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Find below a picture of our USA house's electrical panel (There is a box enclosing the panel that for the time being can only be removed enough to get an angled look/shot of the panel, not straight on, sorry).

Adding up the amperages on all the breakers (even the double pole ones, which I presume are to get 240v at the amperage written on one of the two cream coloured plastic ?paddles?) comes to 450A (700A if both numbers on each double pole breaker are inserted into the calc). Just the single pole breakers adds up to a more expected (to me, given the house was built in the 70's) 200A.

Further complicating the self-determination of total allowable amperage of the feed into this panel is the top two banks of breakers have "Main Disconnects" next to them, but some are instead labeled with high amperage appliances (the rest aren't labeled (?any longer?) as to what they control power to). Lastly, there doesn't appear to be a ?easily accessible? main breaker to definitively determine amperage of our service.

Can anyone help with the question of total amperage able to be drawn through the panel with installed breakers, and/or if needed, debunk some of my assumptions?

Home Electrical Panel

user66001
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  • My car revmeter red lines at 4500 but that doesn't mean it is always revving at 4500 rpm. Think about this and possibly rephrase your question to make sense. –  Oct 05 '17 at 19:08
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    If you were pulling 450A at 220V that would amount to 99kW, I don't think you draw that much power. Did you know you have a meter on the outside of your house that tracks energy? –  Oct 05 '17 at 19:11
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    Those circuit breakers protect the circuit between the breaker and the outlets. To find out the maximum current you may draw for the whole breaker board, you have to go to the upstream breaker board and find the single fuse/three fuses which are delivering power to your house. As you have an electrical water heater, likely 3x50A. I doubt it's more than 3x63A@240V – Janka Oct 05 '17 at 19:41
  • @Andyaka - I am well aware amperage is drawn as needed. As clearly everything would be right with the world through the use of the word "maximum"/it's relatives, let me insert these as relevant now. – user66001 Oct 05 '17 at 20:07
  • @laptop2d - My question implies adding up the double pole (240v) breakers amounts to 150A, with another 200A coming from the single pole (120v) breakers. To my knowledge watt meters don't inform one what the maximum (clearly missing this word out was an issue; now fixed) amperage that can be drawn through it, or the possibly lesser amount based on ratings of other components in the rest of the feed. – user66001 Oct 05 '17 at 20:12
  • @user66001 that's the problem, it implies it. If your asking about paralleling breakers, then say that. You probably have a bigger breaker outside by your meter –  Oct 05 '17 at 20:16
  • @Janka - I trust your implying that there may be spare capacity on the panel, based on what the main fuses (thought that was also a breaker; realise that doesn't make sense now) upstream are rated at. Unfortunately I can't take the cover off the panel right now and can't think of where these fuses might be if not in this panel (panel feed goes upwards through a small fake ceiling, and then exits the basement out to a watt meter and round the house to where the feed comes in from the street). – user66001 Oct 05 '17 at 20:17
  • If we were to assume however that the breaker capacities added up to the feed capacity, what is the correct maximum amperage for our service? – user66001 Oct 05 '17 at 20:18
  • @laptop2d - No other breaker visible. Round meter is a cereal-ish length and width'd box, about 5 inches deep. I am asking about both series and paralled breakers. – user66001 Oct 05 '17 at 20:19
  • @user66001 edit your question so we don't have to guess as to what it is through discussion. Write a clear specific well thought out question –  Oct 05 '17 at 20:20
  • @laptop2d - It seems I know what I want to ask (which I did), but your looking for detail I didn't know/think was relevant. I cannot update my question, providing appropriate detail, unless I know precisely what is wrong with it. *EDIT*: I put a great deal of thought into this question; Calculated the amperage shown on each breaker; Provided a picture. I can't create a car (to borrow an earlier analogy subject) without knowing the components of it... – user66001 Oct 05 '17 at 20:23
  • You have to go to the pole on the street then. There's your house's fuse box. (Usually, the main fuse is near the meter, but you said it isn't.) – Janka Oct 05 '17 at 20:29
  • @user66001 A good question will need zero comments to figure out what the OP was asking. http://electronics.stackexchange.com/help/how-to-ask Please review guidelines for asking good questions. –  Oct 05 '17 at 20:30
  • @laptop2d - Lets examine those limited scope of guidelines: Search, and research - I provided picture, calculated amps, realisation that double pole breakers are to get twice the voltage onto a circuit; Be on-topic - Yep, this is an electronics Q&A and my question is about electronics; Be specific - Yep, first paragraph mentions the photo. Second paragraph lists my calculations. Third paragraph listed issues I have with the calculations I made. Forth paragraph basically asks how can one add together the amperages shown in the picture to get maximum amperage that can be drawn through our panel. – user66001 Oct 05 '17 at 20:44
  • @Janka - If it is 3x63A@240v, does this imply 3x63=189A x 2 (240 - 120v) = 378A? – user66001 Oct 05 '17 at 20:49
  • There should be a label on that panel (perhaps inside it) giving the maximum current it is rated to handle. If one of the breakers in the top section is rated at 100 Amp or more, it may be your "main breaker", and will limit the total current you can draw. – Peter Bennett Oct 05 '17 at 20:50
  • @laptop2d - I have been on SE for a number of years. *Most* questions I see have at least one comment. I am getting tired though of this "go fix what is wrong with that painting". SE has the option for anyone to edit questions. I can't rewrite a question __when I don't know what is wrong with it__. I work in IT and firmly believe a certain level of pointed questions/extrapolation is required in required for any support role. I would never tell a user to go rewrite their request, leaving them to figure out what I was expecting.... – user66001 Oct 05 '17 at 20:51
  • janka is apparently assuming the European 240V 3 phase system, while I suspect you have the North American 120/240V single phase system - which is it? – Peter Bennett Oct 05 '17 at 20:53
  • @PeterBennett - Hi! Made sure there wasn't a rating on the panel visible before bothering the community with the question. There is no breaker having anywhere near 100A written on it visible without taking off the panel cover (I was hoping to find that, as that would have answered the question). I am wondering if the picture in this posting is visible on your end? – user66001 Oct 05 '17 at 20:54
  • @PeterBennett - Good point. Had forgotten that there aren't just US users on SE sites. Question updated. Am more or less sure it is single phase (house, not business), but was going to leave that to the (I trust) more experienced people addressing this question to tell me how I can determine that/determine that themselves. – user66001 Oct 05 '17 at 20:56
  • Can you provide the square footage of your house, the nameplate amperage of any large/fixed loads such as an electric dryer, AC, electric water heater, or electric range, and how many branch circuits feed your kitchen countertop receptacles? – ThreePhaseEel Oct 05 '17 at 22:44
  • To make this simple look at the wire size feeding the panel if the panel dosent have a visible listing labeled (it should have one) . then look at at Harper's answer as it explains this panel quite well. The panel listing value may be the limiting factor or it could be the size of the feeder wire if less than the panel listing . make sure after you learn about the 6 rule to up vote the answer. – Ed Beal Oct 06 '17 at 03:08

3 Answers3

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TLDR: Ask the power company what size your service is. There may also be evidence on the meter or meter pan.

This is a classic "Rule of Six" panel

Where's the main breaker? Electrical service to houses used to be 60A. As it got bigger, there was a problem - large breakers like 100A or 200A were very expensive. A compromise was struck where the "main breaker" could be up to six smaller breakers. That's what you see in the upper part of this panel.

They are not paralleled! Each of the breakers serves a different load.

This panel, like many "Rule of Six" panels, has its own built-in subpanel - that's what you see in spaces 17-28. The subpanel area is fed by one of the six breakers in the "Rule of Six" area. This feeds all other loads in the house.

(Space 15-16 are not spaces, the lids are renovable but there are no busbars behind them.)

Your loads

What is in the upper area? Probably, based on labeling,

  • space 1,3 50A ??????
  • space 5,7 50A air conditioner (a little large? Maybe a heat pump with aux heat?)
  • space 9,11 50A the subpanel below
  • space 2,4 40A range/oven
  • space 6,8 30A ?? likely an electric dryer
  • space 10,12 30A water heater

You notice this adds up to 250A -- there's no such service size. You also notice the subpanel breakers add up to way, way more than 50A, so you can see it's normal to oversubscribe breakers on a service. Actually, breakers are there to protect wires and appliances. Wire size (e.g. 12 AWG) decides breaker size (e.g. 20A).

What size is my service?

As far as the subpanel area, your service is 50A, but hold that thought.

As far as the capacity of the "Rule of Six" area, you'd need to consult with your power company or look for evidence at your meter or your meter base.

EDIT: You could also pop off the panel cover and inspect (carefully, from a respectful distance) the fat wires landing on the main lugs at the top of your panel. Look for a size number right before the letters "AWG". Expect 2, 1, 0, 00, 000, 0000, 1/0, 2/0, 3/0, 4/0 (these are the same as 0 thru 0000). Or right before the letters "MCM" or "KCMIL" expect 250, 300, 350, 400 or 500. Those wires come from your meter and will inform the size of your service, as these are the wires that will burn your house down if you overload them.

EDIT: Your other limiting factor is the capacity of the panel. Search it for marks as to its maximum capacity in amps, typically 100, 125, 150, 200 or 225. Exceeding this is out of the question.

Keep in mind you only have 5 breakers in the "Rule of Six" area and you are full. You can get CH breakers up to 100A ($50) or 125A ($110) in the normal shape (which would force you to abandon one large appliance)... or 150+ amps in "beast mode" packaging, where the breaker is 4 spaces wide (eliminating two large appliances, ouch).

The problem with the "Rule of Six" design is there is no main breaker to keep you honest, so nothing really keeps you from putting in six large things and overloading the circuit and burning those wires. That's why Rule of Six was outlawed. You can't buy a panel like that today, and you shouldn't continue this one in service any longer than you have to. Dismantle it lovingly, and offer it up on eBay. It'll save someone's bacon. But only sell it to a licensed electrician.

Get a better panel and make this a "Rule of One" panel

If it was my house, I would add an additional sub-panel quite near this one, and move all my loads to it. Then I would fit one of those big CH breakers in the "Rule of Six" area and have it only power that subpanel. Put blanking plates over every other space in the old panel, and you now have reduced it to a giant main breaker only.

EDIT: Alternately 2 or 3 subpanels, as I would not be inclined to pay the big bucks for the 150A jumbo breaker.

I am very picky about panel brands. CH is a fine type (other than this one being "Rule of Six"), and I would even consider CH for my new subpanel. Reuse all your breakers!

It needn't be done in one sitting. Find a period when you can do without enough loads out of the Rule of Six area to make room for the new breaker (like A/C in winter). Once you wire up the new panel, at that point both panels are live, and you can move the other loads at your leisure, one per Saturday or whatever works for you.

Harper - Reinstate Monica
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    Finally after all the thrashing from folks that don't have a clue the right answer! Wish I could give this one a +2 – Ed Beal Oct 06 '17 at 02:58
  • Thanks Harper. Was happy to read it all. Sounds like a sound explanation. Will research the info further, and ring our power company, then return here to accept an answer soon. – user66001 Oct 06 '17 at 03:54
  • After sleeping on it, as an aside, I am undecided as to whether oversubscribing breakers is such a good idea. If one is aware that they can draw 15/20A off each circuit, and has determined which mains outlets/lights are wired to each circuit,(as the previous owners of our house did mostly well) one might conclude that they can put heaters on each circuit, which would of course not trip the immediate circuit breakers, but if more than 3 were run at one time, I extrapolate from the above that it would trip the "main lighting" (which I think is more aptly named "sub panel") breaker... – user66001 Oct 06 '17 at 13:31
  • ... taking out ALL mains outlets and lights in the ENTIRE house. Then again this setup allows more circuits/outlets/lights to exist for a cheaper setup. Regarding the idea to reuse the CH panel. I am not at all opposed to reusing things, but having such a large panel only serving as a main breaker seems like an eye sore (more for my wife than I) and space hogger. Also, would you think that if I did this work myself and later needed a licensed electrician to work on something, that that job couldn't be signed off until the CH panel was replaced with a "Rule of One" panel (due to current code)? – user66001 Oct 06 '17 at 13:46
  • Oversubscribing breakers is normal and it works -- *main* breaker trips are an extremely rare occurrence. Granted, yours is a more extreme case, with 200A in the subpanel on a 50 "main" but how often does it trip? Remember there are two poles of 120V each so you can pull 50A from each pole and be fine (a typical heater is 1500W or 12.5A, so 4 per side). *It works.* Oversubscription is more spooky on the "Rule of Six" side, because there is no master overcurrent protection at all. That is the hazard. – Harper - Reinstate Monica Oct 06 '17 at 16:30
  • @user66001 The old panel is grandfathered, so it may continue in service. My advice lets you DIY repair and work *incrementally*. You can't deenergize the top 6 buses -- so to change main panels, the power company would have to turn off the whole house, and that would be for an extended period, and the city may insist on hiring an electrician, and they're all in Florida/Texas/PR right now, and now you're on a slippery slide into "project from hell". You need to permit this work, the CITY inspects, not electricians. And do the work dead nuts perfect. – Harper - Reinstate Monica Oct 06 '17 at 19:12
  • Also, houses have utility space, a practical side-effect of having utilities. Unfortunately we do see a lot of people who find utility space "ugly" and do everything possible to eradicate it. They end up regretting the choice when they try to DIY a project and find they cannot access anything -- and end up paying electricians to do drywall. That's like paying a lawyer to do your laundry. – Harper - Reinstate Monica Oct 06 '17 at 19:18
  • Re-visiting this answer to provide further comments, realised I didn't address some of the questions in your last @Harper - None of the breakers at the top of our board have tripped while we have been living there. As for the heaters, by some guides of heating kW to space, we are looking at somewhere in the vicinity of 100A (8 heaters house-wise) total potential draw at some times of winter, which (unless I am misinterpreting your two pole @ 50A each for the subpanel) seems to be maxing out the wiring and breakers, regardless of whether (as it seems to me) the decisions on what to put on.... – user66001 Jan 25 '18 at 16:08
  • ... each circuit aren't well suited for modern use/switching to room-by-room electrical heating needs. The other consideration with keeping the Rule of 6 panel is the maximum amperage it can handle. Once the current supply amperage is figured out (more on this in my next comment), it may mean to handle our ambitions for electric heating and potential EV charging during the day (more likely at night, though) we need to "heavy-up" our supply. I trust panels have an amperage limit, and I would think that it would be unlikely the house builders would have picked a panel that is rated much over... – user66001 Jan 25 '18 at 16:46
  • ...the supply, no? As for __TLDR: Ask the power company what size your service is__ - I contacted our power companies main customer service dept., they referred me to our local 'Customer Design and Engineering Department' that initially suggested I needed to call an electrician to come over and establish supply ?!? Even after a bit of convincing to look up their records, the person in that dept. I talked to could only tell us what local transformer we were connected to... My search for how to read any numbers on our smart meter to establish max. load was fruitless, but then the... – user66001 Jan 25 '18 at 16:51
  • ... aforementioned person stated that the meter should handle 200/400A service without changing, so that probably won't be helpful either. Any other suggestions bar getting access to the cable from where the power companies feed delimits on the side of our house, to our meter box, or from that to the panel, and measuring it's dimensions??? – user66001 Jan 25 '18 at 16:56
  • @user66001 Yes, fully loading a 50A subpanel with 50A on each pole would indeed "max it out", and that is not legal for continuous loads - you need to derate by 125%, i.e. provision a 62.5A panel. You'd think your power company would be more eager to help you, these will cost ~$1.50/hr to run unless they have a special rate plan. I added content to describe how to get service capacity from your meter-to-panel wire sizes. – Harper - Reinstate Monica Jan 25 '18 at 22:39
  • @harper - I am returning to this task and on reflection I understand now why the AWG size coming into the panel has to be an indication of maximum amps drawn through that cable and panel combination. Though I see we didn't cover off whether oversubscription of breakers is common for everything _but_ the main breaker? Do I presume correctly that oversubscribing the breakers that are there to protect the upline cabling, not the house wiring per se, would not be to code/a good idea? Also, any chance your could confirm my thought that the service sizes match with the panel capacities listed above? – user66001 Mar 15 '18 at 17:22
  • @user66001 Normal branch circuit breakers protect the wire downline of it (e.g. a 20A breaker protects 12AWG wire). The main breaker protects both the panel and the upline service drop from the pole. "oversubscription" allows for the fact that not all lines will be taxed to full load load at once. The main breaker still protects the panel and feed. The problem is, this model falls apart on a multiple-main-breaker panel like this one because nothing protects the feed. – Harper - Reinstate Monica Mar 15 '18 at 17:42
  • @harper - Thanks for confirming my thought on the main breaker, if there is one. Might I also be correct (in essence) that the service sizes for the feed into the house match the available panel capacities you listed in an edit above? – user66001 Mar 16 '18 at 07:15
  • @user66001 that's the real question. The *meter* may be good to 400A, but the *meter pan* is not. The limiting factor is the lowest of a) meter, b) meter pan, c) the service drop (pole to meter), d) the service conductors (meter to main panel) and e) the main panel's rating itself. I really wish you had a main breaker instead of the Rule-of-Six breakers which could pull 250A (I'm sure you don't have 250A of service). – Harper - Reinstate Monica Mar 16 '18 at 14:20
  • @harper. I agree; Would be helpful to have that main breaker, as has been useful in the past, and would not necessitated the need to write up this question and all the subseuqent discussion. Thanks again for all the information. – user66001 Mar 18 '18 at 16:36
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This is likely a split-bus panel where you have multiple main disconnects. The "Main Lighting" breaker (50A) is probably what controls the lower set of breakers, and is your effective capacity if you're considering adding new circuits.

See:

Edit: I previously assumed there must be a separate main breaker, but after recognizing this as a Cutler-Hammer panel (my house must have also been rewired in the 70s) and googling a bit, I found the above answer which should also apply here. Thanks to @Speedy Petey for the helpful answer there, and @Peter Bennett for a comment below that set me on the right path. I'm replacing my prior incorrect answer.

Shimon Rura
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  • I wonder if the two-pole breakers in the top section together serve as a main breaker. (The OP did state that he traced wiring back to the meter, but couldn't find a main breaker). The 50 Amp breaker marked "Main Lighting" then may feed the bottom single-pole breakers. I don't think this would be legal here (British Columbia), but electrical codes do vary with time and location - perhaps this arrangement is/was legal in the OP's Maryland at some time. – Peter Bennett Oct 05 '17 at 21:31
  • Thanks for your answer Shimon, I will need to determine my feed capacity to see if there is need for a heavy up (as I understand it is called in the States), and realise this will also answer the question, but before I can get better access to the panel and it's surrounds I had hoped that someone could clarify with me how the amperage ratings of the breakers shown in the picture combine to determine total maximum amperage able to be pulled through the panel as far as those breakers will allow. – user66001 Oct 05 '17 at 21:32
  • I was trying to reason through a response but holy crap you're right, see https://diy.stackexchange.com/questions/72717/where-is-the-main-shutoff-my-cutler-hammer-breaker-box – Shimon Rura Oct 05 '17 at 21:37
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Adding up the amperages on all the breakers

taken with

and/or if needed, debunk some of my assumptions?

I think I see where you are going wrong with this part, other people in comments have pointed out how to try and track down the main breaker. As for the wrong assumption, you will not learn anything by adding up the amperage of the individual breakers in this box. They can add up to way more than the box can handle, and that is just fine.

All breakers are about protecting a specific circuit for overloading. Most of them are about the circuit from the box out into your house. So a given 15 or 20 amp breaker is determined by the wire in the walls. The wire coming into your house also has a max rated amperage as well and that is what is protected by this main breaker that cannot be found. In my house that is a wimpy 100 Amp breaker.

In the panel I have some big branch circuits like an electric double oven, an air conditioner, a sauna, a big table saw. Just those are almost 200 Amps. So I have to be careful about which things I run together. I cannot run the table saw when both ovens are on from experience that plus the random lights and other things push my total draw over 100 Amps. Which triggers the main breaker to protect the panel and incoming wires.

One thing you could do is call your power company and just ask them what sort of service you have, they will be billing you based on that and should be able to also let you know how much it would cost to upgrade your service to something bigger. I found out is costs enough that I just don't work in the shop when there is lots of cooking going on in the house.

Ukko
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