Split-bus electrical panels

Factual Questions
1

I think I have an understanding of what’s going on, but I want to be sure I’m not missing anything.

My sister has an older house (~1950s) and wants to install an EV charger. I’m giving them a spare mobile charger, so they just need a 14-50 receptacle to be installed.

They took a look at their setup and found a few unusual things. First: there’s no main breaker. They sent me some photos of the main panel, though, and I believe it’s what’s called a split-bus panel.

These are older and obsolete, but basically: the electrical code allows up to 6 breakers (or “service disconnecting means”) to be connected directly to the service conductors. So, the top part of the panel has 6 slots for breakers, which have no upstream main breaker.

One of these slots has a 60 A breaker which is connected to the bottom half the panel, which has the normal collection of 15-amp breakers for lighting, etc. (hence the “split” between top and bottom half)

There are two other breakers in the top half: a 30 A going to the air conditioner, and a 100 A going to a subpanel (which has some heavy loads, like an electric range and washer/dryer). The box has a rating of 150 A.

So already there’s something I find weird: we have 100+60+30=190 A of breakers on a 150 A panel, with no upstream main breaker. However, NEC 230.90 Exception 3 says:

The sum of the ratings of two to six circuit service disconnecting means can exceed the ampacity of the service conductors, provided the calculated load in accordance with Article 220 does not exceed the ampacity of the service conductors.

So it seems ok, assuming the load calculation was done. Which leaves the question of the EV charger.

I think, based on this, that they can install a 50 A breaker in the upper part of the panel (there are 3 free slots) and wire it to a receptacle (which doesn’t need to be far away). And this will be to code as long as they perform a whole-house load calculation and it falls under the limit. Which would take into account the fact that the EV charger is only drawing 32 amps max, for instance, that not all appliances are going at once, etc.

Anything I’m missing here? Anyone else have to deal with these split-bus panels?

2

I think there’s some rule about the max number of breakers needed to completely shut off power. I thought the number is 6 and if so you’re ok with that.

3

Thanks, that’s the limit I’ve seen as well. The panel only has 6 slots in the upper part (the directly connected bus), so there’s no real way to exceed that. It’s just weird that there’s nothing preventing the user from hitting max load on each of those breakers, and therefore exceeding the panel limit.

I’m sure the panel/service connection has plenty of margin past the 150 A rating, but it’s still an odd setup.

4

Paging my pet electrician, @Bob_Blaylock, for his professional expertise.

5

This is correct

6

These must have been popular at one time for small houses or apartments. The ones I saw had a few large breakers for an electric stove, water heater, and dryer, along with a 60amp lower panel. They clearly exceeded the rated amperage for the box if you added up all the main breakers. Don’t know if this is helpful for you but here’s a load calculator for NEC 220 requirements including EV chargers.

7

The fact that the panel is rated for up to 150A does not mean you have 150A service. It could be 100A. You’ll have to ask the utility what the actual service size is.

8

Virtually every receptacle requires GFCI protection under modern NEC editions. 2-pole GFCI circuit breakers are not cheap - like $100-$200. Between that and a quality receptacle that won’t melt down and catch fire, they could be spending almost as much as a hard-wired EVSE would cost anyway (which doesn’t require a GFCI breaker, because EVSEs include GFCI functionality and plain two-pole breakers are more like $10).

That’s assuming it’s not some obsolete panel type that’s too dangerous to continue using and/or doesn’t have trustworthy new breakers available (FPE, Zinsco, Challenger, etc).

9

Good point–thanks. The picture I have isn’t clear. The service conductors look to be substantially beefier than some nearby 100 A conductors (for the subpanel), but of course given the aluminum/copper difference, insulation type, etc. it’s difficult to be sure. Plus that doesn’t necessarily indicate the service limit anyway. I’ll mention that to them.

I think they said they already bought a GFCI breaker, but I’ll mention that as well.

They’re in Virginia, which as best I can tell uses the 2017 NEC. So possibly not actually required yet?

Do you know if there are alternatives, like having a lockable receptacle?

Oh, nice. The form I ran across earlier was just a PDF for manual calcs. That should be much easier.

10

I’m just going to pipe in and say “good luck”.

We had an electrician do an estimate to install an EV charger for our house, with electrical original to the 1982 construction.

They said “We’ll do this if you insist - but we will require in writing that we’re doing this against our judgement and will accept no responsibility for any damages to the house, the charger, or possible fires.”

We’re still waiting on a second evaluation to make sure this wasn’t a case of excessive CYA - but the first group fundamentally wanted to rebuild the main box as an obsolete unbalanced POS. Which, along with the charging options, would run around $7500.

11

I have 100A service (as sper main breaker) and a lot of items are connected. Everything except house hear (natural gas). I added a 50A circuit (note, max 40A charging since continuous load s/b 80% of top rating)

I schedule charging for 1AM when dryer, dishwasher, cooking etc. should be done. I did have an episode where the main breaker popped one night - I’m guessing the A/C and hot water heater kicked in while charging, and maybe a freezer too. I’ve since schedule the Tesla to only charge 26A not 40A and never had a problem.

Just because the dryer or range has a 30A breaker, does not mean it’s drawing that much, especially not usually all at the same time. Your range can run four elements and a stove without popping, but who runs that much at 1AM when they’re charging? (Hot water tank and AC you have less control over0

So it’s like Douglas house in Green Acres - you can run this, and that, and that - but not all at the same time.

12

There are a number of EVSEs that can hook up to current transformers that you clamp on your service wires, and they use the information about how much current the house is drawing (and the service size, programmed in) to tell the EV how much it can safely use for charging, moment-by-moment. Using one of those, it’s entirely possible to have as much as an 80A EVSE on a service as small as 100A, without ever having to worry about tripping a main breaker or overloading a rule-of-six panel.

(Of course 80A is ridiculous overkill for probably 99% of drivers. Even a 15A 240V circuit can replenish well over 100 miles into an average EV overnight.)

13

My 240V 50A circuit can charge my Tesla Model 3 (nominal rate based on EPA travelling 55mph flat road no wind etc.) 58km / about 36mi in an hour drawing 40A. Dumbed down to 26A it charges 36km /24mi per hour. I rarely let it go below 60mi (20%) in daily driving, I’ve never had an instance where it did not finish charging (to 80%) by morning. If youve had a busy day, you can start your car charging early instead of automatically at 1AM.

What I actually did was get the 14-50 receptacle installed in the garage, and found a 50A range cable on Amazon to use to plug in my wall charger rather than direct attach. It also allows the option to use the Tesla portable charger unit if there’s a problem with the regular wall charger.

The guy down the street appears to have an 80A charger, based on the thickness of the armoured cable feeding his charger. But then, he has a Model X and based on the size of his home (maybe twice mine) probably has 200A service.

The key point is - don’t just arrive home at 5:30PM and start charging immediately when you are likely going to be using the range, the dryer, take a hot shower, and possibly have the A/C going full blast.