# Resistive load, EV charger, circuit breaker sizing

Is an EV charger a resistive load? If not, what kind of load is it?

The question behind the question is how to size a circuit breaker for an EV charger circuit. I've reviewed various sources, and it seems that the breaker could be size at 80% to 125% of the wire capacity, depending on load type. 80% to 125% is a big difference: for 6 awg copper at 90 degrees, it's a difference between 60 and 90 amps.

Keep in mind that charging would be a continuous load for hours.

• 80 and 125 are the same it’s how you use the numbers in the calculation divide by or multiply. you cannot use the 90 deg table as you will not find a service panel that is rated for 90 deg. EV chargers are required by code to be rated 125% of the actual load, most breakers are only listed for 80% or the breaker. Now starting to get the idea on the 80/125 if you provide the actual load we could help size it. Feb 19, 2022 at 21:48

EVSE stands for Electric Vehicle Supply Equipment (aka wall "charger").

So you've heard of 80% and you've heard of 125%. You are also aware of the NEC 310.15(B)(16) table (I'm guessing?) but haven't heard the good news about aluminum.

## There's only one load derate - 125%. Apply it once.

This is always in the disfavorable direction.

The rule is if you have an `X` amp load, you must provision `1.25 X` power to it or 125%. That's the rule. So if you have a 32A EVSE, you must "provision" 32 x 125% = 40A - and provision means the wire is at least that large, and the breaker is at least that large.

This 125% rule is repeated over and over in Code for a huge variety of applications. It's repeated so often that many people simply apply it to everything - and then, you get this weird inversion that isn't in Code at all - the so-called 80% derate. By this logic, if you have a 40A circuit/breaker, you can put 80% of that or 32A of any load (such as a tankless water heater or an EVSE). This is technically wrong, but since it only errs on the side of safety and overcompliance to Code, no one cares and no one will correct you. This sets the stage for...

...inadvertently double-applying the derate. For instance you colloquially call something a "40A EV charger" because its spec calls out a 40A circuit and breaker. You know it charges at 80% of that or 32A. But then you go "we need to apply the 125% rule, and 125% of 40A is 50A". No, that's a goof, you unnecessarily applied the derate twice. It derates once, from 32A actual x 125% to 40A.

Where does 125% apply? Only certain loads. But many. First, it applies to all "continuous loads" (anything that will run > 3 hours at a stretch) e.g. "not a coffee maker". It also applies to cord-and-plug connected loads (so yes, a coffee maker lol). Second, for certain loads, they remove your right to argue by imposing 125% - just a few examples are space heaters (baseboard heaters) and EVSE's.

## You don't get to use 90C rating

In order to make use of the 90C thermal rating on wire, four things need to be true.

• The wire itself has to be rated for it - THHN is, UF is not.
• The conduit or wiring route has to be rated for it.
• The source terminals and enclosure must be rated for it - breakers and panels are only rated 75C.
• The destination enclosure and terminals must be rated for it.

Getting all four of those stars to align is hard. So as a practical matter, we invariably end up no better than 75C thermal - just plan on it. With NM and UF, we're also stuck at 60C thermal, but don't use that stuff anyway, it's made of copper. Also, older panels may be 60C rated (thanks Ed).

## At large sizes, aluminum is your friend

America has lots of experience with aluminum wire, and it's always worked great for large feeder. The cheat-sheet is #1 use terminals properly rated for aluminum. #2 set torques with a torque driver. There were some problems with small branch circuit aluminum wiring in the 1970s, but that failed because of what I put in italics. Note that on larger feeder, electricians have always torqued those, and terminals have always been legitimately AL-rated (often made of aluminum).

We see people who are skittish of aluminum because of "folk knowledge" about the 1970s problems, and want to spend on copper "out of an abundance of caution" - but that makes no sense. If we are looking to turn dollars into safety, a rational study of risk would give 100 better places to spend that money - just in electrical, making sure everyhing is GFCI protected that should be.

By the way, EVSE's should not be GFCI protected. Why? Because EVSE's already contain a vastly superior GFCI, and feeding them from a run-of-the-mill GFCI defeats that improved protection. In particular, the EVSE's onboard computer will reset its own GFCI several times, to see if the problem has cleared, which it usually has. That way, you awaken to a recharged car and not a tripped GFCI. UL has approved this and this is part of the spec.

• I agree with the exception of the 90 degree the service panel will end up being the limiting factor in residential (we don’t know what wiring method is being used). Feb 19, 2022 at 22:01
• Harper, I'm beginning to like you. In my deep dive into these electrical topics, I sometimes read materials that leave me feeling dumber. Your responses reliably make me feel smarter. Thank you for your efforts. Feb 19, 2022 at 22:06
• How would I know or what would be evidence of a 90 degree service panel? The house was built early 60's, copper wiring. Over the years, a few circuits have been added, and service was very recently upgraded to 200 Amp, using a new service panel. This work was done by a professional electrician and was inspected. I have no idea if this is helpful. Feb 19, 2022 at 22:12
• Early 60 wiring was 60 degree. Feb 20, 2022 at 1:11
• @ElroyFlynn I'm unaware of any 90 degree panels in the residential space. They would be industrial arcana, with a price to match, so there would always be a cheaper path - a) eat the wire size bump, or b) exit the panel with 75C sized wire, go to an enclosure and use 90C splices (Polaris) to change to 90C sized wire. However if you attempt the latter have all your Code chapter and verse ready, as I'd expect a fight from the inspector. Feb 20, 2022 at 18:45

My understanding is that EV chargers look like a pure resistive load that is continuous for 6+hrs at a time (likely the only thing like it in your house), unlike something like a motor that is inductive.

I mostly agree with the top rated answer regarding derating using the referenced 125% rule once. It's well written, so reference it! However, the temperature rating of the cable does come into play for other reasons to derate.

For example, I'm installing a 48A EV charger. Using the 125% rule, that means I need a 60A circuit (which the manufacturer already calls out, so I just need the 60A circuit, not adding another 25% on top).

However, I want to run that in a 2" conduit. This conduit will have other conductors in it, 6 of which will be carrying large 240V loads (2 pairs for EV, 1 pair for an air compressor), and two of which will be carrying small loads (10amp 120V).

Given that I have a total of 8 current carrying conductors, that means I have to multiply the allowed current on the wire by 0.7. However, given that I'm using 90C wire, I can start with 95A for my #4 CU (in 90C column), rather than 85A (in 75C column). Once I do all my calculations, I have 66.5A max capacity on the cable itself due to the bundling derate. Now, the load center, wire itself, and devices are going to be 75C (or possibly even 60C, though not in my case), so I look up the value there at 85A. Some circuits are given other limits, like 12 gauge limited to 20A, even though the wire within is rated 30A in my case. I put all these limits together, and then have to take the smallest of all that apply MIN(95A, 65A). In my case, that is 65A. That's still bigger than the 60A circuit required, so I'm good.

• The otherwise excellent response by Harper Reinstate - Monica contained slightly wrong information regarding 90C conductors. I'm not allowed to post a comment, so assumed the most appropriate option was answering the question myself. I just didn't go into as much detail as the other answer on the basics, focusing on the portion that was omitted. Since I neither have nor asked a question on my own, I think using a new question to point out the issue would be inappropriate, and pointless. Is the issue here using my project as an example to illustrate using the 90C rating in my answer? Aug 18, 2023 at 12:35

Probably not, it's likely somewhat inductive. But it really depends on the specific make/model of charger and possibly the vehicle as well.

The best resource will be the installation manual for the charger you are installing.

I size the wire to support the worse case current draw of the device,then maybe up it a size if it is close and or if there are code requirements. I double check to be sure the wire size will support that load over the prescribed distance. Then I size the breaker to protect the wire, not the load. Those temperatures are in degrees celsius so you are looking at 194 degrees fahrenheit. Do yourself a favor and follow the NEC ratings. You can find them at: https://www.usawire-cable.com/pdfs/nec%20ampacities.pdf This will keep you safe.

• you cannot use the values straight out of the tables for a EV charger the values are either derated to 80% of the wire size or 125% of the load size you CAN NOT use the 90 degree table ad there are no systems that would be 100% 90 degree rated so your advice is dead wrong it won’t keep you safe. Feb 19, 2022 at 21:56
• Emphasis on dead wrong. Especially when the fire starts. Feb 20, 2022 at 0:28