House to shed Breaker Box?

[Redikilowatt]

My 200 amp main breaker box was likely installed in late 1970s and I've used up the breaker slots. I'm contemplating running underground feed to a new shed. The distance to house main to shed is 90 to 100 ft. Shed is 8x12 and would only need light, couple of outlets, and most importantly (for my wife ) a chest freezer. No need for power tools etc as I have a shop for that stuff.

Looking at my main house box the 20amp pool GFCI breaker takes up 4 slots, the workshop 30amp GFCI takes up 4 slots. There is one "skinny minny" slot empty. Is it possible/reasonable to replace the pool or workshop breakers with one that does the job but takes less slots?

I could run 25' from another shed (yep, I have 3) to the new one but it's fed with 20amp 12awg underground and although it measures 120 volts I fear the voltage drop for the extra 25ft would be too high.

 

[David_NC]

Those double breakers take up two slots, actually. The one marked 'empty slot' is a half-sized breaker. At any rate, you should be able to use a half-size 20A breaker in the 'empty slot' for the shed. Use 12 gauge wire that's appropriate for your installation (direct burial, conduit, etc). You can use 10 gauge, and it'll have less voltage drop. I'm not sure that it'll be a big deal, though. I know that much of the wiring in this house is over 100', like your shed, and it all works fine.
If you think you might need more than what a single 20A/ 120V circuit can provide, you can use 12-3 cable and only use one hot conductor now. Replacing some of the full-size breakers with the half-size ones will free up some space for a double breaker for 120/240 @ 20A. Something like this will work, too. It'll replace a double / 240 breaker that's installed now and give you two more 120V or a single 120/240 breakers:

 

[Redikilowatt]

Those double breakers take up two slots, actually. The one marked 'empty slot' is a half-sized breaker. At any rate, you should be able to use a half-size 20A breaker in the 'empty slot' for the shed. Use 12 gauge wire that's appropriate for your installation (direct burial, conduit, etc). You can use 10 gauge, and it'll have less voltage drop. I'm not sure that it'll be a big deal, though. I know that much of the wiring in this house is over 100', like your shed, and it all works fine.
If you think you might need more than what a single 20A/ 120V circuit can provide, you can use 12-3 cable and only use one hot conductor now. Replacing some of the full-size breakers with the half-size ones will free up some space for a double breaker for 120/240 @ 20A. Something like this will work, too. It'll replace a double / 240 breaker that's installed now and give you two more 120V or a single 120/240 breakers:

Thanks, took a couple of reads for me but now I understand. 12-3 UF wire is 240-volt and voltage drop is lower for 240v than 12-2awg even if I only use 120v

I may do a temporary run with the "old" shed to the new just to see if the voltage drops under 120v. I think it would be ok for lights and a couple of outlets but not sure it would handle the chest freezer.

 

[whoaru99]

Thanks, took a couple of reads for me but now I understand. 12-3 UF wire is 240-volt and voltage drop is lower for 240v than 12-2awg even if I only use 120v

I may do a temporary run with the "old" shed to the new just to see if the voltage drops under 120v. I think it would be ok for lights and a couple of outlets but not sure it would handle the chest freezer.

You don't really need to do a temporary run. There are voltage drop calculators that will tell you what you need to know based on length of cable, wire gauge, and load.

As far as the 12/3 cable, using just one "leg" for a 120V circuit does not provide less voltage drop. The only time you get the reduced voltage drop is when wired for 120/240V and have loads on each leg. Then you get less drop than two separate 120V circuits carrying the same loads.

 

[whoaru99]

Also, sticking to a 20A (max) 120V circuit does make the trenching/burial requirements less stringent. You could use type UF for direct burial at only 18" down.

If you go with a 120/240V circuit or a 120V circuit >20A, the cable has to be buried deeper or put in conduit, etc.

 

[Redikilowatt]

You don't really need to do a temporary run. There are voltage drop calculators that will tell you what you need to know based on length of cable, wire gauge, and load.

As far as the 12/3 cable, using just one "leg" for a 120V circuit does not provide less voltage drop. The only time you get the reduced voltage drop is when wired for 120/240V and have loads on each leg. Then you get less drop than two separate 120V circuits carrying the same loads.

Thanks. Now I understand the 120v vs 240v application. I did find a voltage drop chart/calculator They indicate I need 6 AWG for 94 feet (120v) or 8AWG for 76 ft. The bigger the wire, the higher the cost

Legally you can't break the law, Ohm's Laws in this instance

 

[whoaru99]

Thanks. Now I understand the 120v vs 240v application. I did find a voltage drop chart/calculator They indicate I need 6 AWG for 94 feet (120v) or 8AWG for 76 ft. The bigger the wire, the higher the cost

Legally you can't break the law, Ohm's Laws in this instance

What did you use for load, the full 20A?

 

[Redikilowatt]

Also, sticking to a 20A (max) 120V circuit does make the trenching/burial requirements less stringent. You could use type UF for direct burial at only 18" down.

If you go with a 120/240V circuit or a 120V circuit >20A, the cable has to be buried deeper or put in conduit, etc.

Yeah, I trenched a 24" deep 240v line to my pool pump a few years back. The wire eventually shorted underground and I had to re-dig and put in in conduit Lesson learned!

Oh, I used a chart, not the calculator.

 

[whoaru99]

I get ~3% drop on 100ft of 10ga considering 15A load.

Based on the intended use of the shed the typical load is likely to be less than 15A, probably under 10A.

If I use 10A load, 100ft of 12ga gives ~3.3% drop.

If you wanted to consider the 80% continuous load rule for a 20A circuit, then figure 16A. 16A on 100ft of 10ga gives 3.3% drop. I could live with that being conservative, IMO.

Note the footage I used is one way distance for cable. Two conductors are factored in (200ft of wire).

 

[wlhd1610]

With your proposed load at shed then using 10/3 would be more than adequate.
I've been doing electrical work all my life and the hardest part of all of this is digging the trench.
I always use pvc conduit a little larger than required.
Besides offering superior protection against damage you can always upgrade and pull bigger conductor wires if your needs change.
Can't do that with direct burial wire.
So if you used 1" or 1 1/4" pvc you could easily pull up to #6 or #4 ga THHN stranded later.

Also keep in mind that this is considered a sub panel so you'll need to keep the neutral and ground bars separate.

Bob

 

[whoaru99]

It's your deal but, since you already have a shop for the big work, I wouldn't put in a 240V circuit. 10/2 w/ground for a solid 120V, 20A circuit I'd think you'd be golden.

 

[Redikilowatt]

. . . .

Also keep in mind that this is considered a sub panel so you'll need to keep the neutral and ground bars separate.

Bob

Checked the sub panel I had laying around and noticed it didn't have a ground bar, although it does have a screw hole for adding one. After doing so, do I need to ground the bar to a metal post in the earth? Or could I just connect it to the ground on the wire coming from the house? Pix below and I already have a 20amp and a 15 amp breaker switch to fill the two slots.

 

[whoaru99]

@Redikilowatt

If you're running a simple/single branch circuit from the other panel you don't need that at all, and no ground rod required.

You'd just wire in the receptacles, light, and switch directly from the branch circuit since there's already a breaker at the source. In the shed, when you get to wiring the receptacles, put a GFCI first in line, supply wires to the "Line" terminals then all downstream receptacles wired from the "Load" terminals are protected to satisfy the GFCI requirement. And, yes, per current code, GFCI is effectively required on all receptacles.

 

[wlhd1610]

It all depends on what you want.
If all you want is a single 120 volt circuit that feeds a couple of receptacles and a few lights then do as @whoaru99 says and you'll be fine.
If you think you might need a little more circuit ampacity for several circuits then run a 10/3 at minimum and put in a little subpanel that can feed several receptacle circuits ,some lighting and even continue from there to another little shed.
As I said previously the hard part is the trench and everything is a piece of cake!


Bob

 

[Redikilowatt]

I . . . .
As I said previously the hard part is the trench and everything is a piece of cake!


Bob

Thanks again folks! I did a 80 ft trench, 24" deep with a rented trench machine ~15 years ago. It was easy peasy except I cut through another underground wire . I used a mattox to dig one 24" deep and 25 ft long for the pool switch 6 years ago. Now 74, I won't do that again! For this project renting a trencher for 4 hours will cost as much as the wire, pvc conduit, etc. Maybe I can get my wife to dig the trench since she is only 66

Luckily, I have a little time to sort all this out but AK folks are incredibly helpful

 

[wlhd1610]

Checked the sub panel I had laying around and noticed it didn't have a ground bar, although it does have a screw hole for adding one. After doing so, do I need to ground the bar to a metal post in the earth? Or could I just connect it to the ground on the wire coming from the house? Pix below and I already have a 20amp and a 15 amp breaker switch to fill the two slots.

View attachment 1978723

You can use that subpanel .
You just have to remove the grounding screw from the neutral bus bar and screw a small 3 or 4 terminal ground bar right through the back of the panel.
When you run your white/red and black wires from the house you also pull a green wire with it if you're doing pvc conduit.
If you're using an all in one cable ,like 10/3 flat direct burial or romex or MC the ground wire is already inside the sheathing.
The NEC frowns upon multiple or separate ground rods other than the one(s) required at the main entrance service disconnect.

BTW,
Pool ground rods around perimeter of swimming pools with metal sides either above ground or below are a different story/application.

Hope this helps,

Bob

 

[whoaru99]

Call 811.

 

[whoaru99]

If you put a subpanel in the shed fed with a three wire (120V/240V) then you've changed from a branch circuit to a feeder. I'm pretty sure with a feeder and a subpanel you are required to put a grounding rod at the outbuilding.

With just the basic 2-wire branch circuit there isn't a requirement to have a grounding rod at the outbuilding. You could though if you wanted, put one as an auxiliary grounding rod connected along with the grounding wire of the branch circuit (but not connected to Neutral).

 

[GuyK]

...
If you think you might need more than what a single 20A/ 120V circuit can provide, you can use 12-3 cable and only use one hot conductor now...

I wouldn't do this. The potential later second circuit would have to share the neutral wire with the first, and I don't believe it's allowed by NEC. Use a 12-4 cable instead if considering this strategy.

 

[gadget73]

If used to supply 240 only 1 neutral is needed. Two 120 does need 2 neutral. With 240 the return current will never exceed 20a so it works. Two 120 could have 40a on a shared neutral. Handily 1 240v 20a feed is also 2 120 20a so it works out. Just needs a 2 pole breaker.