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Question DetailsAsked on 4/27/2017

Can I install a 100 amp breaker box in my garage coming from my 200 amp Meter box.

I have spaces in the Meter Box for breakers, I only have 1 space left in the main service panel in the house. Is it necessary for me to put in a earth ground on the new panel or can i attach the ground to the Meter Box?

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Different places have different codes and code amendments, but generally speaking - no (with provisos) you cannot tap a new breaker box directly off the meter panel for two or three reasons stated below depending on your local codes - clarify if needed with local electrical inspector at building department.

1) All electrical services have to have a disconnect at the power supply point - or within from 3-6 feet of supply cable run under different codes, so sometimes main breakers (which also qualify as disconnects for household voltage equipment) can be commonly located at the top of the distribution panel in panel so built (more of a 70's and 80's thing but some still are available that way). Commonly these disconnects are "main breakers" like the spaces it sounds like you have in the meter panel (which sounds like a combo meter/main breaker panel where the meter and breakers are actually in separate sections of the combined panel, usually with separate cover panel for the breakers). But can also be an automatic or manual disconnect switch like you commonly see on large shop/industrial equipment. And no, just pulling the meter does not count as a "disconnect" because you should have electrical insulated (1KV rated) gloves for that so not a homeowner thing to do anyway - plus many of meters (especially newer electronic ones) do not actually disconnect the power when they are pulled, they just tap into it to meter the usage.

2) Under many or maybe most codes you are required to have an appropriately sized "master breaker" protecting all distribution panels - which might be one in the meter box (assuming you have space for two 220/240V breaker pairs, which some do) or can be a master breaker on the distribution/breaker panel itself (shutting off power to the entire panel when tripped). Generally, for the master breaker to be located in the distribution/breaker panel (as noted above) the panel has to be within from 3-6 feet (depending on code provisions) of cable run from the meter base, so there is not a lot of unprotected power run to the panel.

Note some areas do NOT allow the master breaker to be in the distribution panel and I agree with that - because I can tell you from experience that if you have a fault/fire/short in the distribution/breaker panel you do NOT want to be opening a metal door and trying to trip off a breaker in a frying panel, so a remotely located main breaker is safer as it gets you entirely away from the problem area, which if the main breaker needs to be tripped is commonly in the distribution/breaker panel itself anyway. Also eliminates the section of main feed wiring from the meter base to the panel from being both unprotected by a breaker and unable to be shut off quickly in the event of a fault in that feed wire or its connections, plus outdoor main breakers get you out of the house in the event of a major fault - so potentially safer in the event it becomes a fire.

3) Whether you can add another 100A box (beit a secondary panel or a sub-panel off the existing one) in the garage depends on your current loads on the current distribution panel and on the incoming supply/meter panel rating, and in some areas on whether you have available taps or bus connection points in the meter box to come off of with another main feed. Some areas allow taps directly onto the main incoming feed wire or onto the wire coming out of the main breaker at the meter panel, some do not.

I do not like taps onto the wires themselves for reliability and corrosion/current erosion reasons that have caused some burnouts and arcing that I have seen - I prefer ONLY bus taps where each incoming service lead goes to a compression clamp or set screw (not as good) bus bar, from which any feed to distribution panels then come off that bus. Bus bars can be added to some model boxes with enough room, not to others, and some power companies do not allow taps/breakers in the meter panel because they do not want the public messing with their incoming service and meter.

If not feasible or allowed, then it is very common to run a main lead "jumper" from the meter box to an adjacent main breaker panel with one or two main breakers (sometimes more for large demands or large properties with guest house, pool, outdoor hot tub, etc) which then have a main feed to a secondary breaker/distribution panel at each usage area. If putting in an adjacent main breaker panel I recommend AGAINST using the type that connects directly to the meter panel - pay the extra $10-20 and have a separate box put a foot or so away - avoids issues with the power company, and if one has a burnout or arcing then it does not damage the other.

Ditto to split taps off main breakers - legal in some areas and there are even breakers made for multiple tap connections directly on them, but not as safe as separate breakers.

There is also a basic safety issue involved in multi-tapping a breaker or its feed - the breaker has to be large enough to handle the normal loads coming off it, but if multi-tapped that then means that breaker is signfiicantly oversized for each of the multiple panels tapping off it, so your protection level is significantly decreased. That is also what makes it illegal in many areas.

For instance - assume you want a total of two 100A rated distribution/breaker panels which may both carry significant load at the same time - say they are serving a house and an adjacent guest or servant or in-law apartment for instance. Normally the main breaker for each of those panels would be 100A - but connect both off the same main breaker or main feed wire, and you would need 150-200A (depending on the results of the combined load utilization calculation) main breaker there to prevent frequent tripping. But that would mean that potentially, if one panel was pulling little or no power at a given time, that the other panel would then have a 150 or 200A main breaker protecting a 100A rated panel - a bad scene, and the cause of many panel overheating/fire situations. Far better practice to have each circuit on a separate propearly sized breaker, and each panel served by a properly rated main breaker.

This type of master breaker oversizing effect is aggravated by the fact that a normal household breaker can still be within performance spec if it takes (specs vary by breaker type and demand) as much as around 2 minutes to trip out at 2 times its rated amperage, and can take as much as 15-60 minutes with some models to trip at 120-125% of rated load. Generally, they do not even have to trip at all at under 120-125% of rated capacity, so the fact you have a breaker of a certain rating does not necessarily mean your circuits will not actually overload. That is the reason panels and breakers are generally recommended to not be loaded for more than a second or two (like during motor starts) beyond 80% of their actual rated capacity, and to distribute the larger loads between different circuits and to roughly evenly distribute the loads across both legs of the 240V service.

And for unusually high demands, pull the power as directly from the source as possible - for instance for a high-demand detached building or workshop or high-horsepower A/C or such come off a main breaker at/immediately adjacent to the meter panel, not connecting through another distribution panel. I have seen 40, 50, even 60 slot household distribution panels rated at 150-200A with 500-600 or more amps of breakers in the panel - which might have been legal if you calculated the effective overall load utilization (which considers that most circuits will not be pulling anywhere near their rated power at any given time), but I have also seen and heard of such panels smoking and hot enough to be felt from several feet away because normal but heavy household use (range, electric dryer, electric water heater say) coincided with heavy deamnd from A/C or workshop power tools or high power hair dryer or such. This sort of event is common at holdiays when more people are home or lots of guests are over, and people are using a lot more appliances and other demands at one time - and of course is more common in all-electric houses too.


Now the possible solutions:


4) If your meter panel has space (and capacity) for additional main feed breakers, put new 240V paired breakers in there and run a new feed to the new panel - or install a master breaker box adjacent to the meter panel to serve the entire house.

5) A common solution for shortage of breaker space, provided the box is not overloaded overall (distribution/breaker boxes have a total load rating that the bus bars and connections can safely handle, and some have limits on the total demand on the lower portion of the bus bars as well), and that you can move around or free up space where 240V high-amperage breakers are allowed in the box (commonly only in top 3-4 slots on each side of the panel with some brands - others allow 240V breakers anywhere in the box), you can with many or most brands (if your local code allows this) put in two half-wide breakers in place of two single normal-width 120V circuit breakers. Half-wides are still individual breakers with just one circuit connected to them, they just fit in half the space (and are commonly limited to about 20A or 30A and many or most are also not rated for use in a 220/240V application, but with a bit of juggling locations around you can commonly free up a breaker slot where you need it for the second half of a 240V twinned breaker install.

6) One possible other solution, less commonly done for obvious reasons and violates the rule discussed above about having two loads coming off one breaker, but depending on the load on your current panel and its rating - is either replacing an appropriately located 110/120V breaker with a breaker designed for two leads or installing a legal splitter and connecting two lightly loaded circuits to it, now treating them as one circuit with split branches. Legal in most areas, but of course have to be lightly loaded circuits so the combined load does not overload the breaker, and each of the former circuits (now a branch of one circuit) will effectively have an oversized breaker serving it and the two combined loads will have to be less than the breaker capacity. And no, it is NOT legal to up the breaker capacity to the total of the two circuits, because the breaker size is also limited to the amperage that the circuit wiring can safely carry individually, not combined.

Of course, what type of load you are looking at serving from the new panel can influence whether you cram it into the existing panel as a connected load, use a 240V twinned breaker in the current distribution panel as a main breaker for a new sub-panel which would then have breakers and circuit connections for the new load, or bring a new main feed direct from the meter base and main breakers there to a totally new sub-panel with breakers - totally in parallel with the existing panel supply wiring. If the new demand is a a number of lightly loaded circuits any of those can be done in most cases - if a heavy load (large motors or welder in a shop, major long-term loads from electric household heating/cooling or water heater or electric dryer), then my preference is a new home-run to the source (meter/main breaker panel) to avoid putting a lot of high-demand items through a current full distribution box. They can get pretty hot when running shop power tools or welder or electric dryer or steam washer or steam dishwasher, for instance, at the same time as normal evening household loads and heavy electric range use, for instance.

As for the grounding question - under all the codes I have seen, you have to ground the panel to the source panel ground - some areas allow this to be the neutral in a 3-wire 240V lead (assuming using normal single phase utility feed from the transformer off a 3-phase distribution system, not Y-Delta or 3-phase or something like that), some allow connecting metal conduit to carry the ground, others require a 4-wire main feed from the meter/main breaker panel, which is then of course in all cases grounded to the ground rod(s). Note - your connection at the meter/main breaker box would NOT be "to the box" as such - has to go to an official ground bus or ground connection at the box (which will be grounded to the box with the mounting bolts), not just something you juryrig - the box itself cannot be used as part of the grounding, even though it is grounded too. Many locales allow ground cable-to-cable clamp connections for this, some require it go through a bus or terminal bar. You NEVER want an enclosure to be part of the planned grounding path unless the fault is in the box itself, because everything in the grounding circuit can potentially, in the event of a fault, carry up to full voltage.

Regretably, saw a man die that way - industrial plant during construction where a contractor hooked a 4160V circuit ground from the plant to a bus in the switch cabinet which he thought was the ground bus but was actually a cabinet grounding bus - designed only for interconnecting the electrical switch cabinets together and grounding them to a safety ground. That circuit had a fault carrying now amperage, but not enough to trip out the very high (500A or 600A as I recall) amperage breaker on that circuit - worker came in contact with the live cabinet and a grounded conduit.

Note also - in every electric code I have seen, if grounding from the new panel direct to ground is allowed in your area (to a new nearby ground rod), you also have to bond that ground rod to the existing one for the main service, so all grounds coming off a particular service drop are connected and you cannot get a voltage differential (which can at times be pretty high) between the two grounds.

I really think you need an Electrical contractor (or electrical designer if your project involves an architect) to work out the correct configuration - and it is possible existing issues may be identified, or some existing parts may need to be upgraded to current code as part of the exercise as well, to come up to current code and/or to eliminate issues which may cause problems in a resale home inspection down the road.

Answered 3 years ago by LCD

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