-
Want to remove ads? Register an account and login to see fewer ads, and become a Supporting Member to remove almost all ads.
Gen 3 Wall Connector Wire and Conduit Question
- Thread starter burn1010
- Start date
Interestingly, the 4/3 MC is 40% the cost of the 4/3 Romex, and the only place I could even find the 4/2 MC, its 4X the cost of the 4/3 MC. So I ordered in 4/3MC. In addition, I encountered several electricians during this process locally (in local supply houses, Home Depot, etc.), all of whom insisted I was crazy for using 4 gauge instead of 6. When I explained my reasoning (namely, that 6 gauge is rated only to 55 amps and cannot legally be used on a 60 amp breaker), they all simply said well that's how we have being doing it for years and never had a problem.
The issue is that #6 NM-B isn’t rated for high enough temperatures for use as a 60a EVSE circuit. Depending on length, you should size up to #4 anyways.
You can absolutely use #6 THHN in conduit for a 60a.
This has been covered ad nauseam in other threads, and you are making the same mistake of people misusing that code provision. The breaker can go up, so then people think that magically means every single piece of equipment in the whole thing is transformed into being rated at that level. But the cable ISN'T. It's still limited to 55A, so that has to be the lowest common denominator, so you can't use it as a 60A rated circuit. Period.
NM wire and 60 degree ampacity limit
I've seen a number of comments similar to the one that follows relative to the usage of non-metallic sheathed wire (a.k.a. Romex). The ampacity of Types NM & NMC cables shall be that of 60ºC(140ºF) conductors. Do I take the "NM" in the comment above literally, meaning NM-B is excluded from...forums.mikeholt.com
In this case, even with a 60a breaker, the charger is drawing a 48a load, so the round-up NEC 240.4b guideline would apply.
Charger spec:
60a breaker draws 48a load
Wall Connector
The Tesla Wall Connector is the most efficient way to charge a Tesla at home — just plug in your vehicle overnight and wake up to a charged vehicle.www.tesla.com
80% of 55 amps is 44 amps. Less than the 48 amp load of a 60a EVSE. It is not safe.NM wire and 60 degree ampacity limit
I've seen a number of comments similar to the one that follows relative to the usage of non-metallic sheathed wire (a.k.a. Romex). The ampacity of Types NM & NMC cables shall be that of 60ºC(140ºF) conductors. Do I take the "NM" in the comment above literally, meaning NM-B is excluded from...forums.mikeholt.com
In this case, even with a 60a breaker, the charger is drawing a 48a load, so the round-up NEC 240.4b guideline would apply.
Charger spec:
60a breaker draws 48a loadWall Connector
The Tesla Wall Connector is the most efficient way to charge a Tesla at home — just plug in your vehicle overnight and wake up to a charged vehicle.
I don’t even know why people argue this. It’s like $0.20/ft more expensive to get #4 wire, and it is actually rated fully for the circuit. Just buy the right size wire.
Electricians are lazy and cheap, and they probably have plenty of #6 on the truck. It “probably” won’t be an issue.
But it’s one of those things where I actually have to live in the house and use the charger and want to make sure it never burns down and destroys all my stuff, and they don’t - so they don’t care as much that its technically over the line and unsafe.
If they use THHN then it’s going to be fine. But if they cut corners because they are cheap and lazy, there is real risk, even if it is small.
You are continuing with this mistake by talking about it as if it is a standard, intermittent 48A load. If that were the case, like with an oven or a clothes dryer or welder or some other appliance that you turn on and off for short periods, then all it would need to meet is just equal to or greater to that 48, so you could do a 50A circuit.
But that's not true.
It's not just a 48A load. It is a 48A CONTINUOUS load. Because it is continuous, code requires that the circuit rating be 125% of continuous loads. The 55A limit of the cable does not meet that, so cannot be used for that. The conductors must be able to meet 125% of 48A, which is 60A.
Yes, the 60A breaker would be fine if you ALSO use wire that is rated to support 60A. 6 gauge Romex doesn't.
Yes I've been appropriately chastised, redacted my postings, and given a pound of flesh in sacrifice to the forum gods ;-)
Because certain chargers limit you to 6 awg wire, like the Chargepoint Home Flex. Also, 4awg is a pain to bend and work with. Lastly, 4 awg NM is more like $7 a foot more expensive and 6 awg now days.
In which case you should use #6 THHN/THWN. It would suffice for a 60a. You just have to put in conduit for the whole length.
You just can’t use #6 NM-B for a 60a because #4 is too expensive.
Just as an interesting little aside. I don’t understand why the NEC allows a 60 Amp breaker to feed a wire rated for 55 Amps maximum merely because 55 Amp breakers are not available. Isn’t this a little unsafe?
This means that if the load decides to pull 60 Amps for some reason, the wire will overheat yet the breaker will not trip.
If it really is dangerous to pull 60 Amps through 6/2 NMB shouldn’t the code require a breaker that absolutely prevents this?
Because if it were intermittent loads it would probably be totally fine. However when dealing with dedicated continuous load circuits you need heavier gauge wire because #6 NM-B wire doesn't support 48a safely. It is only rated up to 44a continuous.
That is a decent question to ask. This potential dangerous area would involve 56 through 60A. It's a small amount of overage that would be inappropriate but might not trip. And I guess the answer is that code is made to be very safe with a little bit over overcautiousness to it. So this would officially be a 55A rated circuit, and if something goes wrong, what are the chances it will create a condition that accidentally goes into exactly the range between 56 and 60 for a very long extended period of time? Vanishingly small, basically.
I think it would have been better if the Powers That Be would have made this not even a question by just publishing the rating at 50A instead of 55. Then it would discourage this type of thing with people trying to make excuses to use it for 60.
Understood. So let’s dig a little deeper into the continuous load situation and this allowance to serve “55a wire” with a 60a breaker.
Say we have a circuit consisting of 6/2 romex supplying our wall connector. Let’s accept the specification that the maximum safe current that can flow continuously through this wire is 0.8*55 = 44a. Now we need to provide over current protection for this circuit to avoid overheating and fires. Can we find a breaker that allows 44a continuous to flow but does not allow 48a continuous to flow?
Will long term current flow at > 80% of a breaker’s rating cause it to trip? Is this designed into the interruption mechanism somehow?
If this is the case, and the illusive 55a breaker would have offered proper protection, then using a 60a instead would allow the 48a continuous current case, which we claim is unsafe.
Similar threads
- Question
