Would running a new subpanel in parallel meet NEC requirements? If not, what should I do?

Question

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?

Answer 1

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:

_{(source: amprobe.com)}

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.

Answer 2

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.

Answer 3

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.

Answer 4

Your concept is correct - you can daisy-chain any number of subpanels off a feeder breaker, with no additional breakers, as long as the wire and panels can handle the feed breaker ampacity.

However, your plan to attach 2 wires to 1 lug is not acceptable. You can never do that, unless the labeling/instructions for that lug specifically tell you it's OK. Which isn't very likely!

"So what do I do, then?" Use a 3-void Polaris connector, MAC Block connector, or big wire nut (if the wires are in the nut's range). Join together a) the supply wire, b) the onward-to-the-next-panel wire, and c) a pigtail to this panel's bus. It's not much different than pigtailing a receptacle, really, except you have 2 hot wires.

You don't need this on the neutral bar; most neutral bar voids will cheerfully take a #8 or #6.