Electrician questioning installinig a 50 amp circuit

It's been a while, but I would imagine the cabling for the kit is rated for higher than 40 amps.
IMHO, a 50 amp breaker is what is called for in that circuit.

Typically, a circuit should be limited to 80% of it's wiring ampacity.

For non-motor loads, I seem to remember the breaker can be rated for up to 25% higher than the expected current load. So, 40 amp max load X 1.25 = 50 amps.

Any wire-biters out there with more recent resi experience please step up...........

Of course, any electrician worth his weight would have a NEC code book available....

 

I thought it was standard operating procedure for a consistent draw be 20% under the circuit breaker rating.

 

The most the Prime can pull is 16A @ 240v. Needing 50A is tons of overkill. So is 40A as is 30A. My Juicebox 40 happens to be on a 40A circuit, but only for future-proofing. I am guessing that whatever I buy in 2024 will need far more than 40A @ 240v. Honestly, I'd go for the 50A since it should not cost more than a 40A. The real $ is in the labor time.

 

I’m not sure I understand. The product page for the JuiceBox Pro 40 EVSE says it has a NEMA 14-50P plug, which would mate with a NEMA 14-50R receptacle, both 50 A types according to the ANSI/NEMA WD 6 standard (PDF), page 52. If the plug on yours looks like the one in the standard, than perhaps your electrician has made a mistake; if not, could you post photos of the plug and any data plates or labels on the EVSE, please?

Assuming the EVSE does indeed have a 14-50P plug, and the electrical service, panelboard, etc., can all handle the added load from charging, I’m not sure why it would be a problem to connect the EVSE to a 14-50R receptacle on a dedicated branch circuit protected by a circuit breaker rated at 50 A.

Not only standard procedure, it’s required by the National Electrical Code. In the 2017 edition, see 210.20, 625.41, and 625.42. It’s not a coincidence that 40 A × 125% = 50 A.

 

If your electrician can't figure this one out, I'd thank them for their time and find a new one.

The JuiceBox supplies 40A (potentially). It needs a 50A connection. It has a 50A plug. Perfect. This can plug into a 50A socket. OR any socket that can accept 50A or greater. You can plug a 1A load into a 50A socket, or a 20A load, or a 40A load forever. Or a 50A load for a short time. But if you take a unit that can provide 40A continuously you cannot put it on a 40A socket.

Similarly the wiring must be able to support at least the continuous current. 50A wiring is OK. 90A wiring is OK. 200A wiring is OK. 40A is NOT OK. Then on the protection side, the breaker, must be rated at equal or less the ampacity of the wire. So the 100A wiring could use a 100A breaker, or 80A breaker, or 60A breaker or 50A breaker. It is pretty simple.

Good on you to ask, and you are correct. 50A socket, at least 50A wiring.

If you are truly "future proofing" then I personally would wire it with copper able to carry 100A continuously like 3-3-3-5. I say copper because something like an EVSE is the worst enemy of aluminum wiring even though it is so much cheaper. Heat it up close to max load for 4-8 hours then cool down to room. Do that every single day. The Al will hate you for that. Definitely pay for the Cu now.

 

How many vehicles do you think will need recharging at the same time? Remember, your service-panel only has so much capacity.

8 hours from a 40-amp circuit will deliver 200 mlies of range. Knowing that rule-of-thumb, we installed two 40-amp lines in our garage.

Choosing to use 3/4" conduit meant higher gauge wire could be pulled later. For now, our JuiceBox Pros are set (via the software interface) to only allow the 80% draw (32 amps). That 7.7 kW max rate should be fine. After all, many public chargers top out at 6.6 kW. Odds are, a future vehicle wouldn't stop at the charger's max of 10 kW anyway, so having allowed for 40-amp draw (50-amp circuit) wouldn't be much of an improvement. The spec for that port's protocol can actually handle 19.2 kW. Delivering that requires a 100-amp line though, which would be very expensive and brings you back to overall capacity available.

So, don't worry about the numbers or future proofing. 50-amp line could be nice. 40-amp is fine.

 

As an electrical engineer - agree with others here - a 50A Breaker with 50A wires, wired to a 40A device is ideal. When sizing a breaker we think about preventing two main things from happening:

1. Tripping when we shouldn't - this is why breakers are usually sized 110-150% greater than the maximum load under normal use. This prevents tripping when smaller non-damaging voltage transients come through the power. For loads that have a very high inrush current, like motors, the breaker will be sized even larger.
2. Not tripping when we should trip - this is why you don't run 50A breakers when there is only wiring rated for 15A. If you have a 50A breaker and get a short that results in a 30A power draw, you don't trip, which is very bad.

I don't understand what the electrician is having issues with - it's super normal practice to put a 20A breaker on a 15A 120V wall outlet. This is exactly the same thing, just higher voltages and current.

 

You probably meant a 15A breaker on a 12/2 (12AWG) circuit, which would be rated for 20 amps, IIRC.
But +1 for your intended point.

+1 again.

 

Nope I think it was intended as written. The breaker protects the wire. So yes a 20A breaker on wire that can minimally support 20A like 12awg. But then you can put the cheaper builder grade 15A outlets on the wall if you want. People get confused with the slanty slot for 20A outlets.

 

Correct, normal house wiring in most states is:
12 AWG - 20 amp breaker, 15 Amp Plug
10 AWG - 30 amp breaker, 20 Amp Plug (has extra horizontal slot)

12 AWG wire can handle 20A no problem. House wiring is very conservative as it needs to work for decades and decades without problem and the failure rates need to be measured in values of per million houses.

Some old houses have 20 amp breakers on 14 AWG wire, which is unsafe. These should be replaced with 15 Amp breakers. The problem is that any high current devices like vacuum cleaners, are likely to pop a 15 amp breaker in normal use as the current under stall is often just above 15A.

 

Thanks for posting the photos. The plug and cord would have been evaluated as part of the UL listing for the equipment; note the ᴄULᴜꜱ listing marks on the plug itself and on the label. The required ratings for these are determined by the equipment characteristics, not necessarily by the outlet configuration.

 

It's worth understanding the reasoning behind the different wire sizes (diameters of the metal conductors) and insulation types. This brief quote from Wikipedia:

The ampacity of a conductor depends on its ability to dissipate heat without damage to the conductor or its insulation. This is a function of the insulation temperature rating, the electrical resistance of the conductor material, the ambient temperature, and the ability of the insulated conductor to dissipate heat to the surrounds.

All common electrical conductors have some resistance to the flow of electricity. Electric current flowing through them causes voltage drop and power dissipation, which heats conductors. Copper or aluminum can conduct a large amount of current without damage, but long before conductor damage, insulation would, typically, be damaged by the resultant heat.

The ampacity for a conductor is based on physical and electrical properties of the material and construction of the conductor and of its insulation, ambient temperature, and environmental conditions adjacent to the conductor. Having a large overall surface area can dissipate heat well if the environment can absorb the heat. ​

So, that explains the safety consideration.

The other consideration is voltage drop. Because all wire has some electrical resistance, as current flows along, some of that current is turned into heat as noted above. The longer the wire, the more energy is lost along the way and the lower the voltage at the end. So, under some circumstances, larger wire sizes are used for long runs even if one might otherwise use a smaller wire in a shorter typical run. Less heat = less voltage drop.

Electricians are taught how to make the basic calculations to determine the appropriate wire size, type of insulation, and method of installation for any specific application. It's because of all these variables that it's often impossible to give a simple answer to many of the questions asked here on PriusChat.

 

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Just curious, what are the final cost for your installation? I am interested in installing L2 charger similar to yours, but I do also have to install a new 50A service.

 

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Yeah, all else being close to equal I'm pretty sure I'd want to go with the one who understood that you needed 50A service on a circuit that may take a 40A load.