# What is a the maximum breaker size inside a subpanel?

I have an electric car that I'd like to charge faster. I have a charger (EVSE) that will pull 24A. I have a detached garage with 30A service running to it (10-3 wiring, 30A breaker at the main panel). My question is: Am I allowed to have a 30A breaker in my garage subpanel with 30A service?

• "I'd like to charge faster" than what? What are you doing now? What equipment are you using? Do you want 2x your current speed or 3x? Sep 24 at 18:57
• Does the car not charge fully overnight? Sep 25 at 2:19

A 30-amp breaker in the main panel, feeding 10-gauge cable, can have a subpanel at the end with a 30-amp breaker in the subpanel.

However, if you live in the USA, NEC requires high-current devices that draw electricity for a long time to not draw more than 80% of the capacity of the circuit's design. This is called a de-rate.

Your EV charger is one of those always-ish-on devices drawing large currents. Your circuit cable is 10/3, which is 30 amps maximum. 80% of 30 Amps is 24 Amps, which is the maximum current that your existing circuit can allow for the EV charger. However, you cannot change the garage service or EV charger circuit breakers to anything larger than 30 Amps with your existing service. And you cannot increase the EV charging circuit beyond the existing 24 Amps.

To draw more, you must upgrade the service to the garage. And you have to do a load calculation on your whole house service to determine if there is room for more charging power. If you have room in the load calculation, pick the amount of current you want to use to charge the EV. Find out what maximum current that EV charge current is 80% of (multiply your desired charging current by 1.25) then bump up the replacement garage supply cable and circuit breaker in the main panel to the next available circuit breaker size typically available for your panel above the calculated current, and appropriately-matched cable gauge. An new question on calculating this value would be a good idea if you want to go this route.

• Can the EVSE be configured to draw 20A actual? Then you can cable it the way you describe (30A feeder and 30 amp branch), get a 25% increase in charge rate, and have SOME power left for your garage while the car draws 67% of circuit capacity. It's not the 50% increase in charge rate you hope for, but it's an increase .... if the charger is capable. Sep 24 at 17:32
• Thanks for the clarification and concerns. I am aware that I would need to limit usage on the main 30A breaker to 80% (24A) for continuous use. This is within the scope of my use case as there will be nothing else turned on in this detached garage except for an LED light bulb less than a minute a day. I can limit the EVSE to 20A, but this will be unnecessary as I do not need the headroom on a regular basis. Thanks! Sep 24 at 17:41

In addition to the 80% derate, whole house load calculation, etc. all discussed in Triplefaul's answer, you also need to do a load calculation for the garage, or at least put in some consideration to the existing loads.

As I understand it (the question is not 100% clear), you are currently charging using a Level 1 charger (i.e., 15A (12A actual) or 20A (16A actual) at 120V) and want to install a 24A actual (30A breaker required) Level 2 charger. Since that will take the entire garage load allocation, you are immediately heading into dangerous territory.

I'm not too concerned about lights (assuming they are LED or fluorescent) as that load is probably 1A or less. But any significant additional load run simultaneously with the 24A EV charging can cause problems. The 80% derate is because of the way things are designed to handle heat in wires. If you add 6A of other loads running at the same time as your EV is charging, you are now at 30A for the subpanel. 30A for the subpanel will not trip the breaker. But if the 6A load is going for a long time ("continuous") then the effect will be to eliminate the safety margin provided by the derate because your subpanel feed is also 30A rated, just like the EVSE circuit.

This is not much of an issue with a larger feed (e.g., 60A) because unless you are minin' or growin', you aren't likely to have that many other continuous loads. But even something as simple as a 12A plug-in heater to work on your car on a cold day would cause problems here - either tripping the 30A breaker (inconvenient) or burn up your wires (because at a 20% overload a breaker may take a while to trip).

• Good point about load-calculating the garage panel! Sep 24 at 17:42
• Thanks for the clarification and concerns. I am aware that I would need to limit usage on the main 30A breaker to 80% (24A) for continuous use, but that is an important point that it pays to be sure people know about. This is within the scope of my use case as there will be nothing else turned on in this detached garage except for an LED light bulb less than a minute a day. I could limit the EVSE to 20A, but I think that will be unnecessary as I do not need the headroom on a regular basis. Thanks! Sep 24 at 17:48
• @SnappyCrunch Unfortunately, NEC does not recognize the validity of "pinky promises" to not use circuits simultaneously. You need to follow the provisioning rules in NEC Article 220. Or alternately, use a Load Management system to assure the EV never overdraws the feeder. Sep 24 at 18:49
• I thought the purpose of the derate was to allow for the possibility that the tolerance band for a 30A breaker to trip on a sustained load extends below the rated current, and the NEC doesn't want to in any manner encourage sizing of circuits that would cause frequent trips, which would in turn drive people to use unsafe workarounds. Properly designed and sized breakers should trip under any pattern of usage which would overheat appropriate wiring, so derating shouldn't be necessary to prevent overheating, but merely to ensure reliability. Sep 25 at 22:21

## The secret

Nobody runs a 30A subpanel to a garage for a light bulb. You are ignoring other loads. There's no excuse for doing a proper NEC Article 220 Load Calculation on the loads in the garage -- oh wait, THERE IS!

By using a Load Management System, you can run up to the full speed of the feeder. The system installs "CT clamp" meters on your feeder wires. They measure the draw actually being taken by all the loads in the garage, and adjust EV charge rate so that the feeder is never overloaded.

E.G. This means operating a garage door opener is going to momentarily see the EV charge rate drop from 24A to 12A. But that's fine, it recovers immediately after, but importantly, does not trip the breaker at the house. This means you do not need to "tip-toe" around the EVSE when it's running, nor worry about other family members doing something.

This requires a "wall unit" charger, but you're going to need that anyway to unlock intermediate speeds like 16-24A at 240V. Almost no UL-listed "travel unit" chargers (EVSEs) support* intermediate speeds like that. They are generally all-or-nothing 12A or 32A, both of which are inappropriate for home charging. Those units are for travel, i.e. hotels and RV parks. Do not use the cheap Cheese units off Amazon, they are not safe and can burn your house down.

A wall unit can easily be configured to any amp rate. These delete the need for either a socket or a GFCI breaker (NEC is clear that any socket installed for EV purposes must also have a GFCI breaker. Even though the EVSE has a better GFCI inside.) The Wallbox Pulsar Plus supports Load Management with a cable. The Emporia also does, but uses WiFi to "the cloud" for the connection.

## Do you really need 5 times level 1 speed?

People tend to look at amps. Level 1 charging is 12 amps, true, but at half the voltage of 240V.

So 240V @ 16A might seem like "barely more than level 1", but it's actually three times level 1 (when you take overhead loads into account). 4kW charging is not nothing, it will 50% charge even an F150 Lightning in 12 hours. For a more modest EV with a 66kWH pack, you'll regain 80% in that same time.

24A is almost five times level 1 which is overkill for most people, especially those who were marginally able to make it work on level 1.

Simply turning down the power to something compatible with your Load Calculation would save the cost of the Load Management kit.

I shouldn't tell you this, but most level 1 "charger cables" (EVSE) can be tricked into charging at double speed by making a 6" long dongle cord with a NEMA 5-15R (standard socket) on one end and NEMA 6-15P (240V/15A plug) on the other end. Check with fans of your model to confirm this works. However the gotcha with that method is that a 240V socket in a garage needs a costly (\$120+) GFCI breaker feeding it, and that sucks. This is money better put toward a wall unit like the Wallbox.

* The Tesla Mobile Connector has 16-24A dongles. And the Cadillac unit has a 24A dongle, if you enjoy paying Cadillac prices. (supposedly the Caddy dongle will fit the Chevy travel unit). Other than the dangerous garbage from Amazon Marketplace, those are the only units I'm aware of.

• I appreciate the response. Currently the Load Balancing EVSEs are too pricey for me. I have a "travel" EVSE from Amazon, but it appears to be better built than some. It says it is UL listed, and it has selectable amperages at 12, 16, 20, and 24A. The detached garage is 100 years old, but had electricity run to it in 2021. There is nothing else in the garage except a 60W light bulb, and outlets with nothing on them. The garage is only used for storage, and I'm in there less than once a week. So it feels like overkill to downrate myself to 20A or less when there is little reason to. (Cont) Sep 27 at 16:47
• (Cont) I am also not parking and changing there every night, and I drive 50-150 mi/day. So being able to substantially charge from 4 hours or less would be useful. Still, I worry about someone else coming in and plugging something like a heater to a 120V receptacle, tripping the feeder breaker because they weren't aware that they needed to stop the car charging first. So I want a switch that will make it so that either the 120V outlets or the 240V outlet will be energized, but not both. It seems like a contactor would be the best solution for this. What do you think? Sep 27 at 16:54
• @SnappyCrunch Contactor you mean like a DCC? That'll cost more than a proper EMS. Wallbox \$450 at CostCo, current monitor \$300 and Bob's your uncle. Are you sure you aren't naysaying here? Sep 27 at 19:01
• I don't think I'm trying to naysay, I really am trying to work out a simple and safe solution that doesn't cost a lot of money. It feels like a good solution for my situation is for either the 240V or 120V receptacle(s) to be energized, but never both at the same time. That would make it very hard to unintentionally overload the feeder breaker. An industrial cam switch would allow me to switch between the two circuits, but I worry about arcing if someone switches the 240V circuit off while it's pulling 24A. (Cont) Sep 28 at 21:07
• (Cont) A contactor, like the kind used for HVAC or large lighting systems, feels like the right way to switch the circuits. So the 240V receptacle would run through an appropriately sized Normally Closed contactor, and that would be actuated by the 120V from the 120V switched receptacles. So if the 120V receptacles are switched on, that also actuates the contactor, which turns the 240V receptacle off. I hope that makes sense. To me, it seems like the way to go, but I don't know what I don't know. Sep 28 at 21:13