Old house, difficult panel, new circuit question

Question

1962 house with a lot of bad wiring, and I want to add a EV charging circuit without making it worse. I think I understand my options but hoping there is one I am missing. Homeowner not electrician but trying to do things right. USA.

The house has two panels at service entry, the main has 6 largish breakers (no main), and adjacent to it is a subpanel fed with one of those at 100a. Two other subpanels inside the home are also fed from this (so total of 3), the other three breakers appear unused though two have cable attached.

These two panels are outside on a brick wall surface mounted; 4 of the cables from the main one (2 unused) go through two openings in the back of the panel thru brick into a wall behind it, and up into the attic. They are all cable assemblies (I hesitate to say type as they are so old no labels are visible, but like NM-B). These holes have a connector but no conduit, the cables go through raw brick into the wall. The second panel is also mounted on the brick but uses conduit to go up and thru the attic above the brick. I can't fit into the attic and that conduit is very full and for other reasons want to avoid that panel.

The room behind the panels is a garage and where I want to put a charger. It's a charger suitable for hardwire at 48a (60a breaker) if #6 THHN is used in conduit. Max wire size is #6 for connection to it.

I can't run THHN without a new hole in that panel for conduit, and I do not want to go thru the old, crumbing brick and mortar again, or drill into a panel with no cutoff.

I want to pull the unused cables from the panel freeing up space in the current holes and use them. My impression is that leaves me choices of:

1. Use 6-2 NM-B and limit myself to 50a breaker and 40a charge (I guess in theory 55a breaker but I am unclear whether I can find any and hazy on using 60a as next higher).

2. I could use 4-2 NM-B to a small subpanel/junction in the garage where I can connect to 6-2 THHN and do 60a/48a keeping thhn in conduit to the charger. Except 4-2 doesn't seem to exist, and I am not sure I could get 4-3 thru those holes (if I could even find some by the foot, no luck locally or online).

3. I could put the charger outside but that rather defeats the purpose of a garage.

And that's all I can think of.

Am I missing anything? No kind of 75c/60a rated cable I can run into that panel without conduit inside a wall, right? (I'm ignoring options with SE cable as I don't want to run aluminum, and besides not sure that solves any issues with larger sizes).

Hoping for options to get the full 48a charge, but I think I'm stuck with 40a with these limitations. Am I?

Linwood

Update: Adding image of panel by request.

Update #2: Adding image of back side of panel:

Answer 1

I'm going to post this as an answer as no further ideas appear forthcoming.

The question I asked related to how I could get a 60a run from the panel when I could not use conduit, and do so in the existing smallish holes.

I do not believe there is a viable way. NM-B must be used at 60C, and so #6 can't get to 60a, and #4-2 appears not to exist, and #4-3 would almost certainly not go through the hole (and I couldn't find it cut to length in town, though that's logistics not code).

THHN can't be used without conduit. MC was a suggestion I got offline and solves the problem with #6-2 but that jacket would also almost certainly not go through these holes (#6-2 NM-B pushed easily but mostly from being very slick, almost pre-greased). I also am not sure it's legit not to have a connector on the MC armor as it entered the panel.

So I think the answer to my question is there was no other viable choice.

In terms of load calculations and other considerations - all good info, I do not discount it, but that was not really my question. I used NM-B and limited it to 50/40a until I can replace that panel.

Answer 2

Rule of Six Panel

In ye olden times, service sizes were increasing to 100A or more, but large main breakers were relatively expensive. As a partial workaround, the NEC includes Rule of Six. This means that instead of a single main breaker, you can have up to six throws to turn off all electricity in a building. That can actually include multiple small panels (I had that situation until panel replacement a couple of years ago), but is very commonly done as a 12 space panel with 6 double breakers. Each double breaker is one throw, so the rule is satisfied. In some cases it is done as a much larger panel with 6 double breakers at the top, one of which feeds the "lighting section" of small circuits below it. In other cases, such as yours, one of the 6 breakers fees a large subpanel. Rule of Six has some drawbacks - in particular, any change to any of the 6 breakers may require pulling the meter to do it safely, but it saved some money at the time and is still allowed, though not normally used in new installations. A typical setup might include:

• Lighting section (your big subpanel)
• Water heater
• Dryer
• Oven
• Cooktop
• HVAC

The problem is that once you fill up the panel you can't simply stuff in half-size (tandem/"double stuff") breakers because then you lose the "six throws". And you can't generally add large loads to the "lighting section" because it doesn't have enough power available.

In your particular case, it appears you have some of the six breakers available, so allocating one to EV charging should be doable. The question is: How much charging?

Load Calculation

An NEC Load Calculation takes into account many different things, including:

• Required circuits (e.g., at least 2 in the kitchen, at least 1 for bathrooms)
• Cooking equipment (special rules, gets complicated)
• General lighting and other loads, based on the size of the house
• Large appliances such as clothes dryer
• Larger of heating or air conditioning (since you won't use both at the same time)

and some other details. The result is a single number. Your panel or service needs to be able to supply that amount of power.

One Load Calculation is always needed to determine utility service size. If that number is lower than your existing utility service then the balance can be used for new loads (e.g., EV charging). If not, then additional work is needed - see below for options.

An additional Load Calculation is needed if you are working with subpanels. In your case, the 100A panel should have a Load Calculation to determine how close it is to capacity. For example:

• Utility Service = 200A
• Total Load Calculation = 150A
• Subpanel Load Calculation = 70A

In this case, you could add 50A of EV charging (which is 40A actual, 50A circuit size due to 80% derate) in the main panel. Or you could add 30A in the subpanel.

Note that Load Calculation is NOT:

• Add up breaker handles - that will generally produce a far higher number
• Utility bill average power usage - that will produce a number that is way too low

What might get you close is utility bill peak power usage. But many utilities only provide that for commercial buildings. In addition, that doesn't account for continuous use derate and it doesn't account for changes in usage. For example, maybe you have never had a really hot or cold Thanksgiving Day or similar with lots of cooking on electric appliances and HVAC and guests around so all the lights and TVs and everything else is on at the same time.

If your Load Calculation(s) work out OK to give you enough excess capacity then you can add the EVSE, but only up to the limit. A 48A actual/60A circuit EVSE can normally be installed/configured for a much smaller size - typically anywhere from 15A/20A on up. Most people can do just fine with a 30A or even a 20A circuit for EV charging, which can often avoid needing electrical upgrades.

Heavy Up

One option if you don't have enough capacity is an increase in utility service size, also known as a Heavy Up. This typically involves a new panel, in your case replacing the Rule of Six panel and the big subpanel with one large main panel, typically 200A rated. But the cost to do this may increase significantly depending on state and local requirements. While you generally don't need to install AFCI and GFCI on circuits that didn't require them previously, you will likely need to get grounding (ground rods) up-to-date, possibly a shutoff next to the meter (but your panel may be close enough to the meter and outside, satisfying that) or other things.

Load Sharing

Another option is Load Sharing. This is basically a setup where the EVSE (and possibly generalized to other large loads) essentially watches total usage and allocates power based on currently available power. You could have 100A service and as long as < 40A is in use, which is most of the time, especially when everyone is sleeping, allocate a full 60A (48A actual) to the EVSE. On the other hand, if you are cooking and drying clothes and running HVAC then perhaps you have 70A in use and the EVSE will only get 30A (24A actual). This has an upfront cost for the monitoring/sharing equipment, but can avoid an otherwise expensive full system upgrade.