Help in understanding capacity of split bus breaker panel with sub panel

Question

My home was built in 1978 (USA, Washington State), and has a split bus panel for the main breaker box. It also has a separate subpanel, which I'm not sure if it is original or added later.

I am a bit confused about how to calculate total capacity. The lower bus has 6x20 amp, 2x30 amp, and 8x15 amp breakers - which would be 300 amps total, and that's all attached by a 60 amp double-pole breaker labeled "submain" (#6). Obviously not all circuits are going to run to full capacity simultaneously, so I guess this ok? Is there some standard rule about how much overage one can have?

Similarly on the subpanel, there's 3x20 amp breakers, and 2x40 amp breakers - so 120 amps total. The subpanel is connected by a 100 amp double-pole breaker (#1 on the main panel). This is less overage than the main panel, but still over the 100 amp connection.

My main reason for asking is that I'm trying to decide if I have capacity on the subpanel for a 50 amp electric vehicle charger. I think that would be possible, as long as I'm not charging while running both the range and cooktop at the same time. Is that the right way to interpret this?

In general, how much total capacity does this system have, how much is being used, and how much is available?

(Note, there is no main breaker - inside or out.)

Answer 1

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.

Answer 2

Obviously not all circuits are going to run to full capacity simultaneously, so I guess this ok? Is there some standard rule about how much overage one can have?

To figure out how much power is used in your panel, you need to do a service Load Calculation. For a dwelling this is typically done with NEC 220.82. This has the catch-alls and fudge-factors you are thinking of.

In fact this Load Calculation is absolutely essential for any additions to the electric service - and that is because of the split-bus panel. You may have noticed that the six main breakers sum to 300A, considerably more than the 150-200A of service you do have. There is no main breaker to "keep you honest". The only thing that makes this safe is the Load Calculation. However, this is vulnerable to people "just adding stuff", without revisiting the Load Calculation, and then you can have an overload. This has probably already happened and your panel may already be overloaded.

In the Load Calc, most loads fall into catch-alls or a nice 40% fold-down. However HVAC counts for hard 100% and EV charging counts for 125% because of the extreme impact this type of load has - it is the hardest load a home will ever power. People are cavalier about it and this is the path to melted stuff or a fire, even if a split-bus panel wasn't involved.

EV charging sanity

Every EV newborn seems to collect the same misinformation about EV charging - only 2 speeds exist, level 1 sucks, you need a 50A circuit with 6/3 Romex with neutral to a Leviton RV socket etc. No. EV charging is much smarter than that. Technology Connections has an excellent video on this subject, and let me cue up the "sizing" part.

Now I'm going to proceed assuming your panel is maxed out and has not a scrap of headroom available.

Option 1: power it out of the subpanel.

In this case, you determine the parts of the 220.82 Load Calculation which apply to the loads in this panel. Since none of them are HVAC, they are all 40% loads, that is, they appear in the Load Calculation as only 40% of the loads they actually are. Add up the loads and take 40% of that. Suppose the loads add up to 12.5 kW. 40% of that is 5 kW. That is 21 amps.

EV charging is a 125% load, and it takes a circuit breaker at that 125% value. So in our example that would mean a 21A ( well, 20A) breaker. That is 125% of 16 amps, the speed the EV's onboard charger could actually charge at.

However, what keeps the range and EV from going at once? Your pinky-promise? Not enough.

This is the actual role of that thing you call a wall charger, the EVSE. It tells the charger (which is actually on the car) how much power it can safely take right now. On most units that is a fixed number. But certain EVSEs are capable of energy management, such as the Wallbox or Zappi. This adds a module to the panel with sensors on the supply wires. Now that the EVSE knows the current being drawn by all loads in the panel, it can signal the car to slow charging to keep the panel and service within limits.

What service limit do we set? Well, we figured that out above, based on 40% of the loads. In my example 21A. Divide by 125% (i.e. multiply by 0.8) to get the max EV charge limit, e.g. 17A. With that limit operating, we can let the car charge right up to it. We'll be using this trick again in the other scenarios.

Option 2: charge out of a new subpanel just for HVAC.

In this one, we create a new subpanel entirely for HVAC loads. Two furnace loads and a heat pump. You already crunched the numbers on these using one of the six methods in NEC 220.82(C).

So we're doing the same trick as the other subpanel, except it's all HVAC loads and these are 100% loads. So you take the number that popped out of 220.82(C) at 100%.

And you do the same thing with the energy management EVSE with the current meters, except you are setting subpanel limit to 80% of the above number. Let's say the sum of HVAC loads is 12.5 kVA, you multiply by 80% to get 10 kVA, and that (42A) is your actual EV limit.

That's a big jump, which is why it's worth creating an HVAC subpanel.

Option 3: run it out of the main panel.

It is yet again the trick with the current monitoring module. The huge advantage here is we get to put it on the whole house. So we really have no upper speed limit except what the EVSE is physically capable of.

The disadvantage is needing to rearrange a bunch of furniture to make room for a fat breaker, and I would imagine, the difficulty of cabling from the main instead of the kitchen sub.

Can energy management deal with the existing overload?

Yes, but with simpler/dumber systems, you need dumber energy management tech. And fortunately a whole bunch of dumb energy management tech is being shoveled into the EV market, by makers who have had these things in their lines for decades, and now see a new market. They're weak for EVs, but are great for dumb storage loads like water heaters or dryers, so that'll work out just fine, and the manufacturer won't mind.

So you can use something like a SimpleSwitch so that when the heat pump comes on, it knocks out the tank water heater. Or when the emergency heat comes on, it knocks out the dryer. So the lesser load (generally the non-HVAC load due to HVAC coming in at 100%) disappears from the Load Calculation.

The other method for improving the Load Calc is more efficient appliances, and Technology Connections has a video on that too.

Answer 3

Call your utility provider - they'll tell you what size service you have. You can't draw more than that at any one time because they won't supply more than that.

In order to figure out your current load, you'll need to do a load calculation. You can search the web for NEC load calculation (you can include the term section 200 if you'd like to be sure you're finding the proper calculations) to find all sorts of info on how to do that properly. Note that properly does NOT mean adding up the circuit rating on all the breakers in your panel(s). If you click on a link that tells you to do that, the link is wrong. A proper load calculation includes the square footage of your house, certain fixed loads, circuit counts and has special provisions for EV charging.

Also, it's very unlikely that you'll need 50A for charging your new EV. Unless you leave the house very early in the morning and return very late at night after having driven the car until the battery is limping home on electron fumes, the standard 120V "travel charger" will probably be more than sufficient. Of course, you'll probably want a dedicated charger to use when you're home but you don't need the massive charge rates. Search this site for "EV Charging" and you'll find loads of reasoning and links to support that.