jjeff
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Re: Help installing NEMA 14-50 in garage

Tue Apr 18, 2017 8:01 am

wwhitney wrote:
jjeff wrote:Another thing I use my 14-50 garage outlet for is back-feeding my main panel from my 40a 240v generator.

Don't do that. Install a manual transfer switch or breaker interlock, along with an inlet. That avoids the use of suicide cords and provides greater security against killing linesmen.

Cheers, Wayne

"suicide cords" :lol: I knew I'd get called on that carpet for that :oops:
Truthfully I've not used my "new" (2 year old now) 40a generator for this purpose. I tried it a couple years ago with a 20a 240v model and it worked quite well, just had to be very aware of what was turned on and I flipped off at the breaker all the 240v circuits. I do NOT suggest this back-feeding for anyone not totally aware of the dangers or how to avoid them :)
I mainly got the generator to power my Leaf in case of an extended power outage, luckily my power has been very stable since getting the Leaf, it wasn't always this way.
2012 SL purchased used 2/'16
2013 S w/QC purchased new
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wwhitney
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Location: Berkeley, CA

Re: Help installing NEMA 14-50 in garage

Tue Apr 18, 2017 9:06 am

jjeff wrote: I do NOT suggest this back-feeding for anyone not totally aware of the dangers or how to avoid them :)

Let me repeat: don't suggest this to anyone or fool yourself into thinking you can do it safely. You are setting up a situation where a single mistake can kill someone. To safely backfeed your house with a generator, you need interlock equipment.

Cheers, Wayne

GerryAZ
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Re: Help installing NEMA 14-50 in garage

Tue Apr 18, 2017 9:34 am

wwhitney wrote:
GerryAZ wrote:By the way, temperature derating in my climate leads to No. 6 copper for 50-ampere circuits that are not in conditioned space.

Good point, for 90C insulated conductors, #8 Cu is only good for 50 amps up to an ambient temperature of 104 degrees F. For ambient temperatures of 105 to 140 degrees F, #6 Cu is required for 50 amps.

Cheers, Wayne


Another factor that many people overlook is that termination points at circuit breakers, disconnect switches, and utilization equipment are usually rated at 75C maximum (some are only 60C). This means you can only use the 75C conductor rating if the panelboard is outside like mine.

Cheers!

PS I will chime in on your side in the "suicide" (or should it be homicide) cord issue. There are retrofit breaker interlock kits for most panelboards and a manual transfer switch is not expensive.
Last edited by GerryAZ on Tue Apr 18, 2017 10:58 am, edited 2 times in total.
Gerry
Silver LEAF 2011 SL rear ended (totaled) by in-attentive driver 1/4/2015 at 50,422 miles
Silver LEAF 2015 SL purchased 2/7/2015

GlennD
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Re: Help installing NEMA 14-50 in garage

Tue Apr 18, 2017 9:35 am

jjeff wrote:
GlennD wrote:It seems to me that labor is the greatest expense. Yes you can use it on a 40A circuit but the cost of upgrading the lines to #6 instead of #8 is minimal. That way you do not need to label the outlet. The ground and neutral can remain #8. It also makes sense to run a neutral wire even though it is not needed for an EVSE. Make the wiring proper for future use. You never know what a future owner needs. A 50A outlet should be wired for 50A.

+1 but personally I'd go with #6 for neutral and if using #6 for the hots. When I first ran my 14-50 outlet in the garage I thought I'd save a little bit of money and omit the neutral, that was a mistake! I later decided I wanted to occasionally use 120v off the 14-50 outlet, only I didn't have a neutral :x In the end it ended up costing me quite a bit more to add the neutral, vs had I just done it from the start :( While it may be rare to ever need 50a @ 120v for the cost of upgrading the neutral to #6, I'd do it :) Oh and yes I understand any draw on the second 120v leg would subtract current from the neutral, still might as well just keep everything #6, well I guess except for the ground which AFAIK can be one size smaller than the rest, or at least it is in romex wiring.....



The neutral was added to complete the 14-50. I personally only use the ground and the lines for my evse. All of the books say you under size the ground and neutral by one size. If I was really drawing 50A at 120V #6 would make sense but the wire is just to make the outlet complete.
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jjeff
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Re: Help installing NEMA 14-50 in garage

Tue Apr 18, 2017 10:44 am

GlennD wrote:
jjeff wrote:
GlennD wrote:It seems to me that labor is the greatest expense. Yes you can use it on a 40A circuit but the cost of upgrading the lines to #6 instead of #8 is minimal. That way you do not need to label the outlet. The ground and neutral can remain #8. It also makes sense to run a neutral wire even though it is not needed for an EVSE. Make the wiring proper for future use. You never know what a future owner needs. A 50A outlet should be wired for 50A.

+1 but personally I'd go with #6 for neutral and if using #6 for the hots. When I first ran my 14-50 outlet in the garage I thought I'd save a little bit of money and omit the neutral, that was a mistake! I later decided I wanted to occasionally use 120v off the 14-50 outlet, only I didn't have a neutral :x In the end it ended up costing me quite a bit more to add the neutral, vs had I just done it from the start :( While it may be rare to ever need 50a @ 120v for the cost of upgrading the neutral to #6, I'd do it :) Oh and yes I understand any draw on the second 120v leg would subtract current from the neutral, still might as well just keep everything #6, well I guess except for the ground which AFAIK can be one size smaller than the rest, or at least it is in romex wiring.....



All of the books say you under size the ground and neutral by one size. If I was really drawing 50A at 120V #6 would make sense but the wire is just to make the outlet complete.

Interesting, while I see the ground smaller than the other wires, I've never seen the neutral smaller than the hot on romex wiring, maybe it's only undersized when pulling individual wires? I mean for one thing it would be kind of a safety issue as people often use 2 wire(white and black) + ground for 240v, labeling the white wire as hot. If the white were actually smaller....well you can see the problem that would cause :)
2012 SL purchased used 2/'16
2013 S w/QC purchased new
Juicebox Premium 60a L1/L2 EVSE, Ebusbar 16a L1/L2 EVSE
'12 EVSEupgrade'd 20a L1/L2 EVSE, '13 EVSEupgrade'd adjustable 6-20a L2, 6-13a L1 EVSE
Zencar 13, 20, 30a L1/L2 portable EVSE
GE Durastation 30a

wwhitney
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Re: Help installing NEMA 14-50 in garage

Tue Apr 18, 2017 11:22 am

GerryAZ wrote:This means you can only use the 75C conductor rating if the panelboard is outside like mine.

You lost me here, there isn't any such restriction in the NEC.

Insulation temperature will be 60C, 75C, or 90C. Terminations will be 60C or 75C. Ampacity is then the lesser of two values: Tabular ampacity at the insulation temperature derated for conditions of use (ambient temperature, number of current carrying conductors in a single conduit or cable, etc.), or the tabular ampacity at the termination temperature. NM cable is further limited to the 60C tabular ampacity, even though it is required to be constructed of conductors with 90C insulation.

So if your indoor panel has breakers with 75C rated terminations (typical, I believe) and your equipment has 75C rated terminations, then you can use the 75C rated ampacity, as long as the conductor isn't NM cable and either the insulation temperature is 75C and there's no derating required, or the conductor insulation temperature is 90C and the derated ampacity for the conditions of use is not less than the 75C ampacity.

Cheers, Wayne

wwhitney
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Re: Help installing NEMA 14-50 in garage

Tue Apr 18, 2017 11:30 am

GlennD wrote:All of the books say you under size the ground and neutral by one size.

"All of the books" are wrong. For the ground (EGC) use Table 250.122 of the NEC, which tells you that for copper EGCs:

#14 is good for a circuit up to 15 amps
#12 is good for a circuit up to 20 amps
#10 is good for a circuit up to 60 amps
#8 is good for a circuit up to 100 amps
#6 is good for a circuit up to 200 amps
etc

As for the neutral, in a feeder it only needs to be sized for the calculated load, not to be smaller than the minimum size EGC per the above table. "Downsize the neutral by one (even) size" is just a rule of thumb, and certainly could only work for feeders above 30 amps.

For a branch circuit, as I mentioned I believe downsizing the neutral is at best a gray area.

Cheers, Wayne

GerryAZ
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Re: Help installing NEMA 14-50 in garage

Tue Apr 18, 2017 12:18 pm

wwhitney wrote:
GerryAZ wrote:This means you can only use the 75C conductor rating if the panelboard is outside like mine.

You lost me here, there isn't any such restriction in the NEC.

Insulation temperature will be 60C, 75C, or 90C. Terminations will be 60C or 75C. Ampacity is then the lesser of two values: Tabular ampacity at the insulation temperature derated for conditions of use (ambient temperature, number of current carrying conductors in a single conduit or cable, etc.), or the tabular ampacity at the termination temperature. NM cable is further limited to the 60C tabular ampacity, even though it is required to be constructed of conductors with 90C insulation.

So if your indoor panel has breakers with 75C rated terminations (typical, I believe) and your equipment has 75C rated terminations, then you can use the 75C rated ampacity, as long as the conductor isn't NM cable and either the insulation temperature is 75C and there's no derating required, or the conductor insulation temperature is 90C and the derated ampacity for the conditions of use is not less than the 75C ampacity.

Cheers, Wayne


You explained it better than me. You are correct--the lesser of the wire rating with all detracting factors or the termination rating. It is probably different in other locations, but most residential panelboards on houses in Phoenix are outdoors so the 75C (or 60C) termination maximum becomes the limiting condition.
Gerry
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