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Old 09-23-2012, 03:41 PM   #1
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Shed subpanel


Hi all,

First off I want to state that I will have my brother-in-law, an electrician, alongside me doing this project. I just want to get a better idea of what I want, what it'll take in terms of cost and labor, and learn. I am already comfortable working with simple electrical work but I've never installed a subpanel or for that matter add a breaker to my main panel.

Background: 200A service main panel in garage. Currently there are three unused slots, but I believe there may be more. I say this because there is a generator subpanel right off the main panel that allows me to switch between city power and a generator to power the well, kitchen, family room & master bedroom. I believe the generator panel was added at some point later because the previous 2 pole well breaker was relabeled as "GEN PANEL" and other labels were scratched out (kitchen, family room, master bedroom). The breakers are still there though, so not entirely sure.

Wants: Run a subpanel out to a shed approximately 120ft from the main panel (from the ground outside the garage by the panel to the ground on the outside of the shed). While I'm drilling holes in my wall and have the panel box open, I'd also like to run a separate circuit out to my front gate for a gate opener. I already have power out there feeding two lights on either side of the gate but oddly enough it's on the same 20A circuit as the entire family room powering an aquarium filter/lighting/heater, a fan/light, multiple lamps & outlets, and three other outside lights to boot.

The goal for the subpanel is to feed the primary shed, a secondary shed, and some outdoor lights/small fan for a horse stall all in the same vicinity of each other. The primary shed, where the subpanel will reside, will be a small workshop that contains a large, old-school table saw and a large planer. I don’t intend on using both at the same time. Other tools that might be plugged in and in-use (some at the same time): sanders, circular saw, shop vacuum, etc. and of course I’ll need two or three fluorescent lights. The secondary shed is just a feed shed that will have a single florescent light and maybe one outlet. This shed actually already has electricity but it somehow daisy chains off of the well circuit (not sure where) and I will be removing that.

As far as the main panel goes, I thought about using two of the empties for a 2-pole 240V circuit to feed the subpanel. The third empty will be used to send for a simple single pole 20A circuit for the front gate openers.
So, the questions…

1) Knowing my goals, what amperage would you recommend for the subpanel 2 pole breaker feeding the subpanel? I've read anywhere from 50-70A.

2) What kind of wire should I run out to the subpanel? Seeing how it's a 120ft run, entirely straight once you 90 off the garage wall, should I use metal conduit? I understand if I use metal conduit I don't have to dig as deep and seeing how it's $2 for 10ft, I'd gladly pay $24 not to have to dig even another 6" down. If so, what size conduit?

3) Do I need a main breaker shutoff in the subpanel or will the feeding breaker in the main panel be sufficient? I've read contradicting answers. I'm looking at purchasing something like what's linked here. So, in this case, I'd have a 100A 240V breaker in the main panel feeding this subpanel. I'd use a 4 conductor wire: two hots on the 240V main panel breaker leading to one hot bus in the subpanel each, neutral going from the neutral bus in the main panel to the neutral bus in the subpanel, and the ground running from the main panel ground bus to the subpanel ground bus? Is that correct?

4) Do I need a separate ground rod for this subpanel? Again I've read contradicting answers. If I do, I just pound it into the ground outside the shed and connected the ground bus to it as well? So the ground bus would be connected the main panel and this separate ground rod? How long does the ground rod need to be?

Thanks for reading my long post and any help. If you have any questions or need pictures please don't hesitate to ask. I wouldn't put it past me to leave out some needed information.

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Old 09-23-2012, 04:38 PM   #2
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Shed subpanel


I am suggesting three #6 copper wires (#10 ground) for the 120/240 volt feed, which will allow 55 amps at 240 volts. But if you are considering electric heat or air conditioning out there I would make that #4 wire for 75 amps (#8 for the ground). Must be wet area rated, such as THWN, even in a conduit. Or use 6-3 (or 4-3) copper direct burial cable.

If you use a 100 amp breaker at the main panel for the shed feed then you would need #2 copper or #O aluminum feed wires.

3. The shed subpanel must have a "main" switch or breaker at the top, or you would need another box with a master disconnect for the new feed entering the shed complex. This is in addition to the breaker set back at the main panel. The wiring as you described it is good.

4. Yes, you need two eight foot ground rods at least 6 feet apart at the subpanel, in addition to the ground wire going back to the main panel.

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Last edited by AllanJ; 09-23-2012 at 04:52 PM.
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Old 09-23-2012, 06:21 PM   #3
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Shed subpanel


Thanks for your help AllanJ.

Thanks for bringing up the A/C. Yes, long term I'd like to build a permanent workshop with A/C to replace the two sheds and simply reuse the same subpanel.

What would be your preference for the 120ft run? Running it through conduit or using UF cable? Looks like it would be cheaper to use UF cable. What depth does the conduit need to be run, or the UF if I go that route? I guess I'd need to call the city? I live in central Florida.

Thanks again!
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Old 09-28-2012, 10:04 PM   #4
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Hi guys. Making progress... purchased all the conduit and fittings today. According to NEC I have to use 1" conduit for four #4 conductors. That's going to be a big hole through my wall! Should be purchasing the wire next week and getting it done that weekend. Thankfully I shopped around instead of dealing with Home Depot and Lowes... local electrical warehouse has #4 for almost $0.50/ft less!

I am struggling to find a suitable load center, but I've only looked at Home Depot and Lowes. They have 70A load centers but they only have one or two spaces in them and I'd like at least six. They have a very nicely priced Square D 100A with 10 spaces. I thought about purchasing this and a replacement 60 or 70A main breaker for it, but they don't sell main breakers to substitute it out apparently. Is it safe to use a 100A subpanel on my #4 feed wires if my feeding breaker in the main panel is 60 or 70A? Theoretically it should trip if the total pull is greater than 60/70 correct? I don't really like this option though... even if it is safe, it seems a bit confounded. Open to suggestions!
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Old 09-29-2012, 05:46 AM   #5
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You can buy a 100A main breaker panel. The protection for the wiring will be in the main panel so the main breaker in the sub panel will act simply as the required disconnect.

Why are you running 4 #4's? You only need 3 #4's and a #8 for the equipment ground.
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Old 09-29-2012, 06:20 AM   #6
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Sorry, I'm running a #8 for ground.

Thanks for the clarification. I don't really like it, but nice to know I can go with the 100A subpanel if I can't find a suitable 70A one.
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Old 09-29-2012, 06:23 AM   #7
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You will not find a 70A main breaker panel. You could buy a main lug panel and feed a 2P 70A breaker (not using the lugs in the panel), but many times that is more expensive than just buying the 100A main breaker panel.
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Old 10-01-2012, 07:16 PM   #8
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Ordered the wire today! Excited to get going. I'm not renting a trencher, will start tomorrow and try to trench 20ft or so a night after work.

I discovered my well was being serviced by romex in the ground from the back of the house -- a whopping 4-5" deep. Since I will be trenching alongside the well run I've decided to run a second conduit and 10AWG THWN wires to replace the romex. As I checked out the connection coming out of the house I noticed it only had two conductors and was 240V. Checking the breaker in the main panel, the white cable was being used as the 2nd pole. I thought 240V circuits required two hots, a neutral, and ground as what I'm running for my subpanel?

The neutral is used as a potential difference between the hot so current can come back, correct? In a 240V circuit the two hots are fluctuating (frequency?) between positive and negative so there's always a potential difference for the current to return back. In the subpanel scenario, there might be 120V circuits that require a neutral, which is why the neutral run to the main panel is required? Am I correct in my understanding?
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Old 10-01-2012, 07:57 PM   #9
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Quote:
I thought 240V circuits required two hots, a neutral, and ground as what I'm running for my subpanel?

The neutral is used as a potential difference between the hot so current can come back, correct? In a 240V circuit the two hots are fluctuating (frequency?) between positive and negative so there's always a potential difference for the current to return back. In the subpanel scenario, there might be 120V circuits that require a neutral, which is why the neutral run to the main panel is required? Am I correct in my understanding?
240V can be 2 wire as well if it is a balanced load. Electrical motors are the most common example of a balanced load. You are right that in a 3 wire 240V the unbalanced load is carried on the neutral. Ranges and dryers for example are an unbalanced load.

With your shed, yes that would be correct. If you only ran a 2-wire, you wouldn't be able to get 120V and 240V. If you look inside your panel, voltage red to black will be ~240V and voltage black to white and red to white will be ~120V.

ETA: Id like to add this for clairfication. Remember that AC power and DC power operate differently. Positive and negative is DC. I have an example. Say on Black (L1) you have a load of 10A. On Red (L2) you have a load of 5A. The current on the neutral isn't 15A, it's 5A. This only works on 120/240V single phase systems (like at your house). You can apply this to a larger scale as some wires in your panel are under the black line and others are under the red line. Since it is impossible for your panel to always be balanced, you need a neutral.

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Old 10-01-2012, 08:19 PM   #10
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Thanks. I wanted to clarify because I was afraid they might have somehow rigged it based off the fact that they ran romex out to the well underground!
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Old 10-01-2012, 08:25 PM   #11
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Originally Posted by brandondavie View Post
Thanks. I wanted to clarify because I was afraid they might have somehow rigged it based off the fact that they ran romex out to the well underground!
Yeah, the romex ran there is a serious problem. At least you found it now can make it safe. The is cable like the USC stuff you were looking at, but it needs to be way deeper then that.
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Old 10-01-2012, 10:49 PM   #12
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Originally Posted by brandondavie View Post
Sorry, I'm running a #8 for ground.

Thanks for the clarification. I don't really like it, but nice to know I can go with the 100A subpanel if I can't find a suitable 70A one.
There's really no reason not to like it. It's standard practice to use the main breaker in a subpanel as a disconnect only, and feed it from a smaller breaker. In fact, it's a little bit better this way. If you have a 70A feeder breaker and a 70A breaker in the subpanel, either one might trip on an overload and you wouldn't know which one to go check and reset. With a 70A feeder breaker and a 100A "main" breaker, you know that the feeder breaker is the one providing overload protection and it will be the one that trips in case of a problem. The 100A breaker acts only as a manual disconnect. Trust me, it's not worth ANY amount of effort or cost to do it differently. This is how it's done.
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Old 10-02-2012, 06:50 AM   #13
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Fair enough. I'm pretty much stuck with it anyway; I checked at the electrical warehouse I bought the wire at yesterday and they didn't have what I was looking for either. I'll pick up the $50 100A 10 spot panel at Home Depot later this week.

When I connect the two ground rods to the ground bus in the subpanel, I use #6 bare correct? I have two clamps and can clamp them to both in a continuous run. I guess it needs to be in its own conduit so I will have two conduits feeding the subpanel up through the shed floor, one with the 4 THWN wires and a smaller one with just this ground in it?
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Old 10-02-2012, 08:13 AM   #14
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The #6 ground wire (grounding electrode conductor) must go non-stop from the panel to at least one ground rod.

It must be protected from physical damage but does not have to be in a conduit assembled and attached per the NEC rules for current carrying conductors.
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Old 10-03-2012, 07:43 PM   #15
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Bought the remaining supplies today and trenched 30ft. Trying to trench a little each evening after work so this weekend doesn't hit me too bad.

I've attached a picture of the HomeLine 100A subpanel I bought. I think it was a great deal: good brand, only $50 and it includes five 20A breakers in addition to the 100A main breaker.

My question is in regards to the neutral. In the attached picture I have two red arrows pointing to what I believe are the neutral bus bars. They are both neutral, correct? My hesitation comes from the instructions where it says screw the provided bolt through a hole in the bottom silver bar joining the two neutral bars to bond the neutral and ground together. That makes me think that one bar might be ground and the other bar might be neutral, but I doubt it. I bought the specified ground bus bar kit and installed at the top of the box. Do you think this is the optimal place for it? Most of the runs will be coming in from the bottom of the panel (including the feeder cables). Putting the ground bus bar on either side doesn't make much sense to me because then circuits on the other side has to go all the way around the box.

I want to stress again that I will have my brother-in-law (electrician) verify all the wiring before any breaker gets flipped. He is already set to come out Sunday afternoon. I just want to learn.
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Last edited by BrandonD; 10-03-2012 at 07:46 PM.
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