Mrbrock
Active Member
But he’s got capability to have 5 chargers so more is better right?
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new wall charger confusion
- Thread starter shyboy
- Start date
I asked why they did not use #6 MC, but they choice was 4/3 NMB. 120 foot run for solar inverter. I guess was easier for them to run in garage attic and inside some walls. As long as it met spec I did not care
I just need to met the 60 amp code spec, which #6 thhn 90c easily meets I believe?
One gen 3 installed. I love it charges at like 45 miles per house with 48 amps. 2 more gen 3's ordered. Not sure I will change out the last 2 14-50 plugs now for gen 3 charges. 3 gen 3's give me lots of flexibility
Not allowed to consider the 90C spec because neither end equipment(Neither HPWC nor circuit breaker) is rated for 90C.
Eric33432
Member
And I am amazed at the number of people who still think it is OK to install #6 NM cable for a 60 amp circuit capable of continuous 48 amp charging after all the different threads on this web site have explained this over and over again. And I am amazed at the number of licensed electricians that think this is OK.
It isn't.
If you are installing a Tesla Wall Connector and want 48 amp charging, use #6 copper in conduit or #6 copper MC cable, or use #6 copper NM (Romex) with a 50 amp breaker and 40 amp charging. #6 is much easier to work with than #4; I would not use #4 NM for a wall connector, #6 is stiff, #4 is stiffer. If you must use NM, just install 50 amp breaker and go with 40 amp charging.
If you have a long run and want to use aluminum wire, transition from the correct size AL wire using a sub panel or a junction box with Polaris connectors to CU wire to connect to the TWC. And be sure your electrical service is adequate. You can check that by downloading Mike Holt's Electrical Toolbox app.
All the different reasons for this are stated in the US National Electrical Code and may be complicated if the wiring is going through a hot attic, behind insulation, in wet areas, bundled with other conductors, consideration is given to terminal temperature rating, etc.
I also have seen comments in other threads where people suggest that #6 NM insulation will "melt" if it is run at 4 amps over its rating. Don't worry, it won't melt.
For situations where #6 copper NM has already been installed, I would not rip it out; consider operating the wall connector at 44 amps or less. In the case of a Tesla wall connector, that means configuring the TWC for a 50 amp breaker, which will result in 40 amp charging. If you already have a 60 amp breaker, it will protect the cable from a short circuit, and if the wall connector is configured for 40 amp charging you will not overload the circuit, but you should probably replace the breaker with a 50 amp breaker to keep someone from someday seeing the 60 amp breaker and re-configuring the TWC (or another charger that a future home-owner might install) for 48 amp charging.
Whilst it will not melt, if you do operate the cable over its capacity, what may happen for wiring that is consistently run at a temperature over it's limit is the insulation will slowly degrade and break down over a period of many years and become brittle and crusty, especially if the wire is placed in an area with a high ambient temperature such as an attic. NM cable is is designed for safe operation of the cable at temperatures of up to 60˚C / 140˚F. But it must be derated if it is operated in an area that will reach more than 30˚C / 86˚F for prolonged periods of time.
Therefore, to determine the temperature of the wire, you need to consider the ambient temperature for the wire or cable, and consider the temperature gain that the current through the wire will add to the ambient temperature, and be aware that when the wire or cable will be operated continuously (defined as 3 hours or longer) it should not be operated at more than 80% of its capacity (or derated by 125%).
The amp rating for NM cable (or any cable) is for a cable that is not exposed to ambient temperature of more than 86˚F for prolonged periods of time. So if run in an attic that gets up to, say, 120˚F pretty much all summer long, does it need to be derated? While this is ignored by a lot of electricians and inspectors, derating should be considered, especially if the circuit is in operation during the hot months. (Note that 60 amp circuits run through attics using #6 NM cable for air handlers containing 9,600 watt electric heat strips are not derated because heat is only used when the attic will be well below 30˚C / 86˚F.)
To derate for temperature, use table 310.15(B)(1) (2020 NCEC):
1) To derate NM cable for operation at 120˚F, for a 60 amp circuit, you divide 60 by 0.58 = 103.44 amps. You would need a cable capable of 103.44 amps, but there is no NM cable made for this amount of amps, thus a cable larger than the largest available NM cable that is made would be re required. (0.58 is the derating factor for NM cable for operation at 120˚F, you must use the 60˚C column in table 310.15 (B)(1) for NM cable. If you want to see this table, search for it on the Internet.)
2) In the above case, THHN/THWN-2 wire, installed in conduit (or MC cable containing THHN/THWN-2 conductors) should be used. To derate THNN/THWN-2 cable for operation at 120˚F, for a 60 amp circuit, you divide 60 by 0.82 = 73.17 amps, thus #6 wire is still adequate since THHN/THWN-2 wire is rated for 75 amps. (Keep in mind the rating of THHN/THWN-2 wire is 75 amps at 90˚C, but the terminals of the circuit breaker to which the wire will connect are only rated for 75˚C, so the entire circuit is only capable of 65 amps.)
What if your attic reached 140˚F for prolonged period of time in the summer?
Using THHN/THWN-2 wire in conduit or MC cable, table 310.15(B)(1) says it would need a derating factor of 0.71. 60 ÷ 0.71 = 84.50. In this case you should use wire capable of 84.5 amps, which would be #4 THHN/THWN-2 wire, which is capable of 95 amps. You could also use SE, THW, THWN, or other wires rated for 75˚C since #4 is still capable of 85 amps.
I hope this is helpful for those that care about installing their wall connectors properly, and being sure their electrician does it right. It seems most electricians have forgot much of what they have learned, or have installed so many #6 NM circuits for air handlers with 9600 watt electric heat using NM cable that they just do not think about it, or are guys working under a real electrician's license. Electricians are required to complete some continuing education every few years, hopefully the powers that be that decide what that education will be will realize that a large number of EV charging installations are being done incorrectly and include EV installation training in the continuing education.
THHN meets the 90˚C rating, and is good for 75 amps, but breakers are not rated for more than 75˚C. As I discussed above, the 90˚C rating of THHN/THWN-2 is most often used for derating the wire in high ambient conditions or where there are more than three conductors in a conduit or raceway. But you are OK assuming your THHN is installed in conduit, it is good for 65 amps when connected to a breaker with 75˚C terminal ratings, which almost all breakers are.
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It seems that's not quite how that's done. NEC 334.80 tells how to apply that derating in 310.15 to NM cables.
"334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60 degree celcius conductor temperature rating. The 90 degree celcius rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60 degree celcius rated conductor."
Here was a good document that describes it and has the derating tables:
You're correct on your point about the 0.58 factor, but you get to derate from the 90 degree amp value column as your starting value. So it doesn't end up with the "there is no NM cable made for this amount of amps". 3 gauge NM-B would qualify. The 3 gauge value in 90 degrees is 115A. Derate by the 0.58 factor, and it is 66.7A. That is usable for a 60A circuit. The calculated value can't be used above the 60 degee column, but that's 85A, so this 66.7A is below that and is what you get to use.
Here is another page from the Mike Holt forum talking about it:
NM Derating
What does NM(Romex) fall under when identifying it (exmample:THHN,THWN)in the codebook? Do you have to derate when you bundle them up in a dwelling?
forums.mikeholt.com
I hate how complicated derating factors are.
Eric33432
Member
Rocky, thanks for catching that mistake. And so sweetly! I knew that at one time and forgot.
I would still rather run #6 MC than #3 NM-B. Especially with the arthritis in my hands!
Too bad it has been too long for me to correct that in my post.
Here is a really good video about conductor protection, by a guy who is a really good teacher:
He also has a very good video that explains why the round-up rule does not apply to NM cable when used for a continuous load.
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As long as your wire at the lowest temperature rating of the whole system is sufficient, you’re good.
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