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A couple of years ago I had a licensed electrician add a second 100 amp load center to my home (the original 100 amp load center, still in use, could accept no additional breakers).

He fed the new load center from the meter. The meter is CL200 rated.

Conductors for each panel come from the meter base. I pulled the new panel cover and see conductors connected to the main lugs going into a conduit connecting the new panel and the meter base box. Both panels are in the same structure, a single family house. The original panel is in the garage; the new panel is just outside the side door to the garage - say 5ft apart. Both panels are main breaker types. The new is a Siemens PW2020B1100 w/copper bus in type 3r enclosure. The wire size between the new panel and the meter base is 3/0.

Can I assume that the electrician verified that the meter and base could support two load centers?

  • I would not assume even thought he should have, he may have expected you to coordinate with your utility company for service increase – spicetraders Dec 25 '16 at 18:05
  • I believe that there is no question that the meter can support 200 amps of service because I think all the meters for residential service are the same. At least I think the meter for my 150-A panel is the same as the meters in the 200-A service in the neighborhood of larger houses next to me. Usually one adds a "subpanel" which is connected through a double pole breaker in the original, now "main" panel. Are you sure that this is not how the second panel is connected, rather than a parallel connection to the meter? – Jim Stewart Dec 25 '16 at 18:09
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    The question is whether the supply wire to your building can handle it, and whether it even needs to. That is a question for the power company. – Harper Dec 25 '16 at 18:12
  • The power company would be replacing the meter right away if it was undersized. Most of the time a licensed electrician would contact the utility to verify the feeder / Transformer can handle the additional load and a permit pulled for the new panel. Almost all the time when a new service to the meter is installed the meter is pulled to kill the power. This requires breaking the seal on the meter and many times the power company will verify the meter is still in calibration or replace the meter with a meter that has been calibrated. I would not be concerned. I agree most meters are the same – Ed Beal Dec 25 '16 at 18:15
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    I've learned that meter ampacity doesn't always indicate meter base ampacity. So, I can see that the answer to my question lies with the power company. Thanks for everyone's help. – Frank Dec 25 '16 at 18:48
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100 + 100 ≠ 200

First off -- you do not need a 200A service to feed two 100A panelboards, provided the total load as determined by NEC Article 220's calculations does not exceed the service ampacity. This is a consequence of the how 230.90(A) Exception 3 interacts with 230.40 Exception 2 and your specific setup (bold running text mine for emphasis, italics theirs):

230.90 Where Required.

Each ungrounded service conductor shall have overload protection.

(A) Ungrounded Conductor. Such protection shall be provided by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor. A set of fuses shall be considered all the fuses required to protect all the ungrounded conductors of a circuit. Single-pole circuit breakers, grouped in accordance with 230.71(B), shall be considered as one protective device.

Exception No. 1: For motor-starting currents, ratings that comply with 430.52, 430.62, and 430.63 shall be permitted.

Exception No.2: Fuses and circuit breakers with a rating or setting that complies with 240.4(B) or (C) and 240.6 shall be permitted.

Exception No.3: Two to six circuit breakers or sets of fuses shall be permitted as the overcurrent device to provide the overload protection. The sum of the ratings of the circuit breakers or fuses shall be permitted to exceed the ampacity of the service conductors, provided the calculated load does not exceed the ampacity of the service conductors.

Exception No.4: Overload protection for fire pump supply conductors shall comply with 695.4(B)(2)(a).

Exception No.5: Overload protection for 120/240-volt, 3-wire, single-phase dwelling services shall be permitted in accordance with the requirements of 310.15(B)(7).

(Note that exception 5 has to do with the 310.15(B)(7) allowances for residential service conductor sizing -- they need to be taken into account when determining the total ampacity of your service and the ampacity of your individual sets of service entrance conductors, but stop there.)

A tale of a misplaced panelboard

But, there's more! While what the electrician did (putting a second main panel on your service) would have been OK when done properly as it would fall under 2014 NEC 230.40, exception 2 (bold running text mine for emphasis, italics theirs):

230.40 Number of Service-Entrance Conductor Sets.

Each service drop, set of overhead service conductors, set of underground service conductors, or service lateral shall supply only one set of service-entrance conductors.

Exception No.1: A building with more than one occupancy shall be permitted to have one set of service- entrance conductors for each service, as defined in 230.2, run to each occupancy or group of occupancies. If the number of service disconnect locations for any given classification of service does not exceed six, the requirements of 230.2(E) shall apply at each location. If the number of service disconnect locations exceeds six for any given supply classification, all service disconnect locations for all supply characteristics, together with any branch circuit or feeder supply sources, if applicable, shall be clearly described using suitable graphics or text, or both, on one or more plaques located in an approved, readily accessible location(s) on the building or structure served and as near as practicable to the point(s) of attachment or entry(ies) for each service drop or service lateral, and for each set of overhead or underground service conductors.

Exception No.2: Where two to six service disconnecting means in separate enclosures are grouped at one location and supply separate loads from one service drop, set of overhead service conductors, set of underground service conductors, or service lateral, one set of service-entrance conductors shall be permitted to supply each or several such service equipment enclosures.

Exception No.3: A single-family dwelling unit and its accessory structures shall be permitted to have one set of service-entrance conductors run to each from a single service drop, set of overhead service conductors, set of underground service conductors, or service lateral.

Exception No.4: Two-family dwellings, multi-family dwellings, and multiple occupancy buildings shall be permitted to have one set of service-entrance conductors installed to supply the circuits covered in 210.25.

Exception No.5: One set of service-entrance conductors connected to the supply side of the normal service disconnecting means shall be permitted to supply each or several systems covered by 230.82(5) or 230.82(6).

However, your electrician screwed up when he put the second panel on the outside of your garage, as that ruins the grouping required by 230.40, exception 2 and 230.72(A):

230.72 Grouping of Disconnects.

(A) General. The two to six disconnects as permitted in 230.71 shall be grouped. Each disconnect shall be marked to indicate the load served.

So, in any case, you'll need to have the electrician move the second panelboard inside to a spot next to where the first one lives, or replace the first panelboard with an exterior unit that can be mounted next to where the new one lives.

  • I'll be adding more to this answer once we get enough info to do a set of Article 220 calculations for Frank's situation. – ThreePhaseEel Dec 26 '16 at 3:05
  • I assumed the electrician had the work inspected and passed by the city. The original panel is in the garage next to the door out of the garage exiting to the the side of the house; the new panel (and the meter) are immediately to the right as one exits. Perhaps the city considered that sufficient grouping. I'll check with city inspectors. I'll keep y'all updated. – Frank Dec 26 '16 at 3:50
  • Re this case of a panel located on the exterior wall of the garage: My sister's very well designed and built tract house the electric panel is a two part affair in the wall of the attached garage. An inside panel contains only 120-V 20-A breakers with no main; outside under the meter (a 30 ft walk away) an outside panel contains a main breaker and breakers for all the 240-V appliances. Why would one do it that way? In the event of an emergency with the dryer or kitchen range one would go into garage, open the panel, see no 240-V breaker, realize/remember it is outside, activate garage door – Jim Stewart Dec 26 '16 at 11:38
  • @JimStewart -- the inside panel is a subpanel in the case of your sister's house, I bet... – ThreePhaseEel Dec 26 '16 at 12:20
  • OK now that you say it, I realize that the inside one is a sub-panel. But why would you want to have the main panel outside where to shut off power one has to fight through shrubbery, and fiddle with a tricky box closure maybe in the dark, cold, and rain? And have electrical components more exposed to the elements? – Jim Stewart Dec 26 '16 at 12:57

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