#4 AWG Copper good for 100A subfeed?

[jeff48356]

I'm going to be running a 100A subfeed underground through PVC conduit into a newly-built detached garage where the customer will eventually need to use an electric car charger outlet, which calls for 50A. I decided to sell them on a 100A panel in the garage. I've seen some tables in the NEC where it limits #4 copper to 95 amps (or even 85 amps). Yet, there is another table elsewhere that permits 100 amps if used for a feeder.

Would you go with #2 or #4 AWG for this installation? I prefer to use copper, rather than aluminum conductors.

 

[310 BLAZE IT]

310.15(B)16 is the table you use 75c copper column. You only want to do this type of work if you are a certified electrician if your state requires it. No book in front of me right now. Remember feeders are sized by load but small stuff it makes sense to match it to the breaker

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[infinity]

I'm going to be running a 100A subfeed underground through PVC conduit into a newly-built detached garage where the customer will eventually need to use an electric car charger outlet, which calls for 50A. I decided to sell them on a 100A panel in the garage. I've seen some tables in the NEC where it limits #4 copper to 95 amps (or even 85 amps). Yet, there is another table elsewhere that permits 100 amps if used for a feeder.

Would you go with #2 or #4 AWG for this installation? I prefer to use copper, rather than aluminum conductors.

#4 copper is good for 85 amps which means that you can go up to the next standard size which is 90 amps. If you want to use a 100 amp OCPD you will need to increase your conductor size #3 copper.

 

[310 BLAZE IT]

#4 copper is good for 85 amps which means that you can go up to the next standard size which is 90 amps. If you want to use a 100 amp OCPD you will need to increase your conductor size #3 copper.

Well that explains why I always see 90A mains... Saves from going to a #2 essentially because a #3 is rare for some reason. You'd think it would be common with all the 100A panels around

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[infinity]

Well that explains why I always see 90A mains... Saves from going to a #2 essentially because a #3 is rare for some reason. You'd think it would be common with all the 100A panels around

We use #3 all of the time for 100 amp feeders. Seems to be pretty available from a larger supply house.

 

[roger]

All the suppliers I use stock #3 THWN.

Roger

 

[310 BLAZE IT]

All the suppliers I use stock #3 THWN.

Roger

You know it's cool when your coworkers push their 'field experience' down your throat and it's all just a load of BS passed from one non-field-worker to another

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[petersonra]

All the suppliers I use stock #3 THWN.

Roger

We generally upsize it to number 2 because it's easier to come by. The boss at our shop thinks it's not worth having to buy number 3 since they don't use that much of it anyway. I think if there was a case where we were going to use a bunch of it it might be worth getting because it's a little less than number 2 but most of the time we buy 10 or 20 feet at a time and the difference in price isn't really that significant overall.

 

[roger]

You know it's cool when your coworkers push their 'field experience' down your throat and it's all just a load of BS passed from one non-field-worker to another

I don't know what you're saying but you might need to use better supply houses or tell the ones you do use to improve their stock.

Roger

 

[kwired]

We generally upsize it to number 2 because it's easier to come by. The boss at our shop thinks it's not worth having to buy number 3 since they don't use that much of it anyway. I think if there was a case where we were going to use a bunch of it it might be worth getting because it's a little less than number 2 but most of the time we buy 10 or 20 feet at a time and the difference in price isn't really that significant overall.

Do you have a common application other then feeders that could use 3 AWG which require 2 AWG?

For motors I seem to need 4 AWG, frequently, on several different applications, 3 AWG mostly just is needed on 60 HP 480 volt three phase motors, but then usually jumps to needing 1 AWG, very seldom do I need 2 AWG unless ampacity adjustments would be requiring it.

 

[JFletcher]

Original question having been answered, I think I'd have tried to sell a 125A subpanel with #1... 90A is gonna be blown if they add a spa in addition to that charger. Labor is virtually the same, materials and o/a job cost not that much more.. and if you went with AL feeder, probably cheaper than your Cu 100 (90A) panel install.

 

[kwired]

Original question having been answered, I think I'd have tried to sell a 125A subpanel with #1... 90A is gonna be blown if they add a spa in addition to that charger. Labor is virtually the same, materials and o/a job cost not that much more.. and if you went with AL feeder, probably cheaper than your Cu 100 (90A) panel install.

I bet it don't trip as much as you might think. Unless you come home, plug the nearly completely discharged car in and go straight to the spa, that risk is even less.

 

[Little Bill]

I bet it don't trip as much as you might think. Unless you come home, plug the nearly completely discharged car in and go straight to the spa, that risk is even less.

If you want more risk, just stand in the spa and plug in your charger!

 

[petersonra]

Do you have a common application other then feeders that could use 3 AWG which require 2 AWG?

For motors I seem to need 4 AWG, frequently, on several different applications, 3 AWG mostly just is needed on 60 HP 480 volt three phase motors, but then usually jumps to needing 1 AWG, very seldom do I need 2 AWG unless ampacity adjustments would be requiring it.

It seems to me that most of the number three I would use is for equipment grounding conductors. Since most of that is inside of a cabinet there isn't a whole lot of it.

 

[infinity]

My last project had over 720 apartments and each unit had it's own 100 amp panel. All of the feeders that were less than 100' (600+) were #3. Switching from #2 to #3 (and using a smaller EGC) saved a big chunk of change.

 

[petersonra]

My last project had over 720 apartments and each unit had it's own 100 amp panel. All of the feeders that were less than 100' (600+) were #3. Switching from #2 to #3 (and using a smaller EGC) saved a big chunk of change.

Not much difference between 10 ft of number 2 and 10 ft of number 3 as far as costs go.

 

[JFletcher]

I bet it don't trip as much as you might think. Unless you come home, plug the nearly completely discharged car in and go straight to the spa, that risk is even less.

Blown as in reached 90+A, not blown as in tripped breaker. The charger likely is 40A draw, and even a 60A spa only draws 48A for 15 minutes - after that, the timer kicks you out and cuts off the jet pumps and blower. ofc you can cut it right back on, but at 94-102*, 15 min in the spa is enough. 88<90, but close enough with other common future heavy loads (on demand water heater, large shop compressor, air conditioned garage, etc.) that I would have mentioned it to the HO.

Not much difference between 10 ft of number 2 and 10 ft of number 3 as far as costs go.

x 600 runs it adds up more. Back when I was doing commercial v/d/v, the difference in top of the line cable and equipment was over 3-fold difference of the bottom stuff. You can pay 80c for a cat6 jack, or 7$ a piece. Wire is about a 2 fold difference.

 

[jeff48356]

Blown as in reached 90+A, not blown as in tripped breaker. The charger likely is 40A draw, and even a 60A spa only draws 48A for 15 minutes - after that, the timer kicks you out and cuts off the jet pumps and blower. ofc you can cut it right back on, but at 94-102*, 15 min in the spa is enough. 88<90, but close enough with other common future heavy loads (on demand water heater, large shop compressor, air conditioned garage, etc.) that I would have mentioned it to the HO.

I've actually wired a luxury garage with all that stuff, last year. We ended up having the POCO install a separate 200A service drop and meter, just for the outbuilding.

 

[JFletcher]

I've actually wired a luxury garage with all that stuff, last year. We ended up having the POCO install a separate 200A service drop and meter, just for the outbuilding.

Yep. and there isnt much room to pull a 125A subpanel from the main if the main is already loaded up pretty well... only option is what you wrote.

 

[kwired]

Blown as in reached 90+A, not blown as in tripped breaker. The charger likely is 40A draw, and even a 60A spa only draws 48A for 15 minutes - after that, the timer kicks you out and cuts off the jet pumps and blower. ofc you can cut it right back on, but at 94-102*, 15 min in the spa is enough. 88<90, but close enough with other common future heavy loads (on demand water heater, large shop compressor, air conditioned garage, etc.) that I would have mentioned it to the HO.



x 600 runs it adds up more. Back when I was doing commercial v/d/v, the difference in top of the line cable and equipment was over 3-fold difference of the bottom stuff. You can pay 80c for a cat6 jack, or 7$ a piece. Wire is about a 2 fold difference.

The conductor will handle well over 90 amps and never have any problems. Terminations will fail before the conductor does. I'd guess you could put it on a 125 maybe even 150 amp breaker and never have a conductor failure outside of at a termination. NEC is a little conservative on conductor ampacity. This makes the 90 amp overcurrent device the true limiting factor, but even it will hold a slight overload for quite some time.