Extending a Circuit to Cabin - Subpanel

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Old 10-23-08, 03:51 PM
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Extending a Circuit to Cabin - Subpanel

Hello all,

I am adding a new circuit to my main breaker box to feed power to a new cabin in my backyard. I have already buried 10 gauge 2 phase cable to supply power to the cabin. My maximum draw will be approximately 20 amps; I've purchased a 2-pole 30 amp breaker for my main box, since I hope the extra capacity won't hurt. In the cabin I have a Square D 100 amp subpanel; I'd like to have the power come into a 30-amp 2-pole main breaker, then have 2 single pole 20-amp breakers for 2 circuits inside the building.

An electrician at Home Depot suggested I run power this way: at the main box, wire both the hot and neutral (black and whilte) wires from the new circuit into the 30-amp 2-pole breaker, wire the ground to the ground bus bar. In the cabin, wire the hot and neutral (black and white) into the 30-amp 2-pole breaker, wire the ground into the neutral bus bar (there is not a separate ground bus bar, although the ground bonding screw is installed). Then wire each 20-amp circuit with the hot (black) wire to the single pole breaker, the neutral (white) and ground to the bus bar.

Is this correct? Should I have separate ground and neutral bus bars? Is connecting the hot and neutral wires to the 30-amp 2-pole breaker on either end safe? Thanks in advance for your advice.
 
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Old 10-23-08, 04:02 PM
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If I understand you correctly, you do not have a red wire running between the buildings? Just a black, white and bare inside of a #10/2g cable?

Also please let us know:

what state you are located in

how much distance between the main panel and the subpanel

exactly what type of cable did you bury

are there any other utilities between the main building and the cabin which are metal such as water pipe, phone wire, cable tv, etc?

what kind of appliances/loads you plan on using in the cabin
 
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Old 10-23-08, 04:18 PM
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Originally Posted by ibpooks View Post

what state you are located in

how much distance between the main panel and the subpanel

exactly what type of cable did you bury

are there any other utilities between the main building and the cabin which are metal such as water pipe, phone wire, cable tv, etc?

what kind of appliances/loads you plan on using in the cabin
- I'm located in WA
- appoximately 100' from main to sub panel
- Direct bury cable, 10 gauge, 2-phase (black, white, ground...no red cable)
- no utilities between house and cabin (cable is already buried 24" deep, ok I did hit 1 irrigation pipe during trenching, but went around it)
- max load will be just under 20 amps total

Initially I had planned to simply add a new circuit (there's room in my main panel) and extend it to the cabin. But I want to have 2 circuits inside the cabin, so I separate the lights and outlets. Hence the Home Depot guy's suggestion to bring in 220v (if I go from double-pole to double-pole breaker with both the black and white wires, does this mean it's 220v?) then have 2 20-amp single-pole breakers within the subpanel.
 
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Old 10-23-08, 04:23 PM
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He lives in Washington state.

WOA: don't listen to Big Box personnel when it comes to electrical matters.
 
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Old 10-23-08, 05:17 PM
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With very few exceptions the personnel at big box mega mart home centers are notoriously inaccurate with the information they so freely pass out among the unsuspecting customers.

There is no such animal as "2-phase" cable. The two (insulated) conductor cable you have is not sufficient to use for a 240/120 volt feeder to the cabin. Furthermore, unless this cable is type UF (underground feeder) it is not suitable for burial and likely not even suitable for use outside.

You need a FOUR wire (three insulated and bare equipment ground) type UF-B cable between the house and the cabin. You need an approved ground rod (maybe two) at the cabin. You need a separate equipment ground bus bar in the circuit breaker panel in the cabin along with removing the "bonding" screw from the neutral bus in that panel. If you have a "main lug only" panel then you need a circuit breaker "hold down kit" to attach the 30 ampere circuit breaker you intend to use as a main breaker.

But most of all you need to read a few books on the subject BEFORE you do any work.
 
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Old 10-23-08, 06:09 PM
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Hi Furd,

Thanks for your reply. The cable is UF 10-2G, so it is buriable (and more importantly, aleady buried). So let's forget the HD guy: according to an electrical book I did read, this cable is sufficient for simply adding a 30-amp circuit breaker to my main panel and running it out to the cabin. I do have a newly-installed single ground rod kit already installed from the subpanel as well.

1. Is the existing UF-10-2G sufficient for a maximum draw of 20 amps? Total run is just under 100 feet.

2. If so, can I have 2 circuits inside the cabin (can I split the incoming circuit in 2), or am I limited to 1?

3. It sounds like I need a ground bus bar kit for inside the subpanel and I should remove the ground bonding screw, correct?
 
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Old 10-23-08, 06:24 PM
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You have two options with out redoing what you have. Use a single pole 20 amp breaker to supply lights and receptacles in the shed. At the entrance of the feeder you would install a junction box and a 20amp single pole switch. (A light switch would work.) From there you would run #12 NM (Romex) to lights and receptacle. You made need an AFCI if this will be used as a bedroom'

Alternate plan, more power, more flexibility.Use a 30 amp single pole (120v) breaker and a small subpanel at the cabin. Main down side on this is most subpanels are 2 pole 240v. One way around this is to connect two pigtails to the incoming hot wire then run one pigtail to each pole of the subpanel main breaker. Subpanel would need to be clearly labeled 120v only to avoid confusion in the future.
 

Last edited by ray2047; 10-23-08 at 07:31 PM.
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Old 10-23-08, 09:33 PM
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What Ray describes is completely workable. If you have not yet installed the 100 ampere panel in the cabin I suggest that you return it and instead get a switch rated at 20 amperes to use as the disconnect in the cabin and also get a 20 ampere single pole circuit breaker for use at the service panel in the house.

However, if you still want to use the 100 ampere panel in the cabin then you need to get the circuit breaker hold down kit and install it to hold down the 30 ampere circuit breaker you will use as the main breaker. At the house you want a single pole 30 ampere circuit breaker; DO NOT USE A TWO-POLE BREAKER IN THE HOUSE PANEL!

At the house panel you connect the black wire to the circuit breaker, the white wire to the neutral bus and the bare equipment ground wire to the equipment ground bus if there is one OR to the neutral bus if there is not a separate equipment ground bus.

At the cabin you will connect the black wire to one of the two connections on the 30 ampere circuit breaker (leave the other empty) and the white wire to the neutral bus. You will have to purchase a separate equipment ground bus for the panel and then connect the bare equipment ground to this new bus. You will need a #6 bare or green insulated wire to go from the equipment ground bus to the ground rod that is driven into the ground near the panel. [Note that #6 is way bigger than you would normally need for such a small amount of power but it is required for the ground rod when no other means of protecting the wire (such as conduit) is provided.]

You will also need a short piece of wire to connect the two main bus connections in the panel. This needs to be insulated and a color other than green, white or grey. The size will be dictated by the minimum size requirement of the main lugs and that will be printed on the label on the inside of the panel door. Don't forget to remove the bonding screw or lug from the neutral bus.
 
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Old 10-23-08, 09:51 PM
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Thanks Ray for the idea and Furd for the detail, this plan sounds acceptable (I know it's not ideal) for safety and capacity. If I go with the 2nd idea (a 30-amp breaker in the subpanel), then I can still add two single pole 15 or 20 amp breakers and run 12 gauge wiring for the circuits inside the cabin, correct? Or am I limited to a single circuit within the cabin to the subpanel?
 
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Old 10-23-08, 10:02 PM
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Originally Posted by yagottabekiddin View Post
Thanks Ray for the idea and Furd for the detail, this plan sounds acceptable (I know it's not ideal) for safety and capacity. If I go with the 2nd idea (a 30-amp breaker in the subpanel), then I can still add two single pole 15 or 20 amp breakers and run 12 gauge wiring for the circuits inside the cabin, correct? Or am I limited to a single circuit within the cabin to the subpanel?
Depending on the load on each circuit you might even be able to use more then two breakers.
 
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Old 10-23-08, 11:46 PM
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You will be limited to only 120 volt circuits and a maximum of thirty amperes non-continuous. But other than that you can install as many 15 or 20 ampere circuit breakers as the panel will hold.
 
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Old 10-24-08, 03:18 PM
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Hi Furd,

When you said:

Originally Posted by furd View Post
You will also need a short piece of wire to connect the two main bus connections in the panel. This needs to be insulated and a color other than green, white or grey. The size will be dictated by the minimum size requirement of the main lugs and that will be printed on the label on the inside of the panel door. Don't forget to remove the bonding screw or lug from the neutral bus.
...do you mean the neutral bus bar to the newly-installed ground bus bar? My panel has 2 main lugs but no other main busses (other than the neutral and the ground).
 
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Old 10-24-08, 03:45 PM
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The subpanel has lugs for two hot wires and you will only have one black hot wire. What to do is to cut two 6" pieces of the black #10 wire and wirenut them to the incoming black wire with a large wirenut. You then have "split" the incoming hot to both lugs of the subpanel. This makes the panel 120V-only, but it allows you to use all of the breaker slots in your subpanel.

The incoming white wire goes to the isolated neutral bus and the bare ground wire goes to the ground bus which should be securely fastened to the metal subpanel box.
 
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Old 10-24-08, 05:40 PM
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Originally Posted by ibpooks View Post
The subpanel has lugs for two hot wires and you will only have one black hot wire. What to do is to cut two 6" pieces of the black #10 wire and wirenut them to the incoming black wire with a large wirenut. You then have "split" the incoming hot to both lugs of the subpanel.
But I'm not wiring the incoming hot (black) wire to the lugs; I'm wiring it to one side of the 2-pole breaker. Should I split it first, then have 1 wire in the breaker and the other into a lug on the panel? Or wire both into the breaker?
 
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Old 10-24-08, 08:05 PM
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Ah yes, after the split run one wire to each pole of the 30A back-fed breaker. This will provide power to both "sides" of the subpanel.
 
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Old 10-24-08, 10:20 PM
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Originally Posted by ibpooks View Post
Ah yes, after the split run one wire to each pole of the 30A back-fed breaker. This will provide power to both "sides" of the subpanel.
Just so the OP isn't confused this is one of two ways that will work. Furd's suggestion of using only one pole of the breaker and joining the two bus bars for the hot side with a jumper between the lugs works equally well. That can be the beauty and confusion of wiring: There are multiple ways to remove the outer covering of a cat.
 
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Old 10-24-08, 11:47 PM
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Thanks to everyone for their help. I know what I'm doing this Saturday...
 
 

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