Electrical – Understanding Capacity of Split Bus Breaker Panel with Sub Panel

There are a whole bunch of issues here, but let's boil it down to 3 of them:

Load Calculation

The proper way to figure out how much power is allocated, and therefore how much is available, is a Load Calculation. This is a relatively complex way of adding up:

• Square feet of a building - this gets an allocation for lighting and general receptacle loads
• Fixed loads - e.g., HVAC - the larger of heating or air conditioning because you won't run both at the same time
• Cooking - Oven, cooktop, range, etc. There are some complex rules which help because there is an assumption that you won't normally turn on all burners, bake and broil in both ovens, etc. all at the same time.
• Other large loads - e.g., water heater (if electric), dryer (if electric), etc.
• Standard allowances for kitchen countertop receptacle circuits

etc.

You don't do a load calculation by adding up breaker panels - that would normally be way more than it needs to be.

You don't do it by putting a clamp meter on your feeder - that is not a reliable way of calculating the load, unless you do that all year long to find out the peaks (which you can often get from your utility web site).

An example, which to me looks good, of a load calculation web page worksheet is here. You are supposed to do a load calculation any time you have a major change in your service/usage - my electrician did one as part of my heavy-up even though it was abundantly clear there wouldn't be an issue (heavy-up for panel replacement and generator, not to add new circuits).

In order to figure out how much capacity you have available, you need to do:

• Load Calculation on your entire service. Not obvious what service size you have since you don't have a main breaker. You may have to ask your utility, or it may be indicated on your bill somewhere. If this does not have enough extra capacity (or is already oversubscribed!) then you will have to do a heavy-up or look into ways to automatically turn off some loads when needed.
• Load Calculation for each panel involved. The main panel itself is normally the same as the utility service (unless it is large service (320A/400A) that feeds two panels) but in your case it gets a bit complicated due to Rule of 6. But if something is going to be added to a subpanel then a load calculation on that subpanel needs to be done based on the feed breaker for that subpanel.

So that tells you how much capacity is available. If it is 50A, great. If is only 20A or 30A, you are also OK for EV charging - more below.

Rule of Six

You have a Rule of Six panel. That means you have up to 6 double breakers (or other "pulls" - I had fuses to be pulled out + breakers) to turn everything off. Generally obsolete, but if working OK with a still-available, not dangerous, panel then perfectly fine. One of those 6 breakers controls the bottom of the panel. In your case, as is typical, that is a 60A breaker - 60A being the largest inexpensive breaker for many panels for a long time. (Expensive large breakers is one of the rationales for Rule of Six panels - no need for a 100A or larger main breaker.)

So if you add anything to the "lighting section" you would need to do a load calculation first, but you already have that full with half-size breakers, so nothing going on there.

The other top 5 breakers are 3 x 30A, 1 x 50A and 1 x 100A. According to a comment, the subpanel is connected to the 100A breaker. That should be fine for the existing loads because of the way load calculations handle multiple cooking appliances (2 x 40A in your subpanel) but you may be pushing towards the edge of your capacity on the subpanel. You can probably add 20A or 30A to the subpanel, but almost certainly not 50A.

You really need to find out what your utility service is before doing anything else. If it is 100A, which was quite common in Rule of Six days, then you may already be in trouble.

EV Charging

This is the driving force here, which is quite understandable. The typical sizes thrown around for EV charging are 50A and 60A. Why? 50A is a standard that comes from the RV world, which has resulted in 50A being a common travel charger size. 60A is the maximum possible capacity on many home EV chargers (EVSE is the official term). But the reality is that most people (a) don't need that much capacity and (b) don't have that much capacity available. You almost certainly don't have the capacity - I would be surprised if you could squeeze out much more than 20A of capacity after doing proper load calculations. But you also most likely don't need that much capacity. A 20A circuit charges at 16A (80%) - that's just under 4 kW. 4 kW is roughly enough to add 12 miles of charge to an EV (varies by model). If you charge for 8 - 10 hours while you sleep, that will add 100+ miles per day. The average US miles per year is 13,500 but varies by state. That's less than 40 miles per day! 100 miles per day of charge is more than enough for most people for a typical commute, errands our town, etc.

So the bad news is you are likely due for a heavy-up. If you have 100A service then I really think that makes sense. If you have 200A service then you probably have enough capacity to add EV charging to the mix, but you will likely need a new panel or some very careful reconfiguration of the existing panels to make it work.

But the good news is that you don't need to work in 50A, just 20A, or 30A if you want to splurge, will be plenty.