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Old 10-11-2007, 06:20 PM   #1
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Adding a subpanel to a sub-panel and other questions


Just found this site and have a few questions.

I recently had a new metal building built that will be both garage, RV storage, shop and office. I had a licensed electrical contractor do the initial wiring to bring service from my house to the building.

Here is what he did:
  1. He installed a 70a breaker in the main panel in the house.
  2. Ran three wires to the building. Two hots and neutral.
  3. Installed a subpanel in the building.
  4. Installed another 70a breaker in the subpanel.
  5. Connected the two hot wires to the 70amp breaker and the neutral to the neutral/ground bar.
  6. Installed a rod outside the building and ran a ground wire to it.
  7. Installed one 20a circuit for a plug and one 20a circuit for a light with switch.
Since this is new construction, I'm pretty open to putting what I want and need in the rest of the building. I have a couple of questions now.

Questions:
  1. What are the pros and cons of using "slim" double breakers to get more lines in? Some have told be they are not as good because they can't handle the heat as well? I can double my circuits (subpanel has room for 8) if I go that route.
  2. Can I put two 220v heaters on one 20a circuit. My calculations say they should draw a little over 18a. Or should they be on a separate circuit? Or go to a 30a breaker?
  3. I've read the neutral and ground should be separated at a subpanel. If my memory is right (and I'm not at the building now to confirm), but I believe they are tied together at the subpanel. What are the exceptions to this rule? Did they do it right, or should I confirm with the electrician who did the work? It was inspected by the county and they passed it.
  4. Since this is a shop area, I'm going to put in LOTS of outlets. Not that they will be used at the same time, but to keep tools plugged in, no extension cords, etc. If I want to put in ANOTHER subpanel next to the original one for more circuits - can I do that? A local bulding supply chain (that rhymes with toes) said I could - but I'm not sure how the best, safest, and legal way to do it.
Any help is appreciated.

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Old 10-11-2007, 10:21 PM   #2
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Adding a subpanel to a sub-panel and other questions


You did not say how the feeder wires were installed out to the new building. Overhead or underground? It is a shame that the contractor put such a small breaker box in. Before I added another sub panel, and worried about using twin breakers (some people call them slims), I would turn off the breaker back at the house and remove the 8 circuit panel and replace it with a 20 circuit Main Lug Only (MLO) panel. Reuse the 70 Amp DP breaker as a main and have plenty of room for full sized breakers. Twins or slims are more money than regular breakers, besides getting twins for 220V (then called Quads) is tough and very costly.

As for the heaters and all other loads, you only load the breaker up to 80% of it's rating. So you will need a 30 AMP DP breaker [25 amp breakers are not to common]

As a sub panel from the house / main service, the neutral should not be bonded to the panel or ground rod, add a ground bar to your panel.

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Old 10-12-2007, 07:34 AM   #3
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Adding a subpanel to a sub-panel and other questions


If there are no other metallic paths between the two buildings (phone, cable, etc.) then the three wire feeder to the subpanel is acceptable and the neutral and ground bars should be bonded. Since this is a detached building, you MUST have a disconnect at the sub (you cannot use a MLO panel here). The installed grounding rod is correct, and the metal of the building should be bonded to this also.

There is no problem with adding another subpanel, but it might be better to swap out the existing with a larger one if you really need more circuits. Personally, I think 8 should be enough.

What is the wattage of the heaters?
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Old 10-12-2007, 09:38 AM   #4
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Adding a subpanel to a sub-panel and other questions


My tip to replace the existing panel with a 20 circuit MLO panel.... continues on to say... to use the 70 amp DP breaker as the main breaker.

The reason is, they do not sell a load center with a main breaker smaller than 100 Amps.

If the HO was to install an additional sub panel, he would have to buy a Double Pole breaker to feed the sub panel with plus the sub panel. It is cheaper and neater to just change out the panel with a 20 circuit MLO and use the 70 amp DP breaker as the main.
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Old 10-12-2007, 10:47 AM   #5
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Adding a subpanel to a sub-panel and other questions


Sub panels require a 4 wires and a main breaker. 2 hots 1 neutral and one ECG. The ground "ECG" should not be bonded to the neutral terminal strip. A lug panel cannot be used as the primary sub panel in this scenario. The additional sub panel can be a lug panel if it is in sight of the main subpanel and contains no more than (I don't know how many) breakers.

There is an exception, by making the sub panel a main service panel. But I am not certain the on the rule?

You can go to 100 amp lug panel and mount a 70 amp main breaker in it if you want. Just install the breaker as any other and connect the feeder wires to it. Mark it "Main Breaker"
Do not connect neutral and ground to together. You may need a ground terminal strip if one did not come with the panel. Your panel may have two terminal strips running horizontally along the sides of the interior. These are usually bonded from the factory. Just remove the the bar that connects the two and use one for grounds and one for neutrals.
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Old 10-12-2007, 10:59 AM   #6
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Adding a subpanel to a sub-panel and other questions


Quote:
There is an exception, by making the sub panel a main service panel. But I am not certain the on the rule?
250.32 (B)(2)

Allows a detached building to be fed with three wires and have the panel neutral and grounding bus to be bonded. There must be no continuous metallic paths between the buildings.

Sorry michael, I missed the part about reusing the 70A breaker. It is also permissible to feed a subpanel that contains a 100A disconnect from a 70A breaker at the main panel and still comply.
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Old 10-12-2007, 11:04 AM   #7
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Adding a subpanel to a sub-panel and other questions


Housekeep, you are correct about being able to use a 100 Amp main breaker being fed from the 70 AMP DP breaker back at the main house. The only problem I have with that practice is, if the main should trip, it would trip first back at the house and require going over to the house to reset the breaker.
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Old 10-12-2007, 11:17 AM   #8
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Adding a subpanel to a sub-panel and other questions


True, unless it is a dead short, then it's a crap shoot as to which would trip first.
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Old 10-12-2007, 03:55 PM   #9
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Adding a subpanel to a sub-panel and other questions


Can I chime in? Although this installation is code compliant according to 250.32(B)(2), there are dangers involved.... The OP and everyone who ever lives in this house MUST be sure there are never any continious metal paths between the two buildings. There, imho, are NO circumstances where I would install this sub this way. Anyone agree? That pesky "parallel neutral path" scares me....alot...
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Old 10-12-2007, 03:59 PM   #10
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Adding a subpanel to a sub-panel and other questions


I agree Andy, and I would not install a sub in a detached building this way. However, the OP asked if it was done correctly, and according to the NEC it was.
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Old 10-15-2007, 08:52 PM   #11
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Adding a subpanel to a sub-panel and other questions


Thanks for all the info. Sorry I didn't chime in earlier, but I've been out of town.

This is a new metal building built on another lot next to the one I currently own (that has the house on it). Since I built the house and now the shop, I can guarantee there is no path between the two buildings.

The service to the subpanel was installed underground with #4 aluminum wire. The reason I want more circuits is because I would like to have lots of outlets so I don't have to use cords or move equipment around. The load at any one time should be very small. Other than the heaters, and my table saw, the load should be fairly small. I guess I can put lots of outlets on one circuit, but thought this was a better way to go. I may rethink this after reading everyones comments.

Thanks much!
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Old 03-30-2011, 11:07 AM   #12
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Adding a subpanel to a sub-panel and other questions


Quote:
Originally Posted by HouseHelper View Post
250.32 (B)(2)

Allows a detached building to be fed with three wires and have the panel neutral and grounding bus to be bonded. There must be no continuous metallic paths between the buildings.

Sorry michael, I missed the part about reusing the 70A breaker. It is also permissible to feed a subpanel that contains a 100A disconnect from a 70A breaker at the main panel and still comply.
Hi HouseHelper, I have this setup at my home (200 amp main - 100amp sub in same dwelling, and detached garage w/ 60amp DPST breaker as main disco, bonded neutral+ground w/ground rod outside of garage), and I am looking for some clarification, since I've had so many conflicting answers on this topic, (to bond or not to bond the ground and neutral in the garage panel) and I would like to get a "final answer" on it.

1) When you referenced, "There must be no continuous metallic paths b/w the buildings", is this to be understood as, "There must be no continuous metallic paths b/w the buildings, OTHER THAN THE 3-WIRES"?

If YES, then,

2) What do I need to do if I DO have continuous metallic paths? (I have the 3-wire (hot-hot-Neutral) buried in plastic conduit from house subpanel to the detached garage (house subpanel has separate grounds and neutrals), and, I have another buried plastic conduit from house to garage - holding wiring for a 3-way switch and ADT Alarm wiring (power comes from garage back to 3-way switch in the house mudroom to control outdoor lanterns on garage) ?

Sorry so wordy, but I've learned that every little detail matters when it comes to getting electric right.

Thanks in advance


Last edited by Juneau; 03-30-2011 at 12:43 PM. Reason: Remove my email address
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Old 03-30-2011, 11:29 AM   #13
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Adding a subpanel to a sub-panel and other questions


Its good this thread is this old. 2007! If the OP had posted this exact question today and his locality is now on the 2008 code cycle or newer the install would not be compliant. The 2008 code removed the exception and a four wire feeder would have been required.

Juneau.
On a three wire feeder to a detached structure, the neutral, grounds and grounding electrode conductor (wire to ground rod) are all connected together. Exactly like the main panel.
You cannot have any other metal paths other than the three wires. Note: This was only allowed until the 2008 code cycle. Or you locale had not adopted the 2008 as of the install date.

Your only recourse is to remove the other metallic paths or update the feeder to four wires. I know of no exceptions as this a safety issue. Maybe someone will have some ideas, that will be more helpful to you..
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Old 03-30-2011, 12:11 PM   #14
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Adding a subpanel to a sub-panel and other questions


Thanks JV,

So, if I add the 4th wire:
What kind of wire? Does it matter if bare or insulated? Same gauge (#4)?
Can I leave the other buried run in place?
Does the bonding requirement in the garage panel change?
Does the tiny (18-22 gauge, 4-pairs?) ADT alarm wire count as a continuous metallic path?
I am considering removing only the 3-way light switch wiring. I need the security alarm more than the convenience switch these days.
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Old 03-30-2011, 02:23 PM   #15
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Adding a subpanel to a sub-panel and other questions


Quote:
Originally Posted by HouseHelper View Post
I agree Andy, and I would not install a sub in a detached building this way. However, the OP asked if it was done correctly, and according to the NEC it was.
That depends on which code cycle he is on. 2005 ok 2008 NOPE



WHO KEEPS digging UP these OLD threads arrrrrggggg! ahoy mate

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Last edited by Saturday Cowboy; 03-30-2011 at 02:25 PM.
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