Running wire to power this 20x10 shed for an office. What wire do I need?

Submitted by skinnywolfe t3_ytjee6 in DIY

As the title says,

I am planning to make this shed into a 20x10 office. With a PC in it to office work during the day, with rendering and gaming at night. PC pulls a maximum of 350 watts. Other than that the only thing in there is a set of hanging LEDs to go on the ceiling, and a monitor and TV.

What kind of wire do I need to run from my house in the underground conduit? I have already predrilled and wired outlets inside the shed to local code with 12/2 Romex.

Thanks!

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idiotdetector70 t1_iw4mgmb wrote

You could put in a 40A Load Center in the shed then wire you shed circuits to the breakers in the Load Center. No need for 100A panel and feed.

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skinnywolfe OP t1_iw4yid5 wrote

Is it a problem if I keep the 100a breaker since I already bought it?

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idiotdetector70 t1_iw515ol wrote

It shouldn't be a problem to take it back and exchange for a 40A. If you put in a 100A breaker you will need to feed out with at least 100A wire. The breaker's job is to protect the wires running through the walls/conduits from catching fire. Whatever size breaker you put in means the wire size has to exceed the amperage of the breaker.

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waun t1_iw50yan wrote

It’ll be massive overkill. If it was me, unless I was planning on leaving room for expansion (ie also spec’ing the wire from the house appropriately based on potential future current load) I would return the panel and save some cash.

I did something similarly, but I upgraded my primary panel from 100A to 200A - and so I had a free salvaged 100A panel that I turned into a subpanel.

It came in handy for me because I was able to add HVAC into my outbuilding when COVID hit; I had the extra capacity (including wire gauge) to install a mini split and resistive heating.

If you’re only ever expecting to use the items listed in your post, you definitely won’t need the 100A panel though (and the requisite 100A wiring).

Note that most municipalities will require an electrical inspection whether you as a homeowner or a licensed electrician does the work. NEC (electrical code) in North America will also have a lot to say on the depth of buried cable etc between your house and shed, and how you mark it. Not following code and not getting an inspection is a recipe for excess fire and electrocution risk, and risks insurance not paying out / you may have liability in certain cases.

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spedfarm t1_iw4yq60 wrote

I see no mention of climate control, a small heater/AC can easily cause overcurrent in these situations, always better to have more than you need,.

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peachfuzz0 t1_iw4h80w wrote

Consult an electrician to be sure but I believe you need a sub panel from your main panel to your shed.

You could also consider installing an off grid solar setup with batteries.

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skinnywolfe OP t1_iw4hguz wrote

Right! I did buy a 100amp sub panel that I will hook up once it's time to rock and roll.

Solar is intriguing but south and west of the shed is pretty tree covered. Didn't now if that would put a damper on my generation potential

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peachfuzz0 t1_iw4i3it wrote

100A is a bit overkill for a shed, you can get away with 40 or 60 but if you have it already then keep it. 100A requires 2ga al or 4ga copper, which is not cheap.

If you can diy it, solar is a fun project. Plenty of YouTube videos about it.

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skinnywolfe OP t1_iw4iahj wrote

Would it be more beneficial to grab a smaller amp box in this case if it means being able to use a more cost friendly wire?

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peachfuzz0 t1_iw4injb wrote

Likely yes. 40a requires 8ga wire and that would give you 2 15A 120v circuits so roughly 3600W.

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soniclettuce t1_iw4vcrs wrote

The rating of the panel doesn't matter, just the size of the breaker that feeds the wires. If you want cheaper wire, just stick a smaller breaker upstream. And since you have conduit if you ever decide you need more, it should easy to pull new wire through. (Aluminium wire is pretty cheap though, as long as everything is rated for al connections, it would be worth looking into).

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horsemagicians t1_iw5iwgu wrote

You can but everything really comes down to matching breaker size. Just as long as everything at minimum matches the breaker then you’re fine. You could run 2 ought wire to it if you really wanted but nothing lower than 8 gauge.

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jackdawson1049 t1_iw5gjnk wrote

To figure current from wattage divide the number of watts by the voltage. 3600 watts / 120 volts = 30 amps

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Hattix t1_iw6maji wrote

Use peak voltage, not RMS. The peak voltage of a US AC supply is 170V (peak of the cycle), so if OP specifies for 30A, your example of a a 3600 watt nominal load is 4 ohm reactance or resistance.

Solving for current through that 4 ohms at 170V gets 42.5 amps. If specified for 30A + 5A slack and so running through a 35A breaker, that breaker would trip.

You can also just multiply current by root(2) (1.41) to get the peak in the AC cycle, but I like to show how the figure is derived.

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jackdawson1049 t1_iw71259 wrote

Funny, I was taught to use RMS.

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Hattix t1_iw8a6ji wrote

A breaker will trip on a peak current. It's not much of a 30A breaker if it lets 42.5A past now, is it?

However, don't just believe some random yahoo on the Internet with electrical safety, test it out in a controlled and safe setup.

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JonJackjon t1_iw8p437 wrote

> A breaker will trip on a peak current. It's not much of a 30A breaker if it lets 42.5A past now, is it?

A thermal magnetic breaker trips at different rates.

Magnetic circuit trips the breaker when a short or near short is experienced. They are rated to trip at 2 to 5 times the rated current. Whether 2, 3, 4 or 5 times depends on the type of breaker.

The thermal circuit will trip the breaker when enough heat is generated due to the current flow. Since is it heat related the current is sensed as RMS.

This is a safety and a local code issue. Please do not suggest solutions where you don't know the complete situation nor the local code requirements.

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jackdawson1049 t1_iw8pufx wrote

I was trying to give OP a quick and dirty example to get them in the ballpark. We haven't even considered voltage drop in the wire or the start up current draw. If I was installing this circuit based on the 3600 watt example I would be designing, based on experience, for a minimum 50 amp breaker.

Now I admit, I'm not an electrical engineer or even an electrician. I'm just a lowly PMEL tech who calibrates and repairs all of the test equipment.

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tacodorifto t1_iw4hc3k wrote

I know that you need to know how much wattage you will be pulling as a whole. Plus how long the cables need to be. Depending on those that will tell you what size wire you need.

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skinnywolfe OP t1_iw4hrna wrote

It would need to be about 80-100 feet of wire to account for bends through the conduit and into the attic to tie into the main panel.

There would never be more than 1000 watts of power used at any given time. More like 600W

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tacodorifto t1_iw4ij7l wrote

You also need to look at total amps. I think amps is more important.

But someone who knows more can chime in.

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skinnywolfe OP t1_iw4inqj wrote

I probably should have posted this in an electrician sub on second thought

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Patrol-007 t1_iw55osk wrote

A heater uses up to 1500 watts by itself. Extra power is handy for outside electrical tools and electric snowblowers etc

Make sure to follow the local code for your area, especially with burying the correct cable and protecting if (surrounded in gravel, pressure treated board on top, orange flagging tape…..).

A friend put a shovel through the 240v unprotected wires going to garage, while digging window well. Fortunately the breaker tripped

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Zed-Leppelin420 t1_iw5t17r wrote

Run 8/3 Romex in conduit. On you main feeding your sub put a 40 amp breaker. Your sub can be a 100 amp no problem cause your feeding it with a 40 so it’s technically a 40. If you wanted to do it cheap buy 12/3 run it on a split 20 and that will do what you need at 1/4 the cost. No sub panel needed just run direct to plugs and lights but get 20 amp plugs and switches

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tsaico t1_iw4yf3l wrote

I just did this, final electricial was three 20 amp off of a dual 40 amp subpanel from the pool equipment. One is for lights and outlets (four total, one on each wall), one for the lights and networking equipment, and the last for the air conditioner and heater.

​

Almost the same size, 14x14

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PewMogel t1_iw5sd81 wrote

You would need UF-B direct burial cable. Which you can also run in non-metallic conduit for extra protection. It should be 18”+ deep if in conduit, 24” otherwise. You’ll also need to keep the wire in conduit from below grade into the building. By the way, 12/2 is rated 20A max and its a violation to use a breaker greater than that. This isn’t too complex, just a branch circuit to a detached building.

I am doing something similar with a shed, but need a bit more power and wanted to run only a single cable. And I do not need the extra capacity or complexity of a sub-panel. So I am using a ‘Multi-Wire Branch Circuit’ which is 12/3 plus ground. This will split a 240V connection into two 120V circuits. The only downside is that this requires a 240V breaker (or two tied 120v breakers) and both circuits must be powered down together. There are also some details concerning GFCI protection. Not a deal breaker for me.

Disclaimer: I am not an electrician, etc.

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ignatzami t1_iw6316f wrote

Stop.

First, call an electrician. Second, pull permits.

This is not a quick DIY job. This is a fire hazard waiting to happen. Please, do it correctly.

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Brubouy t1_iw4tdfb wrote

Are you adding electrical heating or cooling? Microwave? Refrigerator?

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skinnywolfe OP t1_iw4tgbz wrote

I am adding none of those things. It's a fairly simple setup when all said and done

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ilikefluffydogs t1_iw4wpj8 wrote

Consult an electrician or read your local electrical code, you should find all the information you need there. I recently ran conduit from our house to our garage for a subpanel and we buried 1" rigid conduit 18" below ground. It's a PITA but is up to code, safe, and should last a very long time.

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ttraband t1_iw4isqv wrote

I would plan for at least three 20 amp circuits. Lights and small incidentals (phone chargers) on one, computer/displays on another, third for future growth (fridge/cooler/coffee machine?). That way, if something overloads a circuit and trips that breaker, you still have power at some of the outlets.

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skinnywolfe OP t1_iw4j1ux wrote

So I have already drilled and wired outlets inside the shed for computer/monitor on one, accessories on another, and a separate circuit for the hanging overhead light

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ttraband t1_iw4k1db wrote

I’d still add a third circuit (outlet or two) for future expansion, unless the walls are going to be open/a, then it’s easy to add at any time. Often rooms are wired so outlets on parallel walls are one circuit, adjacent walls are differential circuit.

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tsaico t1_iw52mjf wrote

Honestly, just do a pull string if you are not doing the 3rd now. Save yourself the cost now, but make it as easy as possible to do it when the time comes. Also, do no smaller than 1" sched 40.

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adjuster_cody t1_iw4nx7j wrote

I legit have a contractor grade extension cord ran to my 10x12 shed that is insulated for work.

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skinnywolfe OP t1_iw4o50s wrote

From an exterior outlet on your house?

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adjuster_cody t1_iw4oad8 wrote

Yep. Have a Samsung odyssey 49” and a small window AC unit. Zero issues for 2 years now sine the hurricane.

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skinnywolfe OP t1_iw4ocmv wrote

How many outlets does the cord have?

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horsemagicians t1_iw5j7nd wrote

Just to point this out. Extension cords aren’t allowed to be used as a permanent source of power. If it results in a fire you could have a battle with insurance trying to cover it. Don’t go with this plan.

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