Birken Vogt
Senior Member
- Location
- Grass Valley, Ca
Actually you could just put it on a 60 amp breaker anyway.
44 amps continuous = 55 amps calculated
#6 NM is OK at 55 amps
60 amp breaker is next standard size up.
Is #6 NM on a 60A breaker code compliance for a Tesla 48A (continuous load) EV charger?
- Thread starter sunbear
- Start date
Actually you could just put it on a 60 amp breaker anyway.
44 amps continuous = 55 amps calculated
#6 NM is OK at 55 amps
60 amp breaker is next standard size up.
I don't think this is one of the available settings on the Tesla charger.
duh of course thanks
So its really a manufacturer issue or two issues
1) to squeeze every last amp out of #6 NM can Tesla just update their firmware to add a 44A charge setting for a 55A circuit? Or is it set with dip switches I wonder?
2) They need to clarify in their installation instructions that 4/2 NM cable is OK for the max setting.
Unless the customer drives a heck of a lot of miles, or has a very short charging window (due to varying rates, etc.) 40A will be more than fast enough. A Tesla sedan gets 3+ miles per kWh, so 40A (7KW) adds 21+ miles each hour on charge.
sunbear
Member
- Location
- New Jersey
- Occupation
- Engineer
In my view, if the customer requested the capability to charge at 48A, the electrician should not pull a fast one and deliver a 40A charging solution. It's dishonest.
I don’t disagree with that, but nothing about what the customer requested, or what the EC agreed to was mentioned in your original post (#34).
Agreed that 40A circuits are usually sufficient for EVs. I consider anything that can do a full battery charge in 12hr to be sufficient, most people will almost never need that much charge in a day anyway. I'd say any battery under 90-100kWh is likely fine on a 40A circuit, and that's most EVs on the market right now.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
I agree. I also wonder if anything was actually specified in writing, or what it was. It is confusing that the circuit ampacity is required to be greater than the charging current. ECs need to be clear what they are quoting.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
It is because the NEC uses the word ampacity for both the run of the circuit, and at the terminations. Say we switch to a different word for sizing conductors at terminations, call it termination size, so that ampacity just applies to the run of the circuit. Then the conductor sizing rules would be:
1) Ampacity must be at least the calculated load (no 125% factor for continuous use, ampacity is already a continuous rating).
2) Ampacity must be sufficient for the OCPD used (240.4, including 240.4(B) when applicable); so the OCPD sizing rule (125% of continuous load plus 100% of non-continuous load, unless the OCPD is 100% rated) makes an impact here.
3) Minimum termination size is based on the termination rating (60C or 75C) and the corresponding column of Table 310.16 (no adjustment or correction), which needs to be at least 125% of the continuous load plus 100% of the non-continuous load.
The latter part of (3) is a reflection of the product listing and testing standards. They specify a procedure for the size of wire used for doing the temperature rise tests. Then based on the observed terminal temperature, the termination may also require a higher temperature rated insulation than the termination rating. (See footnote 1 for an elaboration on this).
Then if 334.80 were phrased so as to limit circuit ampacity but not termination size, there would be no problem with the install discussed in the OP. And no problem in practice, as the install in the OP would perform fine, it's just not NEC compliant with the rules we have (which are not phrased as sharply as they could be).
Cheers, Wayne
Footnote 1: As a thought exercise, not how I imagine it happens in practice, the manufacturer could do the temperature rise tests with various size wires connected. First, try the 75C rating with the wire sized accordingly. If the terminal temperature does not exceed 75C (and always assuming other required temperature checks elsewhere in the appartus comply), then it can get a 75C rating. If the terminal temperature is between 76C and 90C, then it can get a 75C rating with minimum 90C insulation. If the terminal temperature exceeds 90C, no 75C rating allowed. Then try the 60C rating, and you can get a result of 60C rated; 60C rated with minimum 75C insulation; 60C rated with minimum 90C insulation; or a compelete failure, the manufacturer has to redesign the thermal behavior of their equipment. I expect all these tests are carried out at the maximum permissible installation ambient temperature the equipment specifies.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Wow long post to talk about stuff I didn't mean to refer to. Sorry I wasn't clear.
I simply meant that it can be confusing from a contractual point of view, especially given the customer should not be expected to understand the technical details. E.G. a contract document stating the scope of work includes a "50 amp circuit for EV charging" is ambiguous and potentially misleading in as much as it does not clearly specify if it's referring to NEC ampacity or the max compliant charging current. An honest EC would specify both, e.g. "50 amp circuit capable of a max EV charging current of 40 amps according to the NEC". And if the customer wants more then you can revise the quote upward.
If the quote says '50A circuit' and the customer simply *assumed* that's capable of 48A charging then legally the contractor might not be obligated to change it out, but the customer is arguably justified in leaving them a bad review.
If you quote a 50A circuit, then it means by code a circuit that is protected by a 50A breaker. You're not quoting a charge rate.
That said, this should be made clear to the customer at least verbally.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
To some familiar with the NEC and UL requirements you would be correct.
However to someone not in the industry you would be wrong.
A customer that asks for a 50A feed for a load, like an EVSE, expects you as the professional to provide 50A. If you are not sure if the requested 50A circuit is for a continuous load you should clarify it with the customer or be ready to supply the worst case.
It is disingenuous to assume a residential customer knows the nuances of 100% versus 80% loading requirements. It is stuff like this that gives contractors a bad name and makes customers want to avoid them.
Right, as I said simple explanation is sufficient. I was responding more to the comment about contractual obligation: I don't see how a contractor would be "contractually obligated" to give them a 50A charge rate if the contract literally stipulates a 50A circuit and nothing else. Legally, a 50A circuit is a 50A circuit. If I write a contract for that, then it stipulates the exact breaker and wire gauge under the equipment schedule. Any contractor who is writing loose contracts without schedules and with "charge rates" in them deserve to lose a lawsuit I guess.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
It's not even *that* simple. I mean, 50A works out because it's a standard ampere rating. If I quote a 55A circuit I can protect it with a 60A breaker.
(Side note: I learned in this thread that a 55A breaker is a thing. But it's still not a standard ampere rating!)
That's pretty much what I said. You may be legally able to say one thing but the customer would be justified giving you a bad review for not spelling everything out. Is the cost of NM vs MC worth a bad yelp review? I think not.