Circuit Breaker Amperage

[Turlejay]

Hi Guys and Gals

Sorry if this has been covered before. My electrician asked me what the reasoning for a 50amp circuit breaker for the nema 14-50 connector when the max draw is 32 amps. Is there an underlying current draw that's not advertised? Thanks in advance.

Jay

 

[jeadams]

32 amps is the maximum that the mobile charger that came with the car can provide. Load for a circuit breaker should be only 80 % of it's rated capacity. So for a 50 amp breaker continuous load should not exceed 40 amps.

 

[Turlejay]

So by that reasoning 40 amps should be sufficient?

 

[Big Dog]

otoh, you might want to future-proof the line by installing a 60 amp circuit (48 amp charging) just in case you ever want to add a Tesla Wall Charger later. The additional costs for parts is minor if the electrician is already onsite.

 

[jeadams]

So by that reasoning 40 amps should be sufficient?

Yes, a 40 amp breaker would supply the mobile charger with 32 amps.

 

[Turlejay]

otoh, you might want to future-proof the line by installing a 60 amp circuit (48 amp charging) just in case you ever want to add a Tesla Wall Charger later. The additional costs for parts is minor if the electrician is already onsite.

We aren't staying in this house very long, so less is more for us right now, thanks

 

[ivy_]

We aren't staying in this house very long, so less is more for us right now, thanks

The reason for the 50 amp plug is that there isn't a NEMA 40 amp plug.

 

[Rocky_H]

You're getting a scattering of answers that are all basically correct, but are maybe not putting the whole picture together.

Since the appliance being plugged in will draw 32A continuous, a 40A breaker would be acceptable by electric code. It does allow for that exception of a 50A outlet type on a 40A breaker for this reason and because there is not a 40A outlet type.

But the reason why a homeowner would be asking for a 50A rated circuit anyway is basically just to do it the better "more right" way instead of the bare allowable minimum. If a future buyer of that house wants to use that existing 14-50 outlet, it would be helpful if it is installed with the full regular expected capacity, rather than just with the thinner wire of the 40A exception case. What if they want to use an RV that could use more current? What if your UMC breaks, and you find a fantastic deal on a Juicebox or a Tesla corded mobile connector or Tesla brings back the pluggable wall connector or something that can plug in and pull 40A continuous? You'd be irritated that the initial bare minimum is now a limiting factor.

Or what if a friend or relative with an electric car (or truck?) comes to visit at some point a few years from now, and they have a 40A cable and could use the extra charging speed?

 

[Turlejay]

You're getting a scattering of answers that are all basically correct, but are maybe not putting the whole picture together.

Since the appliance being plugged in will draw 32A continuous, a 40A breaker would be acceptable by electric code. It does allow for that exception of a 50A outlet type on a 40A breaker for this reason and because there is not a 40A outlet type.

But the reason why a homeowner would be asking for a 50A rated circuit anyway is basically just to do it the better "more right" way instead of the bare allowable minimum. If a future buyer of that house wants to use that existing 14-50 outlet, it would be helpful if it is installed with the full regular expected capacity, rather than just with the thinner wire of the 40A exception case. What if they want to use an RV that could use more current? What if your UMC breaks, and you find a fantastic deal on a Juicebox or a Tesla corded mobile connector or Tesla brings back the pluggable wall connector or something that can plug in and pull 40A continuous? You'd be irritated that the initial bare minimum is now a limiting factor.

Or what if a friend or relative with an electric car (or truck?) comes to visit at some point a few years from now, and they have a 40A cable and could use the extra charging speed?

Thank you for the input, you have been very helpful. Obviously I want to do what's best here. My issue is honestly I'm not sure what my existing fuse box can handle and don't really want to invest tons of money expanding for a house that I don't plan to stay in more than another year. If 40amps is all I can do then so be it. In fact the 30amp option might even suffice.

 

[Rocky_H]

Thank you for the input, you have been very helpful. Obviously I want to do what's best here. My issue is honestly I'm not sure what my existing fuse box can handle and don't really want to invest tons of money expanding for a house that I don't plan to stay in more than another year. If 40amps is all I can do then so be it. In fact the 30amp option might even suffice.

Oh yeah, if capacity is tight, then there's not much good reason to go higher than you need. Even the 30A circuits are still very solid charging speeds.

 

[kkillebrew]

50 amps is the correctly rounded up breaker in this case. There is a reason the plug is designated with the 14-50 designation. I am not sure why an electrician would even ask. If you stated a 32 amp charge, which in the case of an EV must be considered continuous, then a 40 amp break is pushing the limits of inadequacy.

 

[Runt8]

50 amps is the correctly rounded up breaker in this case. There is a reason the plug is designated with the 14-50 designation. I am not sure why an electrician would even ask. If you stated a 32 amp charge, which in the case of an EV must be considered continuous, then a 40 amp break is pushing the limits of inadequacy.

Installing a NEMA 14-50 on a 40 amp circuit is perfectly acceptable and won’t cause any problems when used with a 32 amp EVSE. There’s nothing inadequate about it.

As others have pointed out there are many considerations to be taken into account when deciding on a charging strategy, but implying there’s something wrong with using a 40 amp breaker is incorrect.

 

[eprosenx]

Thank you for the input, you have been very helpful. Obviously I want to do what's best here. My issue is honestly I'm not sure what my existing fuse box can handle and don't really want to invest tons of money expanding for a house that I don't plan to stay in more than another year. If 40amps is all I can do then so be it. In fact the 30amp option might even suffice.

Ah yes, your logic here sort of makes sense, but this is not how NEC (code) sees it. Whether you install a 40a breaker or a 50a breaker (with adequately sized wire), the “load calculations” still treat that circuit as a 32a continuous load since that is your “intended load”. If you do a 40a breaker and wire then later down the road if someone plugs in a 40a continuous draw device then you are trusting that breaker to blow to protect the wire. Where if you do a 50a breaker and wire but it overloads the panel then you are relying on the main panel breaker to blow (assuming you have one).

So regardless, I would absolutely put in wire good for 50a (if not 60a even!) regardless of if you chose a 40a or 50a breaker so that later of more load capacity is freed up (or the panel is upgraded) the wire can be used at a higher amperage. The difference in cost between 8 awg and 6 awg is pretty small compared to labor.

I personally would go for the 50a breaker and wire if the load calcs allow for your 32a load, but as @Rocky_H points out, if 32a is not ok then a 30a circuit (dryer plug) which is 24a continuous is probably just fine. (Even a 20a 240v circuit is likely sufficient for most folks)

 

[kkillebrew]

Installing a NEMA 14-50 on a 40 amp circuit is perfectly acceptable and won’t cause any problems when used with a 32 amp EVSE. There’s nothing inadequate about it.

As others have pointed out there are many considerations to be taken into account when deciding on a charging strategy, but implying there’s something wrong with using a 40 amp breaker is incorrect.

Look at my other posts - of course if will work.. then they guy moved and they new guy thinks he has an actual 14-50 but he does not.

80% of a 40 amp breaker is 32 - so no matter what you say that breaker will be at the limit of its acceptable use. Tesla, and NEC says the same thing, use a 50, but you know better of course.

 

[Runt8]

Look at my other posts - of course if will work.. then they guy moved and they new guy thinks he has an actual 14-50 but he does not.

80% of a 40 amp breaker is 32 - so no matter what you say that breaker will be at the limit of its acceptable use. Tesla, and NEC says the same thing, use a 50, but you know better of course.

A 14-50 on a 40 amp circuit is an “actual 14-50.” Because of the 40 amp exception in the NEC, you can never blindly assume a 50 amp circuit.

As for your second comment - the entire purpose of the 80% rule is to prevent a circuit from running at maximum capacity for extended periods. There’s no reason to believe that pulling 32 amps on a 40 amp circuit is any worse than pulling 32 amps on a 50 amp circuit. And nowhere does Tesla or the NEC state that a 50 amp breaker needs to be used. The entire reason Tesla moved to a max of 32 amps on the gen 2 mobile connector was because it’s so common to have 14-50s on a 40 amp circuit.