# #6 NM ampacity - wrong all these years?



## mikewillnot (Apr 2, 2013)

I've been using #6 copper NM-b for EVSE circuits for a while now now, where the EVSE draws 48 amps, with a 60-amp breaker. Usually part of the run is in a PVC sleeve. EVSE is a continuous load, and it came up recently that with 48 continuous, conductor ampacity should be 60, and #6 NM is only 55. Inspectors in this area haven't raised any concerns or objections. What the heck?

So it seems that #6 THHN, conduit all the way, would be the correct choice. 
Prices are going up.


----------



## SWDweller (Dec 9, 2020)

The joys of electric vehicles. I went and looked at a home where they had a Tesla in the drive.
They had a 100 amp service and could not get it through their heads that they had to have a service change. Owner got pissed and said he would pay for solar first. I smiled and walked away. I am pretty sure his wife figured it out.


----------



## mikewillnot (Apr 2, 2013)

Inspectors around here are pretty much OK with #2 SER for 100-amp sub-panels (not the entire dwelling load). Suppliers, also, recommend it as the choice for a 100-amp sub. My main cable supplier only started carrying #1 SER when I insisted.


----------



## mikewillnot (Apr 2, 2013)

I guess I could configure the EVSE to 40 amp draw. I can visualize some unhappy customers though.


----------



## SWDweller (Dec 9, 2020)

The whole problem of lower amps for charging is the damage you can do to the batteries. The real charge cycle for longevity of EV batteries is a lot closer to 24 hours than 12.

Then there is Amazon who is going to buy 100000 vehicles from Rivian with a range of 100 miles. 
Sure do not see that happening. 








Amazon's Rivian Prime Electric Delivery Van: 12 Things to Know


What's in the box.




www.motortrend.com





Wonder if the fulfillment centers will need a bigger services for charging?


----------



## mikewillnot (Apr 2, 2013)

SWDweller said:


> The real charge cycle for longevity of EV batteries is a lot closer to 24 hours than 12.


I don't understand this.


----------



## oldsparky52 (Feb 25, 2020)

mikewillnot said:


> I've been using #6 copper NM-b for EVSE circuits for a while now now, where the EVSE draws 48 amps, with a 60-amp breaker. Usually part of the run is in a PVC sleeve. EVSE is a continuous load, and it came up recently that with 48 continuous, conductor ampacity should be 60, and #6 NM is only 55. Inspectors in this area haven't raised any concerns or objections. What the heck?
> 
> So it seems that #6 THHN, conduit all the way, would be the correct choice.
> Prices are going up.


When was it that you had to start using the 60 degree column for NM? Didn't it used to be you could use the 75 degree column for #6?

Edit: Maybe I'm thinking about SEU and SER.


----------



## mikewillnot (Apr 2, 2013)

oldsparky52 said:


> When was it that you had to start using the 60 degree column for NM? Didn't it used to be you could use the 75 degree column for #6?


2017 NEC: 334.80 *Ampacity. *The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor.

Doesn't seem to leave much wiggle room.


----------



## Dennis Alwon (May 9, 2009)

oldsparky52 said:


> When was it that you had to start using the 60 degree column for NM? Didn't it used to be you could use the 75 degree column for #6?
> 
> Edit: Maybe I'm thinking about SEU and SER.



Years ago there was 60C conductors in NM. Then the made NM with 75C wire and finally 90C however, you could never iuse nm at the higher temps-- only at 60C AFAIK... You use the higher temps for derating only


----------



## kb1jb1 (Nov 11, 2017)

SWDweller said:


> The joys of electric vehicles. I went and looked at a home where they had a Tesla in the drive.
> They had a 100 amp service and could not get it through their heads that they had to have a service change. Owner got pissed and said he would pay for solar first. I smiled and walked away. I am pretty sure his wife figured it out.


I have been there. The homeowner pays $80,000. for a car and does not want to spend $2,300.00 for a service change and EV hook up. 

I have also seen several houses that had the 100 amp EVSE , ELECTRIC RANGE, AC, DRYER. all on a 100 amp service. But it works, is what I am told.


----------



## kb1jb1 (Nov 11, 2017)

SWDweller said:


> The whole problem of lower amps for charging is the damage you can do to the batteries. The real charge cycle for longevity of EV batteries is a lot closer to 24 hours than 12.
> 
> Then there is Amazon who is going to buy 100000 vehicles from Rivian with a range of 100 miles.
> Sure do not see that happening.
> ...


And NYC is replacing its school bus fleet with all electric by 2030. I just imagine plugging in 1000 ++ busses at the same time. They have to build several coal plants to charge them.


----------



## kb1jb1 (Nov 11, 2017)

mikewillnot said:


> I guess I could configure the EVSE to 40 amp draw. I can visualize some unhappy customers though.


The problem with adjustable settings on the wall ports is that we are suppose to wire for the largest name tag value. So when the Tesla says 90 amps we are supposed to run a #2.


----------



## mikewillnot (Apr 2, 2013)

kb1jb1 said:


> The problem with adjustable settings on the wall ports is that we are suppose to wire for the largest name tag value. So when the Tesla says 90 amps we are supposed to run a #2.


The latest Tesla model (charger & cars both, over a year now) maxes out at 60-amp breaker, 48-amp charge rate. Cases regularly come up where available service capacity really _requires_ that we dial down the charger. Generally, we downsize the breaker as well, though not the wire. In this case, with a major break in NM ampacity / economics at #6, dialing the charger down to 40 amps / 50-amp breaker might be the right solution -- especially on already done jobs. Going forward, I see a lot more conduit and THHN. Unless some inspector explains to me the non-error of my ways.


----------



## kb1jb1 (Nov 11, 2017)

mikewillnot said:


> The latest Tesla model (charger & cars both, over a year now) maxes out at 60-amp breaker, 48-amp charge rate. Cases regularly come up where available service capacity really _requires_ that we dial down the charger. Generally, we downsize the breaker as well, though not the wire. In this case, with a major break in NM ampacity / economics at #6, dialing the charger down to 40 amps / 50-amp breaker might be the right solution -- especially on already done jobs. Going forward, I see a lot more conduit and THHN. Unless some inspector explains to me the non-error of my ways.


Last month I hooked up a Tesla charger that said max 90 amps. The charge rate can be change in the car. I don’t remember the exact numbers only that I ran the #2 in conduit.


----------



## mikewillnot (Apr 2, 2013)

kb1jb1 said:


> Last month I hooked up a Tesla charger that said max 90 amps. The charge rate can be change in the car. I don’t remember the exact numbers only that I ran the #2 in conduit.


That could only have been a Gen1 or Gen2 charger.


----------



## Quickservice (Apr 23, 2020)

SWDweller said:


> The joys of electric vehicles. I went and looked at a home where they had a Tesla in the drive.
> They had a 100 amp service and could not get it through their heads that they had to have a service change. Owner got pissed and said he would pay for solar first. I smiled and walked away. I am pretty sure his wife figured it out.


Also, if it is like here, I bet the HO bought the Tesla thinking he would save the planet all the while not having a clue that his power is coming from a coal-fired plant.


----------



## mikewillnot (Apr 2, 2013)

Quickservice said:


> Also, if it like here, I bet the HO bought the Tesla thinking he would save the planet all the while not having a clue that his power is coming from a coal-fired plant.


We're a bit off topic, but it's a thin argument you make; while 100% coal might be the exact case where you live, people use the argument everywhere, with no clue where their electricity comes from. Fact is, overall, electricity is the only energy form we have in this country that, while not perfect (yet), over time is getting significantly, increasingly, greener...everywhere. Gasoline is not. And even with the mix of "cleanness" of energy sources that go into electricity, the overall carbon footprint of a the average EV is about 50% of the comparable ICE.


----------



## kb1jb1 (Nov 11, 2017)

mikewillnot said:


> We're a bit off topic, but it's a thin argument you make; while 100% coal might be the exact case where you live, people use the argument everywhere, with no clue where their electricity comes from. Fact is, overall, electricity is the only energy form we have in this country that, while not perfect (yet), over time is getting significantly, increasingly, greener...everywhere. Gasoline is not. And even with the mix of "cleanness" of energy sources that go into electricity, the overall carbon footprint of a the average EV is about 50% of the comparable ICE.


" I know where electricity comes from. It comes from the wall outlet.".


----------



## eddy current (Feb 28, 2009)

Interesting that the NEC has put a 60 degree restriction on NM cable. No such rule up here.


----------



## emtnut (Mar 1, 2015)

eddy current said:


> Interesting that the NEC has put a 60 degree restriction on NM cable. No such rule up here.


I always wondered if it's because their NM is different (has the paper inside)?

That or maybe because their definition of continuous loads are not nearly as strict as ours ... kinda balances it out for some cases?


----------



## kb1jb1 (Nov 11, 2017)

eddy current said:


> Interesting that the NEC has put a 60 degree restriction on NM cable. No such rule up here.


Maybe because the electricians here like to bundle up or cram the cables when running the home feed circuits. I just replaced 4 cables in a 30 foot bundle of 20 plus cables.


----------



## HertzHound (Jan 22, 2019)

I always thought it was because there is no air around the conductors, like MC or conduit. Why not use MC instead of going right to THHN in conduit?

Heres what McGraw-Hill has to say about Romex ampacity.


“


> 334.80. Ampacity. The second sentence requires that NM, NMS, and NMC cables always have their conductors applied to the ampacity of Type”
> “TW wire—that is, the 60°C ampacity from Table 310.15(B)(16). However, the insulation on the conductors must be rated at 90°C. This provides an additional margin for error and helps prevent overheating of Type NM where installed, say, in the attic of a residential occupancy located in the New Mexico desert. The ambient temperature would soar in the summer, unless the attic was air conditioned.
> The last paragraph of 334.80 correlates the use of NM cable to the rule of 310.15(B)(3), which says:
> 
> ...


----------



## micromind (Aug 11, 2007)

eddy current said:


> Interesting that the NEC has put a 60 degree restriction on NM cable. No such rule up here.


It could be that they want the original installation to be overrated so when Uncle Bob says 'put a 30 in, then it won't trip anymore' there's less chance of it burning up. 

I don't know about up north but down south every big box store has an ample supply of single pole 30s in every brand they stock.


----------



## eddy current (Feb 28, 2009)

emtnut said:


> I always wondered if it's because their NM is different (has the paper inside)?
> 
> That or maybe because their definition of continuous loads are not nearly as strict as ours ... kinda balances it out for some cases?





kb1jb1 said:


> Maybe because the electricians here like to bundle up or cram the cables when running the home feed circuits. I just replaced 4 cables in a 30 foot bundle of 20 plus cables.


Does the NEC not have derating rules when bundling multi-conductor cables? We do


----------



## eddy current (Feb 28, 2009)

micromind said:


> It could be that they want the original installation to be overrated so when Uncle Bob says 'put a 30 in, then it won't trip anymore' there's less chance of it burning up.
> 
> I don't know about up north but down south every big box store has an ample supply of single pole 30s in every brand they stock.


That’s a silly reason. Rating NM at 60 does not make a difference if Johnny home owner puts a 30 amp breaker on 14 awg wire.
If they were worried about that, making #10 a minimum on 15 amp circuits would be a better idea.


----------



## eddy current (Feb 28, 2009)

It has got to be because The quality of the NM used in Canada is far superior to the NM used in the US.


----------



## kb1jb1 (Nov 11, 2017)

eddy current said:


> Does the NEC not have derating rules when bundling multi-conductor cables? We do


Over the past couple of weeks I come to the conclusion that the NEC is like the speed limit signs along the highway.


----------



## mikewillnot (Apr 2, 2013)

emtnut said:


> maybe because their definition of continuous loads are not nearly as strict as ours ... kinda balances it out for some cases?


What exactly is the definition of continuous loads in the CEC?


----------



## emtnut (Mar 1, 2015)

mikewillnot said:


> What exactly is the definition of continuous loads in the CEC?


3) The calculated load in a consumer’s service, feeder, or branch circuit shall be considered a 
continuous load unless it can be shown that in normal operation it will not persist for
a) a total of more than 1h in any 2 h period if the load does not exceed 225A;or 
b) a total of more than 3h in any 6h period if the load exceeds225A.
4) A load of a cyclic or intermittent nature shall be classified as continuous unless it meets the requirements of Subrule 3)

Basically, you have to prove that the load meets this requirement.


----------



## Quickservice (Apr 23, 2020)

mikewillnot said:


> We're a bit off topic, *but it's a thin argument you make*; while 100% coal might be the exact case where you live, people use the argument everywhere, with no clue where their electricity comes from. Fact is, overall, electricity is the only energy form we have in this country that, while not perfect (yet), over time is getting significantly, increasingly, greener...everywhere. Gasoline is not. And even with the mix of "cleanness" of energy sources that go into electricity, the overall carbon footprint of a the average EV is about 50% of the comparable ICE.


I fully qualified the statement.


----------



## fredelectric (Jun 26, 2015)

Quickservice said:


> Also, if it is like here, I bet the HO bought the Tesla thinking he would save the planet all the while not having a clue that his power is coming from a coal-fired plant.


It is a common misconception that electric vehicles powered by coal generated electricity are no better for the environment than internal combustion engines. As electrical professionals, I think we should all be welcoming beneficial electrification, whether it is for home heating, transportation, or otherwise. This is not just good for the environment, it is good business for us, with good-paying jobs to offset the jobs that are being lost in coal mining and the fossil fuel industries in general. Here are some things to think about: 1) Unless the grid is nearly 100% coal powered, EVs still have less lifetime emissions than most gas-powered cars (see Forbes article link belo). 2) Coal-fired power plants are not economical to build or operate any more and are being retired rapidly, causing the grid to get cleaner over time. 3) In the future, EVs that are plugged in to the grid can help with grid stabilization using V2G (vehicle-to-grid) technology (see NEC Article 625, recent changes). This will help with the adoption of more intermittent renewables like wind and solar. Take school buses for example - they sit parked during peak sun hours, so they can help absorb excess solar power during the day. There is a bright future for all of us here - embrace it!
Yes, Electric Cars Are Cleaner, Even When The Power Comes From Coal (forbes.com)


----------



## jkvas (Dec 30, 2017)

SWDweller said:


> The joys of electric vehicles. -Oh, wouldn't it be fun if he first had Solar installed, then called you for the service upgrade when his solar panels failed to provide enough power!


----------



## randolph333 (Feb 10, 2015)

mikewillnot said:


> I've been using #6 copper NM-b for EVSE circuits for a while now now, where the EVSE draws 48 amps, with a 60-amp breaker. Usually part of the run is in a PVC sleeve. EVSE is a continuous load, and it came up recently that with 48 continuous, conductor ampacity should be 60, and #6 NM is only 55. Inspectors in this area haven't raised any concerns or objections. What the heck?
> 
> So it seems that #6 THHN, conduit all the way, would be the correct choice.
> Prices are going up.


I recently consulted an engineer on this and he said that for a 60 amp circuit #4 wire should be used and, in the context of my problem, buried conduit, recommended XHHW.


----------



## mikewillnot (Apr 2, 2013)

4/3 NM-b is over $8 per foot. Couldn't find 4/2 anywhere. 
#6 THHN is less than .70 around here.


----------



## Quickservice (Apr 23, 2020)

fredelectric said:


> It is a common misconception that electric vehicles powered by coal generated electricity are no better for the environment than internal combustion engines. As electrical professionals, I think we should all be welcoming beneficial electrification, whether it is for home heating, transportation, or otherwise. This is not just good for the environment, it is good business for us, with good-paying jobs to offset the jobs that are being lost in coal mining and the fossil fuel industries in general. Here are some things to think about: 1) Unless the grid is nearly 100% coal powered, EVs still have less lifetime emissions than most gas-powered cars (see Forbes article link belo). 2) Coal-fired power plants are not economical to build or operate any more and are being retired rapidly, causing the grid to get cleaner over time. 3) In the future, EVs that are plugged in to the grid can help with grid stabilization using V2G (vehicle-to-grid) technology (see NEC Article 625, recent changes). This will help with the adoption of more intermittent renewables like wind and solar. Take school buses for example - they sit parked during peak sun hours, so they can help absorb excess solar power during the day. There is a bright future for all of us here - embrace it!
> Yes, Electric Cars Are Cleaner, Even When The Power Comes From Coal (forbes.com)


What happens to the statistics when you use a gas-powered generator to charge your EV?


----------



## mikewillnot (Apr 2, 2013)

I had an inspector yesterday say that the conductor ampacity (55) has to exceed the actual load (48), not the 125% load (60), and that the 125% applied to the OCPD, not the conductor. This seems inconsistent with 210.19(A)(1)(a), which specifically says "conductor size."

I believe he also mentioned the exception to 210.19(A)(1) and (2) :
_Exception: If the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the branch-circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load. _

He stated that this allows us to use #6 NM-B cable in our application, provided it's on a 60-amp breaker.

I don't want to argue, I'd like to understand the logic - especially since I know that inspectors sometimes follow these forums.

What's an "assembly?" [no definition in the code] -- possibly the NM cable assembly?
And how is its "listed" status, and its rating, known?

Thoughts from anyone who speaks NEC more fluently than I would be appreciated.


----------



## mikewillnot (Apr 2, 2013)

mikewillnot said:


> I had an inspector yesterday say that the conductor ampacity (55) has to exceed the actual load (48), not the 125% load (60), and that the 125% applied to the OCPD, not the conductor. This seems inconsistent with 210.19(A)(1)(a), which specifically says "conductor size."
> 
> I believe he also mentioned the exception to 210.19(A)(1) and (2) :
> _Exception: If the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the branch-circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load. _
> ...


----------



## mikewillnot (Apr 2, 2013)

Just found THIS, on the "exception" interpretation problem. Clarified, it seems, in the 2020 (which is not adopted yet around here).


----------

