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08-10-2009, 04:15 AM   #1
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## Electrical wire size question.

I have a small building that calculates at 66 amp total load ,with a max of 50 amps at any one time...........airconditioner and heater will never run at the same time is the reason that total will never happen,so a calculated 50 amps max,more than likely alot less than 50 ,but I used that number to be safe , with a 66 amp total combined of all draw if everything in the building was turned on high at the same time.

I calculated a water heater electric,a airconditioner window unit and a baseboard heater,plus a few plug sources that might get used to get the total of 66 amps combined.....

Here is my question,I want to use #8 wire for the main panel,so if I wire it in 220 and have 50 amps per leg that means the breaker panel has a total of 100 amps.....?

Much more than I need if it is 100 amps,but I feel better using the larger wire and having too many amps just in case.
I plan on a 30 amp breaker for the water heater and a 15 amp plug breaker and a 15 amp airconditioner breaker ,plus a 15 amp breaker for the lights and a 15 amp breaker for the heater.

EDIT: The run will be approx 100 foot or less.

Thanks for any help.

Last edited by shawnlee; 08-10-2009 at 04:28 AM.

08-10-2009, 06:21 AM   #2
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The wire size is determined by the panel size, proper load calculations determine the size of the panel. But I not sure you are figuring things correctly. #8 is only good for 40A, so that would be too small even with your numbers. A 100A panel require #3 copper, min., add a long run and it should be bigger.

 08-10-2009, 08:19 AM #3 Licensed Pro   Join Date: Mar 2007 Location: SC Posts: 1,571 Rewards Points: 1,000 Use THHN/THWN wires (3-#6, 1-#10 ground) in conduit or 6/3UF direct bury to feed a 100A panel, protected by a 60A breaker in the main panel. If you are dead set on using 8ga protected by a 50A breaker, you must use THHN/THWN wire. __________________ "Life is hard. Life is harder when you're stupid." John Wayne

 08-10-2009, 01:12 PM #5 Newbie   Join Date: Aug 2009 Posts: 1 Rewards Points: 10 Sounds to me like all you need is a 60 amp panel. 3# 6 wires (2 hots + neutral ) and a #10 for a ground to feed it. You will need 2 60 amp 2 pole breakers (1 in each panel ). The one in the main panel protects the wires feeding the new panel and the one in the new panel acts as a means of disconnect because it is greater than 50 feet away. You also need to do this if it is not with in sight of the source breaker ( line of sight ). Depending on the electrical code in your city there may be a few variables but this will cover it. Tony Dolce Meade Electric http://www.meadeelectric.biz
 08-10-2009, 02:20 PM #6 Newbie   Join Date: Aug 2009 Posts: 13 Rewards Points: 10 I little more info ,I am out in the country with no codes ,but I want to do this correctly so it lasts and is safe.I am outside of Rozel in Kansas. I have a 200 amp main box on a pole and no other buildings yet with power. This is going to be a 16 by 24 room to sleep in with a bathroom in it. I can use #6 no problems there. Basically it will be a small hot water heater ,a airconditioner and a base board hydronic heater ,with a few lights and computer plugged in. The main loads will be heat in the winter ,with a few lights and the computer running and a/c in the summer with a few lights running and the computer.......a really small set-up. I was going to run a feed underground in conduit with thhn wire to a main panel inside of the building. Dedicated circuts for the heater ,a/c and water heater ,one breaker for lights and one breaker for recepticles. With the water heater at full rating ,the heater at full rating and the a/c at full rating the load comes out to 45 amps........these are the max ratings for these appliances.Then I figured a light or 2 on and the computer running gives me around 66 with everything on max at the same time,which will never happen....full heat ,full a/c and full hotwater draw,with all the lights on and computer running..... The a/c will be 240 as will the heater and water heater on seperate circuts,lights and recepticles will be 120 on seperate circuts. So I figured a realistic constant load at 50 amps max at any one time ,more than likely alot less in the 35 to 45 range tops ,but rounded up to 50 for a good safe number.....like I said if everything was turned on in the building at once on high ,max would be about 66 amps. I was planning on using 3 wires from main panel and a grounding rod at the new panel. Hope this helps with more info ,I just did not want toverwhelm you guys with tons of info you might not need..... I plan on 3 more buildings like this in the future....one for a kitchen ,one for a living room and one more guest/sleeping room the same as this one. I am not completly positive the main panel is 200 amps,it may be larger ,its a pretty big farm panel with full size line wires coming into it with some pretty big fuses in the top of the panel and a bunch of breakers below those fuses...........it was running 3 mobile holmes a garage and a giant quonset hut along with a well pump and everything looked good ,no discoloration and the panel was not full. Thanks for all the great replies and help!!!!!
 08-10-2009, 05:05 PM #7 Electrician   Join Date: Nov 2005 Location: Winnipeg, Manitoba Posts: 1,298 Rewards Points: 802 You may need to put an arc fault on your bedroom circuit, its code in Canada and wouldn't be suprised if it is in the US as well. and i am pretty sure in the USA the bathroom will need its own dedicated circuit with a GFI on it.
 08-10-2009, 05:26 PM #8 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 66A @ 100' @ 5% drop @ 240v means less than 180 mΩ wire resistance. #8 AWG has ~126 mΩ for this (200' loop) distance. Ampacity is another, more complicated issue.
08-10-2009, 06:52 PM   #9
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Quote:
 Originally Posted by bigtoe1340 Sounds to me like all you need is a 60 amp panel. 3# 6 wires (2 hots + neutral ) and a #10 for a ground to feed it. You will need 2 60 amp 2 pole breakers (1 in each panel ). The one in the main panel protects the wires feeding the new panel and the one in the new panel acts as a means of disconnect because it is greater than 50 feet away. You also need to do this if it is not with in sight of the source breaker ( line of sight ). Depending on the electrical code in your city there may be a few variables but this will cover it. Tony Dolce Meade Electric http://www.meadeelectric.biz

Hard to find a 60 A panel, especially one with enough spaces to be usable. And the big box stores don't usually have the required breaker hold down kit for securing a double pole breaker as a main. A 100 A main breaker panel is very cheap, has the main already installed, and has plenty of space for circuits. He can feed it with a 60 A breaker from the house.

 08-10-2009, 08:03 PM #10 Newbie   Join Date: Aug 2009 Posts: 13 Rewards Points: 10 Thanks everyone ,I think I have it now....... 60 amp double breaker to feed the building ,a 100 amp main panel in the building ,#10 wire for the hot water heater and I will use #12 for the rest. Individual circuts for each major appliance ,one for the lights ,one for the plugs and one for the bathroom with a gcfi... My only other question .......is it ok to use a ground rod at the new panel ,instead of a long ground back to the main panel.......2 hots and 1 neutral in conduit from the main panel to building and a ground rod at the out building from the new panel?
08-10-2009, 08:17 PM   #11
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Quote:
 Originally Posted by shawnlee Thanks everyone ,I think I have it now....... 60 amp double breaker to feed the building ,a 100 amp main panel in the building ,#10 wire for the hot water heater and I will use #12 for the rest. Individual circuts for each major appliance ,one for the lights ,one for the plugs and one for the bathroom with a gcfi... My only other question .......is it ok to use a ground rod at the new panel ,instead of a long ground back to the main panel.......2 hots and 1 neutral in conduit from the main panel to building and a ground rod at the out building from the new panel?
Nope. You need a ground rod at the panel, and you need a real ground back to the house. The ground rod and the ground wire perform two different functions, and you need them both. Also, when you purchase a panel, make sure the ground bar is included, if not, buy one. Alot of panels don't come with one. The neutral in the panel may be bonded to the metal panel can, so remove that connection.

08-11-2009, 06:25 PM   #12
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Thanks everyone for the help and information,this should allow me to do the job correctly and not endanger anyone later.!!!!

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