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if the feed wires into my panel are large enough to support all the loads of all the breakers in my panel... why is the panel rated with a max amp load. I mean... none of the breakers will trip if all the wiring is sufficient. is it a question of heat and that the bus bar metal rails will get hot? but even if it gets warm in there, will that cause breakers to fail or be a fire risk?

2nd Q for extra credit!

my home has an outside panel just next to the meter. the panel has only 1 240v 100 amp breaker which feeds the 'main panel' in my garage and a single 120v 20amp breaker which goes ot garage outlets. this outside panel has a label that says 125A max. my question is can I add another 240v 50amp break in this outside panel (which currently only has the 2 breakers as i describe) ? if I cant then what is the point of this outside panel if almost all of its capacity is just that 100amp breaker for my "main panel in the garage"? is it a code thing that i need to be able to turn off all power from outside the house?

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    Like wire sizes, the connecting parts in a panel are sized also to a specific amperage. Most panels you can add more branch breakers that can add up to more than the main breaker, as long as you don't have full load on all the breakers at the same time. Should add photos of both panels including the labels, to your question.
    – crip659
    Jun 8, 2021 at 18:53
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    Are we talking about a "Rule of Six" panel here? I bet we are. That has up to 6 double-pole spaces, and all of them are hot all the time with no way to shut them off. Jun 8, 2021 at 19:17
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    The "main panel" in your garage is actually a subpanel. The main panel is the one at the meter. Jun 8, 2021 at 19:23
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    Can you post photos of the labeling on the inside of the door for your outside panel? Jun 9, 2021 at 0:09

3 Answers 3

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Question 1 yes if you overload the buss it will get hot and the breakers may malfunction. Zinsco and federal pacific stablock panels were well known for bad connections that caused heat leading to arcing and breaker and buss failure so you do not want to overload the buss.

Can you add another 50 amp double pole breaker? Possibly, I would need to ask a few questions first. Gas or electric heat? AC what size, water heater heater / stove gas or electric. If you say gas heat, stove & water heater,

Done deal I would slap in the 50amp and yes I have on many occasions (common size).

If you say electric,,, not so fast what do you want to use the 50 for? A car charger? ,,, that would be pushing the service but possibly, An on demand water heater , probably not. Running a new circuit for AC probably would.

Basically I would turn a few of the large loads on with some normal lighting and check the amp load if I thought it was going to be close to full load.

Verification of a load test will allow a licensed electrician to put as many breakers in as are needed, (you can do the same but the inspector may push for the testing).

Important: the 3 hour loading can not exceed 100% of the buss or service rating (you said the service was appropriately sized). On top of not exceeding the max buss rating there needs to be 125% of the projected load in most cases (sometimes 100 % is allowed). The measured load + the projected load can not exceed the buss value to add another load.

So can it be done possibly, as long as we don’t exceed the buss rated current we are allowed to add loads, this works out because we don’t run electric heat and cooling at the same time, or electric heat and the electric oven are thermostatically controlled so there is a “diversity” in the load.

I have seen homes with 200 amp total amps on each leg only drawing 50-75 amps it’s this diversity that allows to put more total breakers in than the buss could handle if they are all maxed out.

So we look at most branch circuits they are usually loaded at 80% even less in many cases except for brief periods of time so this is why we may be able to add more but we usually have to measure or calculate. We don’t load the buss more than it’s max because we don’t want fires

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  • "the 3 hour loading" is that how long a load test is performed? Does it not also matter what happens in those 3 hours? For instance, are you supposed to go around the house and switch things on and off (heat A/C etc..)?
    – P2000
    Jun 9, 2021 at 16:56
  • The 3 hour the 125% rule is used less than 3 hour 100% is used I have done this many times it’s not hard to see the difference but most states require a “master electrician” or certified electrical engineer to do the test when it exceeds the “calculations” the actual test can be done in aprox 30 minutes but the values of the loading continual verses intermittent have to be tracked and worst case taken as with with all installs my license is attached to the work (or the person “signing”) that’s why my state calls our highest level a signing supervisor ether general or plant.
    – Ed Beal
    Jun 9, 2021 at 20:31
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Because they're not.

Here's how that works in a modern house.

The square footage of the house, and certain circuits and all the major appliances, are input into a Code-defined formula called a Load Calculation. This is an evidence-based formula that arrives at a likely maximum load.

The Load Calculation allows for the fact that not all loads will be "maxed out" at the same time. Or at least, not for very long.

The Service size must be sufficient for the load to be served (i.e. the product of that Load Calculation).

The Main Breaker is sized to match the service size, and equipment downline is sized so it can be protected by the main breaker.

After that, nobody cares how the loads are divided up onto branch circuit breakers (except for certain breakers e.g. kitchen which factor into the Load Calculation). So if you want to have 3 breakers /circuits per bedroom, have a field day.

Example: For a given house, you crunch the numbers and the Load Calculation works out to 108 amps. You cannot use a 100A service. You can use a 125A service with a 125A main breaker, but you can also use a 200A service with a 200A main breaker.

How you modify a modern home

You really should do another Load Calculation when you add a load. But the fact is, if you don't, the one Main Breaker will assure the service and equipment is not overloaded.

Enter the Rule of Six

Postwar, when large electric appliances were selling like hotcakes and space-age homes were taking 100A service or more, they hit a problem. 100A main breakers were exceedingly expensive, industrial tier exotica.

So they got a brilliant idea: How about having multiple main breakers of smaller, affordable sizes? NEC allowed this, as long as they were all together and took no more than 6 hand movements to shut "all the main breakers" off.

So the "Rule of Six" panel has up to 12 spaces intended for six 2-pole breakers. These bus-bars are always hot and cannot be de-energized. It's very important that the Load Calculation be done carefully, so the up-to-six loads using the "main breakers" do not collectively overload the service wires.

Note that the breaker trip handles may add up to more than the Service Rating; that's OK - this "oversubscription" is accounted for in the Load Calculation. So your 125A house might have a 50A range, 30A dryer, 30A A/C, 30A water heater, (140A so far), 40A for kitchen, bathroom and laundry loads, and numerous 15A and 20A breakers for miscellaneous things.

How a Rule of Six panel house is built

As said, the Load Calculation must be done very carefully, so that the up-to-six main breakers don't (together) overload the service.

But in practice, they stick a typically 50-60A breaker to feed a "lighting panel" for lighting and miscellaneous 120V loads... then your 30A dryer breaker goes in there, 40A range, 30A A/C, 30A water heater, 20A well pump etc.

The Load Calculation has proven this will not overload the panel, even if the breaker trip handles add up to more than the total ampacity.

How "Rule of Six" houses get modified

Down the road, people with such panels want to add/change stuff. Very simple: You re-do the house's Load Calculation with the new things added.

Unfortunately, you have to re-do this Load Calculation every time you change anything. Even if it's in the Lighting panel fed off the 50A breaker - because the approval didn't assume that was drawing full-bore 50A, it assumed it drew what the Load Calculation said it did.

In the real world, what actually happens is people add stuff willy-nilly and skip the Load Calculation altogether. As such, they overload their Rule of Six panel, and cause mayhem.

And that is why they are now banned for new construction.

How to add your thing

Do another Load Calculation with the new load added. If it calculates out to <= 125A, you're good to go. If not, you'll need to address the service issue.

If you really believe in your heart of hearts that the Load Calc is wrong and you'll never exceed 125A, then it's a simple thing: Convert your setup to a modern main-breaker setup. You do that by fitting one 125A breaker in the Rule of Six panel, running 125A wire to another >=125A rated panel (could be the same subpanel you have now)... and then putting ALL loads in that panel. This turns your Rule of Six panel into a Rule of One panel.

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    Thank you for the history lesson, very interesting. But why is the rule of 6 relevant in the OP's case?
    – P2000
    Jun 9, 2021 at 16:58
  • @p2000 , I was thinking the same since the rule of 6 is no longer code compliant for residential and yes the buss rating IS in amps even in the rule of 6 there is an amp rating on ea section, all single buss panels are rated in amps and the maximum voltage usually 250 in the US.
    – Ed Beal
    Jun 9, 2021 at 20:39
  • @P2000 The OP says there are only two breakers, a 100A that feeds the sub, and 20A that feeds the garage. He would need one more breaker if a panel with a single main breaker configuration was present. But 100A breakers in Ro6 is odd, and most Ro6 panels are split bus, and no breaker for lighting section is present, so I would want a little more information before making a conclusion. And even if it is a Ro6 I don't see why he would need to replace the panel if not exceeding 6 mains. Jun 15, 2021 at 5:43
  • @P2000 because OP has a panel with a 100A and a 20A breaker. That is one of two things: a modern "ranch panel" with a meter, main and 4-8 breaker spaces but I've never seen one with only 125A busing. Or, it's an old "Rule of Six" (but not split-bus) which has a meter and 8-12 spaces of "main breaker" area, which are very common with 125A busing. Hence I believe it is the latter. The history lesson is beause of OP's question in the subject line. Jun 15, 2021 at 5:55
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Other answers are good, only addressing some additional points.

The busing rarely get hot enough to melt, but easily can get hot enough to transfer heat to the wire and melt it's insulation

The NEC now requires an exterior disconnect, not necessarily a panel, but panel is one way to satisfy the requirement. In fact some jurisdictions have required an exterior disconnect for a long time. Your owner/builder may have been from an area that did require it and wanted it done how they were accustomed to doing it.

You may have the outside panel as a result of remodel or phased construction. For instance the utility typically only allows a specific length of wire inside the building before the service panel. If a garage is added to the same side of the building as the service panel then you may need to add a new panel on the exterior of a garage and feed the existing as a subpanel.

Or it could be something as simple as the utility only had access to the garage side of the building, and the owner/builder didn't like going into the garage to reset breakers.

Also you can't assume that just because a panel is labeled 125A that it is feed with 125A, sometimes over rated components are used due to planning for the future, availability, pricing, or configuration.

Edit: Bottom line for original question, I think it's unlikely adding a 50A circuit in the garage would be NEC compliant, and most jurisdictions I have worked in would require a load calculation done according to NEC formulas to determine if you have capacity. The more electric appliances you have (range, water heater, clothes dryer, dishwasher, electric heating or cooling) the less likely the calc would come in low enough to add a 50A breaker to a 100A or 125A service. 100/125 is so borderline small to begin with that many jurisdiction won't even allow a new residential service smaller than 200A.

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  • Regarding heat that you mention, box fill is of course limited for this reason. But does the ambient temperature (due to total current) inside the panel affect the breaker's trip points?
    – P2000
    Jun 9, 2021 at 16:53
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    @P2000 Yes ambient temperature affects trip points on inverse time thermal magnetic breakers. I don't currently have access to UL489, but goodsonengineering.com/wp-content/uploads/2017/08/… shows a small chart on it's second page claiming to represent Square D trip time adjustments. Jun 9, 2021 at 19:27
  • @NoSparksPlease thanks. I can elaborate that my confusion is partially caused what im calling the outside 'panel' might be there solely for 'exterior disconnect. but im certain that panel was part of original construction in the 70's . what Im calling the 'main panel' is the 'inside panel' in my garage and has all my breakers except one. Jun 15, 2021 at 3:16
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    @CoolPontiac Utilities/State Codes typically only allow 10 to 15' of wire in a building before the main Service Disconnect. I suspect the owner/builder didn't like having a panel for all the interior circuits at a location that satisfied the 15' rule or didn't want to run all the branch circuit wires through the garage, so he mounted the service panel where the utility/State code allowed, and ran a single feeder to a sub-panel at a location he preferred. Could be he also had some elaborate plan the was never implemented that we could never guess. Jun 15, 2021 at 4:01
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    @CoolPontiac See edit above at bottom of answer. Also if the rating of the busing in a sub-panel and the wires feeding the sub panel are equal or exceed the rating of the breaker feeding the wires that feed the sub-panel then a main breaker is not needed in the sub-panel. – Jun 15, 2021 at 4:18

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