I have a 200 amp breaker box with one open slot ???

Question

I am in process of building a storage/whatever building,12 x 16...I need lights and small a/c possibly a small heater...how big of a breaker is safe?? and I am Planning to use 12/2 underground from house to shed about 75 feet..

Answer 1

To start with, you loose about 2% of your voltage per one hundred feet of wire.
You said the shed is 75' from the house. You didn't say where the breaker box in the house is. You also didn't say how high the building is from the ground or how high you need to go up at the house.
I will almost bet you that if you buy a hundred foot roll of wire, it will be too short.
As has been said, you need at the minimum a disconnect in the building to stay to code and also as said, you are required by code to install a ground rod directly on the oposite side of the wall from it.
If you are going to need a heater and a/c that can only mean you intend on spending time in the building for whatever reason. Whatever the reason is, makes me think you will be useing more electricity than what has been said, (i.e. Tools, T.V. Fridge, etc.)
To start with, you need to figure up the total watts of everything you would want to run at one time, (i.e., small window a/c 1,800 watts or heater, 1,500 watts.+ say 200 watts for lights + 2 H.P. motor for whatever the tool it is you may be useing 1,490 watts= about 3,500 watts.
devides by 120 volts = 29 amps, and I would say that is the minumum you would use at any given time.
If I were to wire it for you, I would run #8/3 wire w/grn to a 50 amp sub breaker box or I wouldn't do the job. You need to bury the cable 18", as has been said, and you need conduit from the sub box, through the wall and down into the ground until the wire turns to go to the house and the same thing at the house.
With this set up, you have a 240 volt circuit where you can run an air compressor or whatever and any 120 volt tools you may want to use can be pulled off the opposit circuit from the a/c // heater plug so you don't get too much drop in the voltage.
Low voltage burns up anything that has a motor in it, including drills, saws and especially air compressors and ac units.
Any hard starting motor, low voltage really works on them and as I said before, I doubt 100 feet will do it.
The new breaker box should be eye level in the wall. Thats 5 feet.
I just wired a building like yours but it was 16' x 24'. but the side where I had to come in with the wire was about 3 feet off the ground so that was 8 feet plus the 18" in the ground and that made 9.5' the owner didn't count on. He also didn't count on the 11' it went up at the house and he had already bought the wire, when he called me. It was close to 20 feet to short and he couldn't take it back because he had it cut to lenght.
Costly mistake. I am not going to splice wire under ground unless it is in conduit and I don't care to do it then.
It will cost more, especially sense the cost of wire went through the roof, but this is what will make you happy with your hard work. There is nothing worse rthan doing a job and being unhappy with the outcome.
At the minumum, use 10/2 with ground for a 120 volt 30 amp discconect.
One last thing, get a piggy back breaker as has been said and connect the last two wires going to the breakers above the open slot to it. That will give you room for a 240 double pole breaker and you will have a spare breaker in case one fails. Just leave it laying in the bottom of the box so you will find it 4 years from now when a breaker does fail, lol.
You can always make room for a new circuit this way.

Answer 2

First off, check the brand of panel you have and then buy what is called a "piggy back" breaker of 20 amps, remove a 20 amp breaker in your panel nearest the empty spot and install the piggy back breaker which is actually 2 breakers in one designed for one space.This will free up to spots. Go with 10/3 or 8/3 to a sup panel in the shed. 10/3 is 30 amp and 8/3 is 50 amp. This will give you 220 volts out to your shed so this should be more than enough power for what you want and then some for future use. A sub panel in the shed is your best bet as you can control your circuits from there instead of the big panel. It may cost a little more but well worth the extra investment and will save you headaches later on down the road

Answer 3

It sounds like your wire size might be a bit small, given the distance and the number of loads that you plan to run on the circuit. The problem you will run into, is with the "whatever" part of your building. If you try to turn it into a workshop with power tools, drill press etc, you will quickly exceed the line capacity of 12 guage wire.

In addition, the fact that you are running the wire underground will mean that it's ability to dissipate heat will be lower. The attached reference table gives amp capacity and wire size for unbundled insulated wire in air.

An alternative that I would recommend is that you consider running a new line from the input side of your breaker box to a new breaker box. I did a similar installation and ran 6 ought cable about 200 feet to a new breaker box. I made the connections with a safety man and the circuit hot, wearing a face shield and insulated gloves. If you know what you're doing it's not that difficult.

but bottom line is, you will not have enough current capacity with a heater or AC unit on the same circuit as the lights, and electrical outlets.

Answer 4

Arthur W had a great answer. You will always find more things to power as time goes on. 12 is good for only 20 amps and that isn't enough to do much.

Be sure to use underground rated cable, Type UF. Bury it at least a foot down, more is better. You don't want anyone digging into it when planting things. Actually, it really should be a lot further down to be legal. Use cable with a ground wire, in addition to the two hot and one neutral.

Answer 5

Ok where to begin, by code you must have a disconnect in the building you are running the electrical to, the disconnect can be a fuse panel, disconnect switch, or a sub panel. (if only running one circuit a disconnect switch would be ideal) Next your running a distance of 75 feet you may start to have a voltage drop at this point i would suggest running 10/2 instead of 12/2, this way you wont have any voltage drop. You need to install wiring suitable for direct burial like UF cable, Although i suggest running 3/4" PVC conduit and pulling wire suitable for a wet location like THWN. Your cable or conduit must be at least 18 inches bellow the grade. This building could then be supplied by a single pole 20 amp breaker from your main panel. At your shed u are also required by code to pound a ground rod and run a ground wire from the disconnect to the ground rod. Also the code states any outlets you install in this building must be GFCI protected. Any advice i have given you here is in compliance to the 2005 National Electric Code, Good luck and be safe!

Answer 6

After a check of total panel load. consider 10/3.it is questionable to power a light, heater or "small a/c" , and a plug or two with one #12.
add the wattages you will use, divide by the voltage, then rate breaker at 125%.
then consider that although voltage drop isn't factored until 100 feet,75 feet can affect output voltage.
lastly light circuits and load circuits are seperate loads, put two pole breaker in a sub panel type box and even drop a ground.

Answer 7

You should think about installing a sub-panel in the storage shed with a few breakers for your needs. These breakers could be run back to a 30 amp breaker in the main panel

Answer 8

Holy Moly, guys. He's talkin' about "a small AC" and "a small heater"!!! He's not talking "storage"; this is human occupancy space / use. Maybe a wood shop or detached office.

At 12 x 16, he'll need a 1/2 ton of cooling for AC and, in electric heat, he'll need to have 4 space heaters (20 amp each) to heat the place.

This will probably end up as a 220, 60 amp line to a sub-panel at the new "shed".

He needs to call in an electrician.

Answer 9

using 12/2 along with a 20 amp circuit breaker will do fine especially if you want to make it a dedicated line. otherwise tap into the closest power junction block (i.e. a garage) just make sure that no more than four feeds are running off that circuit. Hope this works.

Answer 10

I'm a GC, not strictly an electrician, but I know enough to never fly a kite in a thunderstorm with a key tied to the wire.

First of all I'd want to know the "load" on every other breaker in the home, and laundry room, etc.

I'd want to check loads for what you state you want to add to a shed.

I'd want to know that at 75 in length any wire will have more resistance involved than in house wiring usually does.

I might even want to use 14/3 to use an AC unit as well as a heating unit.

I'd want to encase any cable, even in pvc.

I'd want to check with licensed electrical contractors for the feasability.

I'd want to check with my local code enforcement office.

I'd want to check my home owners insurance policy.

Steven Wolf
(TheRev.)