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Background

  • U.S.A. Voltages/power present.
  • My house has a 125A Power Mains breaker at the street.
  • A buried line runs from the street breaker to my main panel on exterior wall outside of house.
  • The main panel has 3 220VAC breakers: a 30A, 40A, and 50A.
  • I have proven the 30A runs to a 3-prong 220VAC Electrical Dryer plug.
  • I have proven the 50A runs to the existing subpanel in the attached garage which has 11 breakers ranging from 15A to 25A (This panel is full).
  • I presume the 40A circuit runs to the A/C compressor unit.
  • The existing subpanel in the garage has two conduits (or knockouts), a small one which contains the power feed from the 50A breaker outside, and a large one which contains the wire bundle that services the fixtures and outlets in the house.
  • The power feed line in the existing subpanel is wired as follows:
    • L is landed on the L bussbar terminal screw (The bussbar is fully populated with breakers)
    • N is landed on the N bussbar terminal screw
    • G is uninsulated, and (oddly) not landed anywhere I presume this isn't to code.

Question

I need to add a new (additional) subpanel in the garage for additional breakers.

Assuming this is up to code, I intend to parallel the new subpanel with the existing one, by:

  • Properly landing the unlanded G in the existing subpanel
  • Landing a new L line which will supply the new subpanel on the same bussbar terminal (screw) in the existing subpanel that the existing L line is landed. This new line will run to the subpanel as a power supply line.
  • Landing a new N line the same way.

If I parallel a new subpanel this way, can I route the tie-in to the new subpanel through the larger knockout which contains the wiring to the outlets and fixtures, or must I route the tie-in to the new subpanel through the same small conduit that supplies the existing subpanel?

What parts of this plan are not up-to-code (assuming NEC 2008, or 2011)?

Regardless of code, what parts are unsafe?

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Is the garage attached or a separate building? If separate, and the garage panel has its own ground (typically a metal stake driven into the ground somewhere near the wall with the subpanel), it may be appropriate to leave the G(round) connection unconnected. I'm not qualified on this detail, so you may want to post additional details to help others: detached or attached garage, brand of panel, etc. –  TomG Oct 17 '11 at 1:45
    
The rules for grounding subpanels have changed over time, and maybe by locale, so it can be tricky. Your AHJ can answer that specific question. It may be that your ground wire is connected in a way you haven't noticed. –  Jay Bazuzi Oct 17 '11 at 6:04
2  
Make sure your additions don't exceed the 50amp limit. It's no fun finishing a project like this, only to discover that a little math up front would have saved a big headache in the future. –  BMitch Oct 17 '11 at 11:26
    
@BMitch This is a good point. I plan on doing a load analysis of my existing circuits. Paralleling a subpanel off the 50A line is "Plan A," if my capacity allows the new circuits. If not, "Plan B," not mentioned above, is to add an new 50A 220VAC breaker to the main outdoor panel, and need the new panel directly from there, rather then off the existing 50A feed. This would require running new lines between the outdoor panel and the existing subpanel, which will be difficult due to their distance and poor attic access. –  JeffG Oct 17 '11 at 11:38
    
Some pictures of exactly what you plan to do might be helpful. Open up both the main and sub panels, and snap pictures. label each breaker with it's size, what it feeds, and it's typical draw (so it is clearly visible). –  Tester101 Oct 18 '11 at 11:16

4 Answers 4

up vote 10 down vote accepted

A sub panel must have the neutral and ground isolated. Panels come with a very long, rather thick (about 1/4 x 20) green bonding screw that connects the neutral bar to the can in the case of a primary panel. You don't get a neutral from your utility, you create one with that bonding screw.

Sub panels should be fed with 3 insulated conductors of appropriate size, and a ground that need not be insulated (but can be, if you want). So the first part of your question is, yes, that sub panel must be grounded, but the grounding conductor should be attached to the can using a ground lug, not by landing it to the neutral bar.

Sub panels must also have a fused disconnect, which means they need to be fed from a breaker, and there can't be anything else on that breaker. Don't double tap. You have two options here:

  • Increase the size of the existing sub panel
  • Put in a new double pole breaker in the existing sub panel and use that to feed your new sub panel. Land the circuits you had to pull out of the existing sub panel into the new one.

As others have noted, you need to watch your loads. If all you need are a few convenience receptacles or a lighting circuit, you should probably be o.k. (hard to tell with what you've given).

Either way, get an amprobe and look at what each incoming phase is pulling in your existing sub panel prior to doing anything. Make sure everything is on when you do. If it's only pulling 25 - 30A on average, you should be o.k. to add a small 8 circuit sub panel. Since it's directly in the line of sight with the existing sub panel, the new sub panel need not have a main breaker since the means of disconnect is right next to it.

A six to eight circuit sub panel runs about $80 without breakers, they typically start at 50A, but you don't have to feed them with 50A. You could feed it from a 30A breaker if all you want are convenience receptacles and lights.

Here is an amprobe being used:

enter image description here

Do that on your sub panel first (one phase at a time) just to be sure you have room to add more. If not, you need to replace your existing sub panel, and an electrician is really your best bet there.

Another good thing to do is measure the draw of the circuits you'll have to move to the sub panel in order to make room for the breaker that will feed it. Obviously, you want to move the circuits drawing the least to the new panel in the end. Some re-arranging might be needed to make that happen.

Since this is a garage, take care what you connect to the sub panel. If you are going to be powering something like a compressor (or anything else with a decent sized motor), carefully consider the locked rotor amperage when determining the load. It will be printed on the motor.

Finally, if any of this sounds overwhelming, call an electrician. If you get into any kind of trouble, call an electrician.

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2  
Great comments. + vote. I forgot to mention that the sub panel must be fed by a dedicated double pole breaker and bonding. Thanks for the better detailed answer. –  shirlock homes Oct 18 '11 at 21:37

I'm not sure i fully understand exactly what you intend to do from the sub-panel in the garage, but I think I understand that you want to parallel off the existing input lugs of the 50 amp breaker to an additional panel. If this is the case, what you are contemplating is called double tapping and is forbidden. You may not connect two wires to any hot lug in any power panel. In smaller size circuits, slicing two hots together with a tag wire with a wirenut and connecting the tag wire to the breaker is OK and common. Doing this with a 50 amp line would require large bung-nuts and insulating tape.

From your description, it sure sounds like your 125 Amp service is getting pretty maxed out. You have three large draws already. Before I would even consider adding more demand to this service, I think you should use an Amprobe on the incoming two line feeds and monitor the peak demands in amps. It is very possible you may too close to the upper limit and should consider an upgrade to a 200 Amp 40 circuit service/panel.

At this point, since I cannot physically test your system, I think it would be very wise in this circumstance to call in a licensed electrician. You are flirting with a potentially dangerous situation if you substaintually overload your main or sub-feed breaker.

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Actually, I was planning on connecting the new sub panel to the hot lugs in the existing subpanel, not the main panel where the 50A braker is, but to the lugs where the 50A braker goes. –  JeffG Oct 17 '11 at 22:38
    
I was going to use L1 and L2 on my Dryer's 220VAC circuit for this, but It's 3-prong, so I have no neutral to make two 120 circuits. The new subpanel will drive (ahem) my Christmas Lights. So the load shouldn't exceed 11000 Watts, and is both seasonal, and runs only after dark for a few hours. I had committed to not drying any clothes during the light show. –  JeffG Oct 17 '11 at 22:41
    
As far as safety, I don't understand what is unsafe about any of this, so long as all breakers' max loads don't exceed the max load of the wires. AFAIK, the breakers will trip before the wires get to hot. –  JeffG Oct 17 '11 at 22:43
1  
I would second shirlock's suggestion of checking the existing load using a clamp-on ammeter (most are decent multimeters and are <$100, and for the work you're doing a good multimeter is an essential tool anyway), just to see exactly what your baseline is now anyway (check a few times, while different things are running). Keep in mind future buyers, too: If I was buying the house, having 3 sub-panels and only 125A service would be a warning flag for me. –  gregmac Oct 18 '11 at 16:18
3  
Check out Tim Post's answer. He is absolutely correct about having to feed a new sub panel from a dedicated double pole breaker. Also bonding the ground is essential. Gregmac also has great points, if I inspected a home with three panels, I would wave red flags to my client. –  shirlock homes Oct 18 '11 at 21:34

You're mucking around with 125V subpanels, which I don't know enough about to comment on. However, here are two other options:

You may be able to replace existing full-size breakers with duplex breakers. The limits on the number of breakers may be printed on the panel somewhere, or you can check with the manufacturer.

You can also replace the existing panel with a larger one, to make more room.

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Both good ideas. I should have, however given more physical details to accompany the technical ones... The wall section the existing subpanel is on is narrow, and there is no space to install a larger one. The existing subpanel has 6 'slots' on the bussbar for breakers. I have 11 breakers. 5 are already duplex. Even if I replace the single remaining simplex breaker with a duplex breaker, I will not get enough new circuits ( i need 5 ). –  JeffG Oct 17 '11 at 11:34

You should call a qualified electrician to inspect this situation before doing anything.

If you are going to use an amp meter, make sure its rated at CatIII 600VAC and has some certification like UL or IEC on the meter.

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Welcome to DIY. Thanks for answering the question but it seems it was answered last year and accepted. A comment would have been more appropriate to express you concern.We do encourage updating answers over time if the accepted one is out of date. please visit meta if you have any questions. –  ppumkin Aug 7 '12 at 8:51

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