Wire Gauge Size for 50 Amps

[FlasherZ]

FlasherZ is the expert here, but you will get a 5 volt (2.1%) drop using #8 for a 70 foot run, and it will get warm. Be sure that you use wire rated to 90 degrees C. If you are pulling wire I would pull #6 then drop is 3 volts.

The voltage drop starts to become a concern for longer runs, but 2% drop isn't something to worry about so much.

Honestly, I personally think the rule is rather outdated for modern type NM cable -- but it's the rule. The conductors in Romex are 90 degC rated -- if you were to cut open a #8 Romex cable and stick the wires in a conduit individually, 50A becomes legal. The rule is there to make you assume that heat can be trapped in the cable and create a safety situation.

- - - Updated - - -

Our run is in the basement of the house ... coolest spot. Never goes about 70f ambient.
Insulated Copper wire, in 3/4" conduit. With 50amp breaker.
Do I care about the additional 2 volt drop?
And I should mention that 1/3rd (final 1/3rd) will be #6 as we already bought it.

Without seeing all of the details in person, I'd say #8 is sufficient, will work, and will pass inspection. I know you live in Chicagoland suburbs, so you're likely subject to the infamous "conduit-only, no-romex" make-work rules.

 

[FlasherZ]

One other point to note, in addition to the other thread: your 3/4" conduit will not support the wiring necessary for an HPWC. Only IMC (heavier metal conduit) can support 2 #3 conductors in a 3/4" conduit - you need 1" EMT (metal) or PVC. Keep that in mind when considering the HPWC as well.

 

[sp4rk]

Yes, I realized that and thanks! I would use 1 1/2" or 2" for those # conductors. I decided against 100amp service for now ... really don't need 60 miles/hour charge for $1,500 upcharge in labour and materials! And the HPWC!
But at least the car is ready if / when it needs it.
Thanks for all your help.

 

[frequencydip]

Your load is a 40A continuous load, which requires a 50A branch circuit, appliance, and receptacle rating.

For wire size, the electrician's favorite answer: "it depends".

8 AWG wire is good for 40 amps for 60 degC systems, 50 amps for 75 degC systems, and 55 amps for 90 degC systems. To qualify for the 90 degC rating, *every* point in the system must be 90 degC rated -- and that can be tough. Typically, for ampacity purposes, you look at 60 degC for Romex (NM-B) and 75 degC for wire-in-conduit.

If you're using Romex (type NM cable), it's only good for 45 amps because by code it must be de-rated to the 60 degC rating (NEC 334). AWG 8 is good for 50 amps only if all connection points, breakers, conductors, termination boxes, etc., are all rated for 75 degC or higher. If you're running through an attic that has a high ambient temperature in the summer, you have to de-rate / correct for that per NEC tables 310.15(B)(2).

This is why many electricians will just forego #8 for 50A and install #6, which gives plenty of headroom for all de-rating needs.

FlasherZ is the expert here, but you will get a 5 volt (2.1%) drop using #8 for a 70 foot run, and it will get warm. Be sure that you use wire rated to 90 degrees C. If you are pulling wire I would pull #6 then drop is 3 volts.

This is why I love TMC, you as a simple question and you get a really in-depth answer that educates you rather than just sending you on your way....

Thanks to all the contributors that have extended my knowledge and understanding...

To throw in my own 2 cents, when I wired my garage I ran #2 copper even though I was only using a UMC at 40Amps. The work involved is more costly and time consuming than the wire, I wanted the ability to upgrade to a High Power Wall Connector or if I bought a future electric car with higher demands Model S, X, R what ever the future holds the garage is already properly wired. If you own your home and plan on staying for a while you may want to make the additional investment.

 

[stevec5088]

I realize this thread is a year old, but it popped up on a google search, and the information provided above is exactly what I need.

I am wiring two bays of my garage with NEMA 14-50 outlets for current and future charging systems. I hired an electrician to install an EV-only charging meter, and a 100 amp sub-panel to the garage. I currently have a Volt and a Spark EV and they don't use much current, but I want to set everything up for faster-charging cars in the future. I am debating whether to run #4 or #6 AWG wire, but the biggest question is this:

Is it ok to build a custom channel to run the wires in across the ceiling of the garage? I am thinking of a 2" by 3" channel built from wood (fun project for my new table saw). There is already a beam crossing the ceiling, and the wood channel would match up to (run parallel and next to) the beam where it protrudes from the ceiling. I don't plan on having the work inspected, but would this sort of thing pass an inspection?

Can I run THHN single-wire in this channel, or should I get the romex (3-wire plus ground) bundled together?

The channel would look way better than a mess of 1" and 1 1/4" conduit stuck on the ceiling.

Thanks in advance!

 

[FlasherZ]

I realize this thread is a year old, but it popped up on a google search, and the information provided above is exactly what I need.

I am wiring two bays of my garage with NEMA 14-50 outlets for current and future charging systems. I hired an electrician to install an EV-only charging meter, and a 100 amp sub-panel to the garage. I currently have a Volt and a Spark EV and they don't use much current, but I want to set everything up for faster-charging cars in the future. I am debating whether to run #4 or #6 AWG wire, but the biggest question is this:

Is it ok to build a custom channel to run the wires in across the ceiling of the garage? I am thinking of a 2" by 3" channel built from wood (fun project for my new table saw). There is already a beam crossing the ceiling, and the wood channel would match up to (run parallel and next to) the beam where it protrudes from the ceiling. I don't plan on having the work inspected, but would this sort of thing pass an inspection?

Can I run THHN single-wire in this channel, or should I get the romex (3-wire plus ground) bundled together?

The channel would look way better than a mess of 1" and 1 1/4" conduit stuck on the ceiling.

Thanks in advance!

Wire may only be run in raceways approved as part of the NEC. THHN wires cannot be run directly in wood channels or on the surface of a wall. NM-B ("Romex") is permitted where it is not subject to physical damage (generally this is interpreted as it is not permitted to be used exposed below 7 ft.). Otherwise, you need to use an approved raceway (EMT/FMC/PVC/etc.) So, in your case, you may be able to use NM-B as long as it doesn't drop below 7' or so, otherwise you're stuck with some sort of a conduit.

 

[Rocky_H]

If you're using Romex (type NM cable), it's only good for 45 amps because by code it must be de-rated to the 60 degC rating (NEC 334). AWG 8 is good for 50 amps only if all connection points, breakers, conductors, termination boxes, etc., are all rated for 75 degC or higher. If you're running through an attic that has a high ambient temperature in the summer, you have to de-rate / correct for that per NEC tables 310.15(B)(2).

This is why many electricians will just forego #8 for 50A and install #6, which gives plenty of headroom for all de-rating needs.

I've been on both Tesla forums for about 4-5 years, and I can't believe I just came across an electric code thing for something as basic as wire size that I have never seen before until today. So, I would like to get some insight from @FlasherZ and maybe @eprosenx . I searched quite a lot on this forum through Google (why does the built-in search suck and turn up no results?), so that's why I'm having to dredge up this old thread, because this was about the only tiny brief mention I could find of this issue, but still no one uses it.

What we've seen all the time is the ampacity table that show the current levels in the 60, 75, and 90 degrees C columns. But there is that thing in the title I hadn't paid much attention to that says that's for ambient temperatures up to 30 degrees C. Someone today pointed me to a reference that includes additional derating factors, and there's this one for higher ambient temperatures that it says is based on 310.15(B)(2).
https://www.lucob.sk/sites/default/files/TK1 11.10-11.11 str. Prudová zatažitelnost vodičov.pdf
http://www.barr-thorp.com/wp-content/uploads/2011/04/digest-176-nec-tables.pdf

Let's take the 50A circuit example that everyone has always been talking about for years. Everyone always has it very simple: 6 gauge romex or 8 gauge wire-in-conduit will be good for a 50A circuit. That's what the main ampacity table says, and that's what's in @FlasherZ 's FAQ.
But for the "up to 30 C" part is only 86 degrees F. Surely most people's garages get hotter than that. So if you apply the next factor up that covers regular kinda hot summer temperatures of 31-35 C (87 to 95 degrees F), you need to multiply by the derating factor 0.94 from the 75 C column. That yields only 47A allowed from 8 gauge wire-in-conduit, meaning it would not be allowed for a 50A circuit.

So finally getting to my question: How in the world can this be? What am I missing? We've been telling everyone that 8 gauge wire in conduit is officially NEC rated approved for a 50A circuit for several years, when it isn't for most cases. I've seen everyone just go by the simple table without ever compensating for summer temperatures. Is there some reason why it usually doesn't need to be applied in residential settings? My world is crumbling here, guys--help me figure this out.

 

[eprosenx]

I've been on both Tesla forums for about 4-5 years, and I can't believe I just came across an electric code thing for something as basic as wire size that I have never seen before until today. So, I would like to get some insight from @FlasherZ and maybe @eprosenx . I searched quite a lot on this forum through Google (why does the built-in search suck and turn up no results?), so that's why I'm having to dredge up this old thread, because this was about the only tiny brief mention I could find of this issue, but still no one uses it.

What we've seen all the time is the ampacity table that show the current levels in the 60, 75, and 90 degrees C columns. But there is that thing in the title I hadn't paid much attention to that says that's for ambient temperatures up to 30 degrees C. Someone today pointed me to a reference that includes additional derating factors, and there's this one for higher ambient temperatures that it says is based on 310.15(B)(2).
https://www.lucob.sk/sites/default/files/TK1 11.10-11.11 str. Prudová zatažitelnost vodičov.pdf
http://www.barr-thorp.com/wp-content/uploads/2011/04/digest-176-nec-tables.pdf

Let's take the 50A circuit example that everyone has always been talking about for years. Everyone always has it very simple: 6 gauge romex or 8 gauge wire-in-conduit will be good for a 50A circuit. That's what the main ampacity table says, and that's what's in @FlasherZ 's FAQ.
But for the "up to 30 C" part is only 86 degrees F. Surely most people's garages get hotter than that. So if you apply the next factor up that covers regular kinda hot summer temperatures of 31-35 C (87 to 95 degrees F), you need to multiply by the derating factor 0.94 from the 75 C column. That yields only 47A allowed from 8 gauge wire-in-conduit, meaning it would not be allowed for a 50A circuit.

So finally getting to my question: How in the world can this be? What am I missing? We've been telling everyone that 8 gauge wire in conduit is officially NEC rated approved for a 50A circuit for several years, when it isn't for most cases. I've seen everyone just go by the simple table without ever compensating for summer temperatures. Is there some reason why it usually doesn't need to be applied in residential settings? My world is crumbling here, guys--help me figure this out.

Great question! And yes you have it 90% correct but you are missing one piece:

When doing the temperature derating for say conduit that goes through a hot attic you get to use the full 90c rating of the wire, not the code mandated derating to 60c for NM cable (romex) or the 75c rating of the breaker or 14-50r or Wall Connevtor terminals.

So that usually gives you a little more headroom to work with.

Also, in most cases we say 6 AWG NM cable for a 14-50 is actually rated to 55a so you have five amps to play with for derating if you need a higher temp. (that is on top of the fact that you can use the 90c rating if the “hot part” of the wire is not at the terminals themselves but just somewhere in the middle)

So yes, when we talk about 8 awg in conduit for a 50a circuit we are right at the limit of the wire so we need to be careful about the temperature derating.

There is also a derating for number of current carrying conductors in a conduit if it is over three (normally does not apply in residential installs).

When I am back at my computer I can go do some sample math and provide examples.

 

[eprosenx]

I've been on both Tesla forums for about 4-5 years, and I can't believe I just came across an electric code thing for something as basic as wire size that I have never seen before until today. So, I would like to get some insight from @FlasherZ and maybe @eprosenx . I searched quite a lot on this forum through Google (why does the built-in search suck and turn up no results?), so that's why I'm having to dredge up this old thread, because this was about the only tiny brief mention I could find of this issue, but still no one uses it.

What we've seen all the time is the ampacity table that show the current levels in the 60, 75, and 90 degrees C columns. But there is that thing in the title I hadn't paid much attention to that says that's for ambient temperatures up to 30 degrees C. Someone today pointed me to a reference that includes additional derating factors, and there's this one for higher ambient temperatures that it says is based on 310.15(B)(2).
https://www.lucob.sk/sites/default/files/TK1 11.10-11.11 str. Prudová zatažitelnost vodičov.pdf
http://www.barr-thorp.com/wp-content/uploads/2011/04/digest-176-nec-tables.pdf

Let's take the 50A circuit example that everyone has always been talking about for years. Everyone always has it very simple: 6 gauge romex or 8 gauge wire-in-conduit will be good for a 50A circuit. That's what the main ampacity table says, and that's what's in @FlasherZ 's FAQ.
But for the "up to 30 C" part is only 86 degrees F. Surely most people's garages get hotter than that. So if you apply the next factor up that covers regular kinda hot summer temperatures of 31-35 C (87 to 95 degrees F), you need to multiply by the derating factor 0.94 from the 75 C column. That yields only 47A allowed from 8 gauge wire-in-conduit, meaning it would not be allowed for a 50A circuit.

So finally getting to my question: How in the world can this be? What am I missing? We've been telling everyone that 8 gauge wire in conduit is officially NEC rated approved for a 50A circuit for several years, when it isn't for most cases. I've seen everyone just go by the simple table without ever compensating for summer temperatures. Is there some reason why it usually doesn't need to be applied in residential settings? My world is crumbling here, guys--help me figure this out.

You bring up a good point about the hot garage issue. First off, my garage in Oregon I don’t think nearly ever gets over that. It is North facing and has no windows. The house is AC’d so even on a hot day the garage should not be that hot.

That being said, let’s say my garage was over 86 degrees F. My panel is in that garage. So I am not sure how to do the derating. Since the high heat is at the location the wire attaches to the breaker, does that mean I have to derate using the 75c limit? Also then, which derating column do I use in the table? The one for 90c (which is what the wire is rated to), or the one for 75c which is what the terminal is rated for?

More research is needed!

I just try to play it safe and oversize the wire a tiny bit if I am right at the limits.

 

[Rocky_H]

@eprosenx I'm obviously not concerned on the 6 gauge case on these installs because as you mentioned, it's got a least a few amps margin no matter what type of wire or cable is being used. It's the 8 gauge one that's rated for exactly 50A in one case that just doesn't have any room to be temperature derated and still pass code.

 

[eprosenx]

@eprosenx I'm obviously not concerned on the 6 gauge case on these installs because as you mentioned, it's got a least a few amps margin no matter what type of wire or cable is being used. It's the 8 gauge one that's rated for exactly 50A in one case that just doesn't have any room to be temperature derated and still pass code.

Yes, exactly. Though to be clear: It does have a small amount of headroom to be derated if the hot part of the wire is in say an attic (not at the ends) since you get to use the 90 degree wire rating which is rated to 55a.

But yes, 8 awg is very tight for a 50a circuit. Hence I am sure why Tesla just tells folks to go with 6 awg.

 

[timk225]

You all need to quit being a bunch of cheap asses and spend the money for 6 gauge wire. I did. I'm running cable now to my new driveway, which is right around 100 feet from the breaker box, using Cerro 6/3 cable which is in open air in the basement but runs through 30 feet of 3/4" PVC conduit for the underground run to the edge of the driveway.

Spend the money for 6 gauge. Be like me.

If you cheap out and do a poor job with too small wire and burn something up, have fun with the fire inspector for your house insurance, see if they pay!

 

[P85_DA]

You all need to quit being a bunch of cheap asses and spend the money for 6 gauge wire.

So why did U cheap out and not run THWN ???​

 

[timk225]

Uhhhhh......... I don't know? This was the only 6 gauge wire on the shelf at Lowe's? Yeah. That's my answer.

And at $250 for a 125 foot roll, it wasn't cheap.

 

[P85_DA]

Uhhhhh......... I don't know? This was the only 6 gauge wire on the shelf at Lowe's? Yeah. That's my answer.

And at $250 for a 125 foot roll, it wasn't cheap.

Lol yah u bought indoor wire (Romex style) for outdoor wet condition use.,u should have got THWN/THHN ... @eprosenx correct me if wrong ...

 

[timk225]

It's in a conduit, sealed up at every connection. It won't, or shouldn't, get wet. The wire remains indoors, relative to the conduit wall, at least.

I just checked the label of the package, it's Cerro NM-B wire:

NM-B cable is primarily used in residential wiring as branch circuits for outlets, switches, and other loads. NM-B cable may be run in air voids of masonry block or tile walls where such walls are not subject to excessive moisture or dampness. Voltage rating for NM-B cable is 600 volts.

Well, who's to say what constitutes "EXCESSIVE" moisture? If the PVC pipe breaks and it gets flooded? Yeah, probably.

Well in any case, it's all in place now, and I'm not changing it. It is indoor for 60% of its length. Close enough. And if something goes wrong, that's what the circuit breaker is for.

 

[GWord]

Lol yah u bought indoor wire (Romex style) for outdoor wet condition use.,u should have got THWN/THHN ... @eprosenx correct me if wrong ...

NEC 334.12 prohibits NM (Romex) cable from being used in a wet location. NEC 300.5 states that all conduit underground installations are considered wet locations. So basically Romex can't be used in underground installations. The branch circuit in question would need to transition to another wire type when making the underground portion of the run.

 

[GWord]

It's in a conduit, sealed up at every connection. It won't, or shouldn't, get wet.

Per NEC 300.5, if the conduit is underground, it is considered wet.

ARTICLE 300 Wiring Methods
I. General Requirements

300.5 Underground Installations.
(B) Wet Locations. The interior of enclosures or raceways installed underground shall be considered to be a wet location. Insulated conductors and cables installed in these enclosures or raceways in underground installations shall be listed for use in wet locations and shall comply with 310.10(C). Any connections or splices in an underground installation shall be approved for wet locations.


ARTICLE 334 Nonmetallic-Sheathed Cable: Types NM, NMC, and NMS

II. Installation

334.12 Uses Not Permitted.
(B) Types NM and NMS. Types NM and NMS cables shall not be used under the following conditions or in the following locations:

(4) In wet or damp locations

The reason for this rule is that water or other liquids can have a deteriorating effect on the cable insulators. Or, at least, the cable isn't rated for exposure to those types of elements. The PVC jacket isn't going to absorb water over short exposure timeframes but any nick or crack in the jacket, or just the exposed ends are going to wick water, and the paper wrap inside will absorb it like crazy leading to longer term degradation or corrosion.

 

[KyleDay]

Hi everyone -

I just moved into a home where the previous owner had a tesla and the NEMA 14-50 socket was already installed. I plugged in and on my first charge I got a "charging interrupted message". I went to check it and the breaker had flipped because only a 30 Amp breaker was placed on this NEMA 14-50 socket and my car was trying to draw 40 amps.

I reduced my charge to 24 amps (80% of 30 AP breaker) for regular charging, but, I can't imagine this is up to code.

I opened the panel and best I could tell, the wiring is 8 gauge wiring from the panel to the NEMA 14-50 socket, with a length of about 10 feet between the breaker box and the socket in the garage.

I'm in Phoenix and temperatures inside and outside the garage regularly get over 100F.

I'm wondering what I should do:

1) Nothing, just charge at 24 amps and be happy.
2) Replace the 30 Amp breakers with 50 Amp breakers and charge at 40 amps.
3) Call an electrician to replace the 8 gauge wire with 6 gauge wire (will be a PITA I imagine), and replace breaker to 50 Amp.

Thoughts? I'm hopeful that option 2 is viable even with my 8 gauge wire as that would be easiest.

Thanks

 

[P85_DA]

Hi everyone -

I just moved into a home where the previous owner had a tesla and the NEMA 14-50 socket was already installed. I plugged in and on my first charge I got a "charging interrupted message". I went to check it and the breaker had flipped because only a 30 Amp breaker was placed on this NEMA 14-50 socket and my car was trying to draw 40 amps.

I reduced my charge to 24 amps (80% of 30 AP breaker) for regular charging, but, I can't imagine this is up to code.

I opened the panel and best I could tell, the wiring is 8 gauge wiring from the panel to the NEMA 14-50 socket, with a length of about 10 feet between the breaker box and the socket in the garage.

I'm in Phoenix and temperatures inside and outside the garage regularly get over 100F.

I'm wondering what I should do:

1) Nothing, just charge at 24 amps and be happy.
2) Replace the 30 Amp breakers with 50 Amp breakers and charge at 40 amps.
3) Call an electrician to replace the 8 gauge wire with 6 gauge wire (will be a PITA I imagine), and replace breaker to 50 Amp.

Thoughts? I'm hopeful that option 2 is viable even with my 8 gauge wire as that would be easiest.

Thanks

Number 2 won’t work without number 3 ..number 1 is a hack but will work as car will remember what u set amps too ..alternative is change outlet to 14-30 and get Tesla adaptor ...