View Full Version : Main Panel- Sub Panel Breaker Size Limit

rusty monkey
04-15-2009, 04:08 PM
Is there anywhere in the NEC that deals with the size of any single breaker in relation to the size of the main breaker? I've searched for previous threads but get conflicting info. Some say it doesn't matter, others say any single breaker in a panel can't be larger than a certain % of the main, say 50% of the main breaker size. I have an attached garage that I will be adding a subpanel to. Currently the house panel has a 100 amp service panel that is full. I would like to go 50A- 60A to the garage.
Could I (hypothetically)use 100A to feed the subpanel, or if I have 50A feeding the sub could I have a 50A breaker feeding a single 220v welding outlet, and still have other circuits in the same panel for lighting and 115v receptacles and still be legal?

04-15-2009, 05:29 PM
Any of the methods you posted is fine and legal. Make sure to use the correct breakers for your panel.

04-15-2009, 08:31 PM
I pondered the very same question some time ago when I was sizing my sub-panel. For some reason I thought that there should be some maximum sum of the breakers as they related to the main breaker of the service entrance panel. I calculated what the house needed and I had 54amps at 120 volts (no 220 in the house). This may seem very low to those in the mid-west or the north, but here in the central coast of California, we don't have the Air Conditioning, or electrical heat, or electric water heaters, and most use Gas Clothes Dryers and Gas Stoves. So when I calc'd with either of the two meathods in the NEC, I didn't have much electical usage. So I first spec'ed out a 60amp sub-panel and then the Plans Checker at the city said why not a 100amp panel. I changed the plans and then I find out that I can't get a 100amp breaker in my 100amp GE panel. I then find that the service to the service entrance panel has a capacity of 125amp and I open another permit for the 125amp service entrance panel to replace the existing. The change out went well and was down for 3 hours. With the new panel I can put in a 100amp 220volt breaker. The real relief is that it is not commercial. If it were commercial then the whole thing would be different. The real thrust of the NEC is to protect the wires in the wall to prevent overheating and to protect the people. If there are nusance circuit breaker trips then the breakers are doing their job. What is bad is that the designer did a poor job and that is not necessarily dangerous, just poor planning. There is nothing in the NEC about poor planning, only standards of workmanship and to protect the wire in the wall.

rusty monkey
04-16-2009, 10:24 AM
Thanks for clearing that up. I don't really need 100amp service to the garage but wanted to know if it was possible with the 100amp main I have. I assume as long as the breaker is sized to match the wiring, the only downside is the chance that a heavy load in the garage could trip the main in the house. However, there is a good chance that I would run a 50amp circuit off the 50amp subpanel. I am the only person working and the main lighting will still be on the house panel, so I don't think tripping will be a problem.

04-16-2009, 10:31 AM
I talked to an electrician about a project that I'm planning--basically the same thing--a sub panel for the garage. His comment was that the sub panel breaker can only be up to 80% of the main panel. So a 100amp panel can supply up to a 80 amp breaker. a 200 up to a 160 amp sub panel. I'm probably going to end up with a 125 amp sub running from the 200 amp main.....

04-16-2009, 05:48 PM
sub panel breaker can only be up to 80% of the main panel.

Unless that's a local amendment, which I doubt, he's wrong. The NEC doesn't care what size breaker you put in your panel as long as the wire is sized correctly, finding a breaker large enough will ultimately be the limiting factor. Most 2 pole breakers top out at 100-125 amps, although some manufacturers make a 4 pole 200 amp breaker, but it's pricey.

tfi racing
04-16-2009, 07:04 PM
I agree with Ace,no mention of that probable old wive's tale in the CEC either.I have installed many 100A breakers for a subfeed in 100A panels.

04-23-2009, 08:05 AM
I talked to an electrician about a project that I'm planning--basically the same thing--a sub panel for the garage. His comment was that the sub panel breaker can only be up to 80% of the main panel. So a 100amp panel can supply up to a 80 amp breaker. a 200 up to a 160 amp sub panel. I'm probably going to end up with a 125 amp sub running from the 200 amp main.....

I think that may be his preference since I see nothing in the NEC about percentages or sums of breakers and totals. It is all about protecting the wire in the wall.

Your municipality area may be different, but I doubt it. I believe most municipalities like to just point to the NEC for guidance and stay out of what could be a legal battle. My local points to National Plumbing Code, National Building Code, and National Electric Code and has very little to say about the technical aspects of how to assemble a proper building.

I think the electrician that you spoke is adding into what he feels is a safer condition. Nothing wrong with that, but it is going to cost you more money.
Do the calc's and make your own decision. A 100 amp panel is capable of a lot service provided you don’t burden it with A/C, Electric Water Heater, Electric Heating, Electric Ovens and Stove Tops.

You might even call the manufacture of your panel and ask them if it is advisable from a safety perspective to put a 100 amp circuit breaker in a panel with a 100 amp service disconnect.

Closing thought: The NEC is only protecting the wires in the wall; you or whoever is designing your system is responsible for nuisance breaker tripping. You have already indicated that your are aware of this...

Good Luck in whatever you decide.

04-30-2009, 11:04 AM
Regardless of code. An electrical system should be designed such that the breaker or fuse trips at the closest point to the load. You can put a 100amp breaker on a 100amp main. It would then be a tossup as to which breaker would trip first if a fault condition occurred.

Ask yourself, do you want your entire house to lose power in the event you accidentally short something out?

04-30-2009, 10:19 PM
I don't know if code says you can't but I bet your main panels does. For example, my 100A main panel in the house says on its label that no branch breakers can exceed 70A. So if you violate equipment specs then I would assume its code violation. I can't imagine any panel beign ok with using a branch breaker the same as its main.

06-15-2009, 11:56 PM
I ran into the same thing 77, after asking in another thread about wire size for a 200 amp service to my shop i went out and looked at the tag in the lid and my 200amp Midwest brand box says "room for 3 single or 1 single and 1 double breaker not to exceed 100 amps" so im going 1oo amp. my 4/0 wire wont fit in the 100 amp circuit breaker so Lowes agreed to take back the 4/0 (110ft) as long as i bought the wire i needed from them. so some boxes are def this way as you said

06-16-2009, 12:39 AM
My main panel and meter are mounted on the garage. The 100 Amp service has a 100 Amp breaker as the service disconnect. The sub panel inside of the house is fed from an 80 Amp breaker. This seems to be the norm in my neighborhood.

My home has the meter and main panel on the alley end of the lot at the garage door. On my block the electric, gas, cable, cable and sewer lines are in the alley. The water service is in the street at the front of house.

06-16-2009, 08:35 AM
Slightly related.....make sure your sub has a single disconnect if you plan to go over 6 breakers in that panel.

I got dinged last week on my inspection....I had 7 breakers in my sub panel but not a single disconnect breaker for the box...

The concern is that if there is an 'oh shit'....anything over 6 breakers may take too long to get the power off....Not sure if it's NEC...but it's too small of an issue to argue with the inspector over.

So, I had to remove one breaker and tie the wire into another one.

The total potential load in my sub is 110A...but I'm feeding it with a 50A breaker....

In my case I like to have plenty of protection and seperate circuits....makes it a lot easier to find the problem when a breaker trips....and...I NEVER put lights on the same circuit as any fact, that may be in the NEC.....Charles? Ace? Can you confirm?

Stuart in MN
06-16-2009, 09:00 AM
The six disconnect maximum is in the NEC. There isn't anything in there about not putting lights on the same circuits as outlets, but it's not a bad idea.

06-16-2009, 09:13 AM
For the six disconnect rule look at the 2008 NEC - Article 230.71
Sorry, but I don't know if the NEC is available for free online or not.