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Old 08-10-2009, 05:15 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.

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Last edited by shawnlee; 08-10-2009 at 05:28 AM.
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Old 08-10-2009, 07:21 AM   #2
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Electrical wire size question.


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.

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Old 08-10-2009, 09:19 AM   #3
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Electrical wire size question.


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.
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Old 08-10-2009, 12:12 PM   #4
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Electrical wire size question.


Shawnlee....always tell us where your from as codes and methods differ.

Assuming USA

There is more to this than you may think.....

If your home is a 100 amp service and assuming your feeding the panel in the detached building from your homes main panel then a 60 amp feeder would be IMO the maximum feeder you should run. Bear in mind though that if you actually have 50 amps or there abouts on any hot wire of the sub-panel feeder this will show up on the main breaker of your homes main panel. So if 50 amps being used at the detached building and 50 amps being used by your home on the same hot leg you essentially are in danger of tripping the main in your home. Hope that was clear. I doubt though from your description of the loads in your detached building this will happen. This all depends on the type of loads your house panel supports such as hot tubs or an all electric home for example. Just have to take a look at what your operating and how many of the bigger loads could be operating at one time.

Run a 60 amp 4 wire feeder (H-H-N-G) to your detached building. Do just like househelper said except don't use #8 use #6 copper as househelper described. Will be very little cost difference. Install a 100 amp breaker panel with a 100 amp main breaker so you have your required main disconnect for your building. You need the 100 amp panel in order to have enough spaces for breakers. Just look them over at the big box and pick one that you like and has enough space/circuits for your needs. I like Square D QO panels and QO breakers but any the big box has will do fine.



Also put any space heating that is 120 volts cord and plug on 12 awg and 20 amp breakers. Make these circuits dedicated to one space heating unit by using single receptacles put nothing else on those branch circuits. Any other type heating 240 volt or fixed in place heating put on dedicated circuits.

Your general use receptacles for hand tools, vacuum sweepers etc...can be 15 amp and 14 awg but 12 awg and 20 amp breakers are better and allow you to use your typical 15 amp plug. You do not need to install 20 amp receptacles that have the T-slot on one side of the plug -in just use the 15 amp ones that you see in your home. If this building is a detached garage your receptacles need to be gfci protected for any branch circuits that are 120 volts.

Lights are fine on 15 amp branch circuits and 14 awg.

Put your a/c window unit on a 20 amp branch circuit and 12 awg make it dedicated...no other receptacles on that branch circuit. This will allow you to upsize your window unit if necessary.

Put your water heater on 30 amp double pole breaker and 10 awg if it's 240 volt.



If underground use UF cable direct buried or thwn in pvc conduit. Conduit is considered better for several reasons but either will work and the cost difference is little. Do not put UF cable in conduit use thwn copper individual wires like described in the 2nd post by Househelper. The supply feeder will be 4 wires including the ground so you will need 2 hots a neutral and a ground. If you elect to use conduit and thwn the hots and neutral need to be #6 and the ground #10 green. The hots can be black or I reccomend one black and one red. The neutral needs to be white. Measure the run accurately then add 10 feet. Any extra #6 can be used to run to a receptacle for a stick welder like a lincoln 225 or other metal working tool. If you try to measure the run too accurately I guarantee you will likely be short of wire.

To expalin the feeder....Your going to have a 60 amp double pole breaker at the main panel in your home. This is going to protect the feeder at a maximum use of 60 amps per leg or hot wire. Either leg has more than 60 amps on it and it will trip the feeder breaker. As for power each leg will supply 14,400 watts of power at 240 volts or 7200 watts per leg at 120 volts. You will have a combination of both, the thing to remember is 60 amps on either hot wire will trip the feeder breaker.
Your panel will be a 100 amp rated panel with a 100 amp main. So if your feeder is protected by 60 amps back at the house then that controls the power usage of the panel...in other words it will trip at 60 amps but the main in the detached building panel will not trip. It will simply act as a disconnect for all power at the building. A disconnect is required to be located at the building even though you can disconnect power at the homes main panel by turning off the 60 amp breaker.

Getting a little long winded here (I'm known for that) so will stop and let others add to this post.
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Old 08-10-2009, 02:12 PM   #5
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Electrical wire size question.


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.
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Old 08-10-2009, 03:20 PM   #6
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Electrical wire size question.


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!!!!!
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Old 08-10-2009, 06:05 PM   #7
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Electrical wire size question.


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.
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Old 08-10-2009, 06:26 PM   #8
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Electrical wire size question.


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.
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Old 08-10-2009, 07:52 PM   #9
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Electrical wire size question.


Quote:
Originally Posted by bigtoe1340 View Post
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.
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Old 08-10-2009, 09:03 PM   #10
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Electrical wire size question.


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?
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Old 08-10-2009, 09:17 PM   #11
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Electrical wire size question.


Quote:
Originally Posted by shawnlee View Post
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.
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Old 08-11-2009, 07:25 PM   #12
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Electrical wire size question.


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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