Instead of #8/2, can I use #10/4 using paired conductors

[MikeD]

Mitchell: I suggest that all of us try to get in the mindset of living as "sustainably" as possible.

Consider that you may soon have a second EV (or even more) before too long. The point is that it may be best to install an EVSE that is merely adequate to sufficiently recharge an EV overnight, i.e. not think one always needs to have a home EVSE that charges your EV (or next EV) as quickly as is possible.

 

[Nubo]

Mitchell: I suggest that all of us try to get in the mindset of living as "sustainably" as possible.

Consider that you may soon have a second EV (or even more) before too long. The point is that it may be best to install an EVSE that is merely adequate to sufficiently recharge an EV overnight, i.e. not think one always needs to have a home EVSE that charges your EV (or next EV) as quickly as is possible.

I've long felt that the usual notion of "future proofing" EVSE circuits is kinda' backwards. Imho, the larger the pack, the less important home charging rate becomes. The role of the battery as "accumulator" becomes more dominant.

From an infrastructure perspective, it might make more sense to encourage more modest rates. What happens when everyone on the block is charging vehicles at 80 amps?

 

[Mitchell]

Trying to decide whether to install heavier cable than needed to the EVSE ...

Since #8 solid copper wire has a resistance of 0.6281 ohm/1000' and #6 is 0.3952 ohm/1000', and my 50' run from the main panel to the EVSE would have a total of 100' of cable for the "electrons" round-trip, installing #6 would reduce the loop resistance from 0.06281 ohm to 0.03952 ohm. At 27.5 amps, this would reduce the power lost in the cable from 47.5 watts to 29.9 watts, eliminating 17.6 watts wasted during charging, which would reduce charging loss by (17.6/6600 =) 0.27%, which barely insignificant.

So the answer is it is only worth installing heavier cable for possible future requirements, it is not worth it just to reduce voltage drop and power lost in the feeder cable.

The CM area of #8- 7 strand copper is 16,510 more than sufficient to carry the required 27.5 A over your required distance of 16 meters without even coming close to the 3% allowable voltage drop. The question is... what size are the lugs on the EVSE , will they accept #6 :?:

I was planning on using a junction box (a grey plastic rectantular one, such as this) a foot or two from the EVSE. The junction box would terminate the unflexible cable from the main panel. The OpenEVSE expects round flexible cables and comes with two round cable glands for strain relief / environmental sealing. One gland is for the J1772 cable and the other for the supply. So I ordered a longer-then-needed J1772 cable, and will cut off a few feet of this very flexible cable and use it for the connection between the junction box and the EVSE.

As a side point, I found that #6/2 NMD cable is available, so no need to waste the third copper wire, better to have heavier conductors used. I've ordered that and will use conduit to protect it in the garage. This should be much better than the armoured #6/3, which would have been almost an inch in diameter, likely really difficult to fish through the finished basement ceiling.

 

[Mitchell]

Mitchell: I suggest that all of us try to get in the mindset of living as "sustainably" as possible.

Consider that you may soon have a second EV (or even more) before too long. The point is that it may be best to install an EVSE that is merely adequate to sufficiently recharge an EV overnight, i.e. not think one always needs to have a home EVSE that charges your EV (or next EV) as quickly as is possible.

You're right, no need for "instant pit-stops" as if we're all in a Formula 1 race. But I wonder that when much larger battery packs become available, people will still expect them to charge in less than, say, 12 hours. So today's 6.6 kw chargers may then seem like a trickle charger and being able to charge at 70 amps might be the "new L2".

 

[Valdemar]

Trying to decide whether to install heavier cable than needed to the EVSE ...

Since #8 solid copper wire has a resistance of 0.6281 ohm/1000' and #6 is 0.3952 ohm/1000', and my 50' run from the main panel to the EVSE would have a total of 100' of cable for the "electrons" round-trip, installing #6 would reduce the loop resistance from 0.06281 ohm to 0.03952 ohm. At 27.5 amps, this would reduce the power lost in the cable from 47.5 watts to 29.9 watts, eliminating 17.6 watts wasted during charging, which would reduce charging loss by (17.6/6600 =) 0.27%, which barely insignificant.

So the answer is it is only worth installing heavier cable for possible future requirements, it is not worth it just to reduce voltage drop and power lost in the feeder cable.

The CM area of #8- 7 strand copper is 16,510 more than sufficient to carry the required 27.5 A over your required distance of 16 meters without even coming close to the 3% allowable voltage drop. The question is... what size are the lugs on the EVSE , will they accept #6 :?:

I was planning on using a junction box (a grey plastic rectantular one, such as this) a foot or two from the EVSE. The junction box would terminate the unflexible cable from the main panel. The OpenEVSE expects round flexible cables and comes with two round cable glands for strain relief / environmental sealing. One gland is for the J1772 cable and the other for the supply. So I ordered a longer-then-needed J1772 cable, and will cut off a few feet of this very flexible cable and use it for the connection between the junction box and the EVSE.

As a side point, I found that #6/2 NMD cable is available, so no need to waste the third copper wire, better to have heavier conductors used. I've ordered that and will use conduit to protect it in the garage. This should be much better than the armoured #6/3, which would have been almost an inch in diameter, likely really difficult to fish through the finished basement ceiling.

Keep in mind that #6 NM cable is 60C and rated at 55A, considering you need to de-rate the breaker for continuous load 20% you are pretty much limited to a 40A breaker, or maybe 44A if they exist, hardly a future-proofing. Running NM inside a conduit can be a code violation, or at least you need to use a larger than necessary diameter conduit to make it compliant. You could strip the outer sheating of the NM and run individual conductors inside the conduit, this is not by code but some think it is acceptable. 30A J1772 cable is often #10, running a short piece from the j-box to the EVSE may technically be a code violation on a 40A breaker if this is a hard-wired setup. I'd try to avoid any splicing from the panel to EVSE to eliminate additional points of failure, this is a new run and you have an opportunity to make it right, saving a couple of bucks here and there is really not worth it in this case if the end product is inferior.

 

[wwhitney]

Keep in mind that #6 NM cable is 60C and rated at 55A, considering you need to de-rate the breaker for continuous load 20% you are pretty much limited to a 40A breaker, or maybe 44A if they exist, hardly a future-proofing.

You derate the load, not the breaker. That is, a 32 amp continuous load requires a minimum 40 amp conductor and a minimum 40 amp breaker. The reason for this is that standard breakers may trip below their rating if used continuously, so you need a larger breaker to avoid this. Then because the breaker is larger, you need larger conductors to ensure the conductors are adequately protected by the breaker.

Running NM inside a conduit can be a code violation, or at least you need to use a larger than necessary diameter conduit to make it compliant.

Not a code violation in and of itself. For a complete conduit system, you need to take the largest width of the NM cable, treat the cable as a circle, calculate that circle's area, and proceed accordingly. If that's the only wiring in the conduit, I believe the allowed fill is 60% of cross-sectional area. The upshot is that the inner diameter of your conduit would need to be 130% of the width of the cable in that case (1 / sqrt(60%)). [Edit: should be 53% fill and 138%.]

However, no such requirement applies for incomplete runs of conduit used solely to protect the NM cable from physical damage. So if you are only using the conduit in the garage, have at it. The above sizing may still be a good idea, though. Also, where the cable enters the conduit, a bushing with a smooth surface is required to prevent damage to the cable.

You could strip the outer sheating of the NM and run individual conductors inside the conduit, this is not by code but some think it is acceptable.

This is code compliant if the individual conductors are marked with their type, e.g. THWN. If unmarked, it is not compliant.

Also, as for the connection to the OpenEVSE, if you use a cord gland, then you are using cord, and you can not hard wire the EVSE. You'll need to use a plug and receptacle. If you want to hard wire the EVSE, then ditch the cord gland and run your protective conduit and NM to that opening. No splicing outside the EVSE required.

The above is all per the NEC; the CEC may differ.

Cheers, Wayne

 

[Valdemar]

Keep in mind that #6 NM cable is 60C and rated at 55A, considering you need to de-rate the breaker for continuous load 20% you are pretty much limited to a 40A breaker, or maybe 44A if they exist, hardly a future-proofing.

You derate the load, not the breaker. That is, a 32 amp continuous load requires a minimum 40 amp conductor and a minimum 40 amp breaker. The reason for this is that standard breakers may trip below their rating if used continuously, so you need a larger breaker to avoid this. Then because the breaker is larger, you need larger conductors to ensure the conductors are adequately protected by the breaker.

I stand corrected, I suppose I wanted to say a 44A EVSE not the breaker. It is still questionable if an upgrade from a working 32A EVSE to a 44A one will ever make sense.

You could strip the outer sheating of the NM and run individual conductors inside the conduit, this is not by code but some think it is acceptable.

This is code compliant if the individual conductors are marked with their type, e.g. THWN. If unmarked, it is not compliant.

Have you ever seen a NM cable with individual conductors marked? My understanding they are about as common as the unicorn.

 

[wwhitney]

Have you ever seen a NM cable with individual conductors marked? My understanding they are about as common as the unicorn.

No, but my sample size is one manufacturer. I have heard there is a manufacturer that does mark them, but who knows?

Cheers, Wayne

 

[smkettner]

Just get what is needed for the EVSE you ordered. All future J1772 vehicles will be compatible.

 

[Valdemar]

Have you ever seen a NM cable with individual conductors marked? My understanding they are about as common as the unicorn.

No, but my sample size is one manufacturer. I have heard there is a manufacturer that does mark them, but who knows?

Cheers, Wayne

Do you know the name? I need to rewire my owen and using stripped NM in the outside section inside EMT might help me avoid or minimize drywall damage. Hopefully the conductors would also be rated for wet locations, but it may be to much too ask...

 

[ElectricEddy]

The CM area of #8- 7 strand copper is 16,510 more than sufficient to carry the required 27.5 A over your required distance of 16 meters without even coming close to the 3% allowable voltage drop. The question is... what size are the lugs on the EVSE , will they accept #6 :?:

I was planning on using a junction box (a grey plastic rectantular one, such as this) a foot or two from the EVSE. The junction box would terminate the unflexible cable from the main panel. The OpenEVSE expects round flexible cables and comes with two round cable glands for strain relief / environmental sealing. One gland is for the J1772 cable and the other for the supply. So I ordered a longer-then-needed J1772 cable, and will cut off a few feet of this very flexible cable and use it for the connection between the junction box and the EVSE.

As a side point, I found that #6/2 NMD cable is available, so no need to waste the third copper wire, better to have heavier conductors used. I've ordered that and will use conduit to protect it in the garage. This should be much better than the armoured #6/3, which would have been almost an inch in diameter, likely really difficult to fish through the finished basement ceiling.

Keep in mind that #6 NM cable is 60C and rated at 55A, considering you need to de-rate the breaker for continuous load 20% you are pretty much limited to a 40A breaker, or maybe 44A if they exist, hardly a future-proofing. Running NM inside a conduit can be a code violation, or at least you need to use a larger than necessary diameter conduit to make it compliant. You could strip the outer sheating of the NM and run individual conductors inside the conduit, this is not by code but some think it is acceptable. 30A J1772 cable is often #10, running a short piece from the j-box to the EVSE may technically be a code violation on a 40A breaker if this is a hard-wired setup. I'd try to avoid any splicing from the panel to EVSE to eliminate additional points of failure, this is a new run and you have an opportunity to make it right, saving a couple of bucks here and there is really not worth it in this case if the end product is inferior.

"Keep in mind that #6 NM cable is 60C and rated at 55A" CEC 2015 T19 (Conditions of use and maximum allowable conductor temperatures...) Non-metallic-sheathed cable (NMD90) ; 90C, T2 (Allowable ampacity for not more than 3 conductors in a raceway or cable...) #6 CU based at 75C TTR; 65A, T13 ( Ratting or setting of overcurrent devices protecting conductors) Ampacity of conductor=61 -70 A; Rating permitted 70. Taps with a smaller conductor are permitted item (b) in 14-100, must have ampacity not less than the computed load, not over 3m long, does not extend past the device and enclosed in a non-ventilated raceway (finished conduit system) or armored cable.

 

[dhanson865]

Mitchell: I suggest that all of us try to get in the mindset of living as "sustainably" as possible.

Consider that you may soon have a second EV (or even more) before too long. The point is that it may be best to install an EVSE that is merely adequate to sufficiently recharge an EV overnight, i.e. not think one always needs to have a home EVSE that charges your EV (or next EV) as quickly as is possible.

Bigger wiring = less power lost to heat aka resistance.

Why not make charging efficient instead of minimizing cable size?

aka way oversized might be unsustainable but undersized is also unsustainable (fire, wasted energy, or redoing past work are all poor use of resources).

 

[wwhitney]

Do you know the name?

No, sorry, not sure if I believe it or if it is still currently true.

I need to rewire my owen and using stripped NM in the outside section inside EMT might help me avoid or minimize drywall damage. Hopefully the conductors would also be rated for wet locations, but it may be to much too ask...

Couple ideas:

Fish smurf tube for the interior portion instead of fishing NM, then make up a a complete raceway system and run THWN-2 condcutors.
Or do the whole thing in UF or SER cable, sleeving the exterior portion in conduit if necessary to protect from physical damage.

Cheers, Wayne

 

[wwhitney]

Why not make charging efficient instead of minimizing cable size?

The NEC is fairly conservative on conductor sizing, so often the minimum size is sustainable. The computation earlier in the thread suggested that the energy savings from upsizing was fairly small. Do you have a computation showing that the lifetime energy savings for upsizing significantly exceeds the embodied energy cost of the extra copper and plastic?

Cheers, Wayne

 

[Valdemar]

Do you know the name?

No, sorry, not sure if I believe it or if it is still currently true.

I need to rewire my owen and using stripped NM in the outside section inside EMT might help me avoid or minimize drywall damage. Hopefully the conductors would also be rated for wet locations, but it may be to much too ask...

Couple ideas:

Fish smurf tube for the interior portion instead of fishing NM, then make up a a complete raceway system and run THWN-2 condcutors.
Or do the whole thing in UF or SER cable, sleeving the exterior portion in conduit if necessary to protect from physical damage.

Cheers, Wayne

Thanks, I'll keep these in mind. Quick research reveals the individual conductors in SER must be rated and typically are XHHW-2 or THHN/THWN, so running stripped SER inside a conduit in the outside portion should be legit, mainly for the easier pull and smaller conduit.

 

[GlennD]

In practice, The EVSE will shut down when the ground current exceeds 20Ma( GFCI). This negates the large size requirement.

What if the GFCI is faulty?

I can not speak for other EVSE's but the OpenEVSE tests the GFCI before charging as a UL requirement so with a bad GFCI you have an error condition.

 

[dhanson865]

Why not make charging efficient instead of minimizing cable size?

The NEC is fairly conservative on conductor sizing, so often the minimum size is sustainable. The computation earlier in the thread suggested that the energy savings from upsizing was fairly small. Do you have a computation showing that the lifetime energy savings for upsizing significantly exceeds the embodied energy cost of the extra copper and plastic?

Cheers, Wayne

Is 8/2 x 2 less or more copper than 10/4?

Either way its common on other threads to see people trying to undersize the circuit to save a few bucks up front. Often someone asks about using something smaller than NEC recommendation, often people also ask about upgrading to a higher amp EVSE later. I'm arguing they should be looking at NEC for multiple scenarios not just NEC for the minimum use case of the current vehicle.

Chevy Bolt 7.7KW charger
Nissan Leaf 2 (doesn't exist yet, unkown charger)
Tesla 3 (doesn't exist yet unkown charger but likely >10KW)

So how many people will install a circuit for a Leaf with a 3.6KW charger and then waste more money and material pulling a new curcuit later because they upgrade to one of these newer cars?

Why should NEC recommendation for a 20A circuit or 30A be the goal if someone will later have a car that will have a 40A or higher charger built in?

I'm cool with the practical limits. If a 30A circuit is what fits cost wise, then fine, deal with the constraint. I just advise against skimping on pulling what you need for a 20A or 30A circuit if you haven't done the math on a 40A as well.

Maybe you end up buying a CPO Model S with the dual chargers and you could charge at 80A but you choose to charge at 30A or 40A due to cost of pulling the higher amp curcuit.

I've already got my 14-50 live and I don't see a reason to upgrade above that but I'm still charging my Leaf at 12A* on that 14-50. Would you say my choice of running cable and breakers for future use was unsustainable?

*openEVSE selcting 12a, reports closer to 11.7A*242V on average, leafspy reports 2.5KW incoming after charger losses.

 

[wwhitney]

Why should NEC recommendation for a 20A circuit or 30A be the goal if someone will later have a car that will have a 40A or higher charger built in?

I can see the argument for upsizing the EVSE for a future car purchase. But for a given size EVSE, I don't see the benefit of upsizing the wires to save on resistive energy losses.

Cheers, Wayne

 

[wwhitney]

Not a code violation in and of itself. For a complete conduit system, you need to take the largest width of the NM cable, treat the cable as a circle, calculate that circle's area, and proceed accordingly. If that's the only wiring in the conduit, I believe the allowed fill is 60% of cross-sectional area. The upshot is that the inner diameter of your conduit would need to be 130% of the width of the cable in that case (1 / sqrt(60%)).

Correction, the allowable fill for a single cable in conduit is 53%. So the inner diameter of the conduit would need to be 138% of the width of the cable.

Cheers, Wayne

 

[Mitchell]

I've picked-up the #6/2 NMD cable. At the widest, it is 0.68" across, so using the 53% maximum conduit fill rule, I would need to use 1" conduit -- though that means I've now got to find a concrete drill bit wider than the 1" x 10" long I have now (to get through the concrete block to the garage -- I can't get my drill to the inside side, so need to do the drilling from the garage side). So it is definately tempting to only use 3/4" conduit if allowed.

The cable is rated for 90 C, so if in the future I want to charge at greater than 40 amps, I think it would be good up to 70 amps. So hopefully I could use it on a 60 amp circuit, which could charge at up to 10.8 kW, a worthwhile improvement over 6.6 kW.

Interesting that a cable gland can only be used with a plug, not hard-wired. Would I be allowed to install a small electrical box along the conduit, perhaps a foot away from the EVSE and splice in that electrical box to a more flexible cable, which can be more easily terminated on the OpenEVSE contactor.

Keep in mind that #6 NM cable is 60C and rated at 55A, considering you need to de-rate the breaker for continuous load 20% you are pretty much limited to a 40A breaker, or maybe 44A if they exist, hardly a future-proofing.

You derate the load, not the breaker. That is, a 32 amp continuous load requires a minimum 40 amp conductor and a minimum 40 amp breaker. The reason for this is that standard breakers may trip below their rating if used continuously, so you need a larger breaker to avoid this. Then because the breaker is larger, you need larger conductors to ensure the conductors are adequately protected by the breaker.

Running NM inside a conduit can be a code violation, or at least you need to use a larger than necessary diameter conduit to make it compliant.

Not a code violation in and of itself. For a complete conduit system, you need to take the largest width of the NM cable, treat the cable as a circle, calculate that circle's area, and proceed accordingly. If that's the only wiring in the conduit, I believe the allowed fill is 60% of cross-sectional area. The upshot is that the inner diameter of your conduit would need to be 130% of the width of the cable in that case (1 / sqrt(60%)). [Edit: should be 53% fill and 138%.]

However, no such requirement applies for incomplete runs of conduit used solely to protect the NM cable from physical damage. So if you are only using the conduit in the garage, have at it. The above sizing may still be a good idea, though. Also, where the cable enters the conduit, a bushing with a smooth surface is required to prevent damage to the cable.

You could strip the outer sheating of the NM and run individual conductors inside the conduit, this is not by code but some think it is acceptable.

This is code compliant if the individual conductors are marked with their type, e.g. THWN. If unmarked, it is not compliant.

Also, as for the connection to the OpenEVSE, if you use a cord gland, then you are using cord, and you can not hard wire the EVSE. You'll need to use a plug and receptacle. If you want to hard wire the EVSE, then ditch the cord gland and run your protective conduit and NM to that opening. No splicing outside the EVSE required.

The above is all per the NEC; the CEC may differ.

Cheers, Wayne