safe thing is to check the temperature rise at the connections. The safe, cheap, and easy wy is to use temperature sensing wax sticks to see of any portion gets hot.

NoEdison

Joined Feb 18, 2017
12
For those circuits,#10 copper wire is allowed for up to 30 amps constant duty, properly installed with the suitable insulation. Any heating issues will be at the connections if they are not adequate. So the safe thing is to check the temperature rise at the connections. The safe, cheap, and easy wy is to use temperature sensing wax sticks to see of any portion gets hot.
Great, thanks!

ThePanMan

Joined Mar 13, 2020
803
For those circuits,#10 copper wire is allowed for up to 30 amps constant duty
my best idea is to run 8-3 wire from a 40A 240V breaker to a sub panel

MisterBill2

Joined Jan 23, 2018
19,466
Certainly that option will also satisfy the code requirements and it may even be a bit more efficient, both in copper costs and in voltage drops, assuming that the connections are all done carefully.
So it depends on where you are in doing the project, and the costs of the materials. If you need to also run a safety ground wire, that will add quite a bit more to the price, depending on the size.. If you will be required to provide GFCI protection then it may be even more expensive to have a 40 amp circuit.

Either approach will pass code and be safe, cost and efficiency will be the variables.