how to calculate how much breaker capacity a 150amp panel has, aside from the physical slots? [duplicate]

i have a 150a panel, i have plenty of space left in it, but my question is about total capacity, obviously the main breaker will trip if more than 150a gets pulled, but ive been trying to get a better understanding how you determine what the limit is for how many total amps in breaker size you can add. for instance, in this 150a panel, can i have 200a total breakers? 400?, etc...

Thanks!

• In most houses people seldom use full breaker capacity on a circuit. Most times they only use about 10 to 50% of a breaker's capacity, if even that. Electric heat, tankless water heaters, EV chargers are usually the most full use of breakers. Commented Feb 21, 2022 at 20:05
• There are many other discussions on this here. I suggest a cruise through search results for variations of your question. Commented Feb 21, 2022 at 20:07
• This question is a bit confusing. There's the capacity of the panel and the capacity of the service. To determine the former, you just look at the data sheet from the panel manufacturer. To determine the latter, you do a whole house load calculation -- the method for which is defined in the electrical code for your jurisdiction. Commented Feb 21, 2022 at 20:12
• The best way is to forget about the numbers just purchase a inexpensive amp clamp meter and measure things, if a majority of your panel is heating turn the furnace all the lights and your oven on and see what it is drawing, this is exactly what I have to do if I want to add to a panel without getting all the load data and running the calculations if your peak usage is under 120 amps (120 on each leg) then you can use 80% of the amount below 120a for additional circuits. If your home was built with lots of branch circuits you may have over 300 amps in total breakers but a actual real load 80a Commented Feb 21, 2022 at 20:29
• @EdBeal that works if you have an existing installation and you want to know what’s actually going on, but if the OP wants to make changes and they need to pull a permit to add branch circuits, the AHJ would expect to see a load calc in the permit application. They do in my area. Commented Feb 21, 2022 at 20:41

I'm glad you have gathered that panels are like checkbooks (having a lot of checks doesn't mean having a lot of money). I like to order plenty of checks, and I like to have plenty of breaker spaces.

Figuring out how much ampacity you're actually using is a tricky but well-solved problem. The numbers on the breaker trips are useless; for instance if you have 40A breaker on an air conditioner and 50A on electric heat, does that count as 90 or only 50? How about a 15A breaker to a 1A refrigerator?

This is solved by a gold-standard way (couple of them) of doing a Load Calculation, relying on nameplate data and standard formulas. On residential:

• Large loads (water heater, A/C, dryer etc.) are based either on nameplate, a standard allocation, or a formula that estimates practical usage.
• Range/oven use a complicated formula
• 1500 VA for each kitchen and bathroom countertop circuit (at least 3)
• 3 VA *per square foot" as a dog-catch for all lighting and small-appliance loads.

What spits out is a number (expressed in VA, similar to watts) which says how big your service needs to be - say 18400 VA. You divide that by 240V and that gives you the service amps you need.

Keep in mind, the emerging tech of "smart panels". They monitor your current, and have the physical ability to interrupt loads under software command - like tanked water heater, dryer, EV charging and the like. So you can put a controller and some smart breakers in, and say "stay under 150A" - that removes the need for a service heavy-up. I'm speaking entirely of self-directed demand-side manamagement.

Of course this creates a business opportunity with the power company for cooperative demand-side management. So far, that's being implemented in very klutzy, ham-handed ways... but there are much better ways to do that, and we think they'll come around quickly to that.

If you are in an area where the utility company records the peak summer and winter demand. You might be able to get that information from them. Peak demand is the highest KW usage that ran for 15 minutes or more. You can usually call them from the phone number on your utility bill.

The only other method is to run a power analysis. Unfortunately, This is usually cost prohibitive for a residence.

The only other solution is to purchase a personal power monitoring meter which can give you that type information.

Both of these last methods will require you to monitor your system for a period of time in order to average out you usage and peak times and should be installed by a professional.

Good luck

To answer your question simply: the panel manufacturer will specify the maximum number of circuits allowed in the panel.

They will also specify what kinds of breakers you can put in the panel.

Finally, they will specify the maximum amperage for the panel — 99% of the time, the panel will come with a main breaker that is rated for the maximum amperage. So if you bought a 150A panel with a 150A main breaker, the maximum sized main breaker is also 150A. If you bought a panel that can have its main breaker replaced with a larger amperage main breaker, then it will say so in the panel data sheet and should be printed on the panel somewhere.

In terms of whether or not the total number of amps of the breakers can mathematically add up to more than the total amperage of the main breaker, the answer is yes. This is where the load calculation comes in. If the load calculation comes out to, say, 100A on a 150A panel, that’s fine even if the simple addition of all the face values of all the breakers is over 150A. And that’s definitely possible if for some reason you have lots of circuits with small loads on them or a small number of non-continuous larger loads like in a house with a basement suite that has a kitchen and a laundry.

• This answer is not close to answering the question. The panel has a listing for maximum number of slots and maximum number of poles. Depending on how the circuits were laid the total numbers on the breaker handles can have little to do with what is added it is not unusual for a “full” panel to have more breaker values than the panel rating. Commented Feb 22, 2022 at 13:31
• @EdBeal that’s exactly what I said. Commented Feb 22, 2022 at 15:56