I own both halves of a duplex (200 amps each) which I've converted into one house. I would like to have one energy bill. Is there a simple way I can easily feed service to the second panel from the first panel from one incoming meter/service?
Electrical – How to combine the two panels in a duplex on one meter
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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.
Lucky you
Your panels are set up very advantageously to put in a generator transfer switch. All the loads you want to switch (but one) are already in a subpanel. This makes this super easy. It's almost like somebody planned it... Except the guy who put the pump circuit in the main panel did not get the memo.
You need to put a different transfer switch in between the two boxes. There are a couple ways to do this.
One is fit the subpanel with a manual interlock switch. This is a listed (offically tested) modification to the subpanel where it puts two breakers opposite from each other with a sliding plate so they can't both be on. Simple and cheap, and you may even be able to retrofit your existing panel. Must be thrown manually.
The other way is to pick a spot along the cable between the panels (or elsewhere if you don't mind rerouting cable), cut the cable, and insert the transfer switch inline. This will work with any kind of transfer switch, including automatic. You'll need to cut the cable on one side with enough slack to work, and the other side will be too short so you will have to replace that "half" of the run. So choose location and cut point very carefully to your advantage.
That one breaker in the wrong panel
The stuff you are imagining, you cannot do anything like that. In that approach, the options are break the law, backfeed the grid and kill linemen; or spend a king's ransom on more heavy cables than the underside of an NYC subway car, in a veritable Gordian knot of transfer switch wiring that nobody will be able to figure out after the fact. And the power company and inspector will absolutely hate it.
The right way is easy, if annoying: move the circuit to the subpanel. Extend the pump circuit, all wires, to the subpanel and land it on a double breaker, ground bar, and neutral if used. Do not continue to use the ground in the main panel, in fact if you make this splice inside the main panel, tape the ground wire with green tape to insulate it from the main panel. It's not the end of the world if it grounds accidentally, but grounds must go to the same panel the hots do.
I know that's a pain, but it's way less of a pain than anything else you could do legally.
Not room in the subpanel?
This happens a lot, some guy drove back from the subpanel store slapping himself on the back for saving $30... by buying barely enough spaces. Don't be that guy. You're going to need 6 right off the bat - 4 for the transfer switch and 2 for the well pump.
Worst case you may need to replace the subpanel, and fortujately that is DIY-possible because you can entirely shut off the panel at the main breaker. Also a great time to be looking for transfer switch friendly panels if you want to go that way. Don't scrimp - slap yourself on the back for buying twice as many spaces as you need today. The bigger panels often come with "bonus breakers" which save you some money too.
Best Answer
Well, if you have 2x 200A service, feeding that from a single meter would require a "class 320" meter (320A continuous, which is 400A derated 80%, so it matches, even if it does not appear that way at first.)
The odds are (since those are a significant step up in expense for the meter can) that you don't have that for the meters you have now. So you'd be looking at replacing the meter, with any upgrades required for the currently adopted code in your area, which include adding an outside disconnect if it's NEC2020. This could be rather expensive, will require permits and (probably) professionals and the inconvenience of having no power midway through the procedure.
As such, it will probably take a LONG time to pay off if the thing you are trying to get rid of is the monthly service fee on the second meter. Combining the bills should be easy, but may not reduce that any - could not hurt to ask, though.