Wire Gauge Size for 50 Amps

[KyleDay]

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 ...

Can you elaborate why #2 won't work? This thread has many posts of 8 gauge wire being used fine on a 14-50 socket. My run is only 12 ft long so maybe that's helpful.

 

[GWord]

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

Assuming your #8 is part of a Romex cable, at 60c and a 100degF ambient derate, the capacity for the cable is 40A. If everything else in the branch is above that rating then you are ok at a 40A breaker. Personally, I would convert the receptacle to a 14-30 and keep the 30A breaker.

 

[GWord]

Can you elaborate why #2 won't work? This thread has many posts of 8 gauge wire being used fine on a 14-50 socket. My run is only 12 ft long so maybe that's helpful.

8awg wire is only good if everything in the branch is rated for 75degC and your ambient rating doesn't exceed 30degC (86F).

 

[eprosenx]

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

What kind of wire is it? NM-B (Romex?)

8 AWG NM cable is rated for 40 amps (32 amps continuous) before any derations.

#1 is no good since you can't trust the car to not get accidentally changed or to change itself post software update, etc...
#2 would absolutely be a bad idea as it would overload the wire *unless* it is 8 AWG in conduit (like THHN wire) AND you do not have to apply a temperature deration to it. 86f is what the max temp is by default, but you are allowed to derate the wire based on its max temp rating (typically 90c) so that might give you some wiggle room. The issue is that I don't know the proper deration application when it is the panel and/or the end device (receptacle) that is exposed to the high heat. They are typically only rated to 75c (though I think my receptacle is cheap and only rated to 60c).
#3 is not a horrible idea, it is likely the most robust

You also could possibly consider upping the breaker to a 40a breaker since even NM wire (rated at 60c) is good for 40a (32a continuous) and then purchase a UMC Gen 2 to use on it which maxes at 32a so no concerns there. But your callout of high temps give me pause. NM cable is limited by code to the 60c rating event though I think it normally is rated to 90c insulation temps. This gives a max of 40a exactly. But when doing derations you are allowed to use the rating of the actual insulation. I don't know about the situation where NM cable is rated to 90c, the breaker terminals and the receptacle terminals are rated to 75c, but the NM wire is artificially limited to 60c by code. Basically what I am getting at is I am unsure if you are allowed to use the 90c or 75c rating of the insulation which has higher limits to then derate back down to allow temps of 100f.

This deration question is an area that exceeds my knowledge, so it may be time to engage a professional.

Note that if you are going to have a professional come out, you might consider a Wall Connector instead (it all depends on your needs and use cases).

 

[GWord]

What kind of wire is it? NM-B (Romex?)
86f is what the max temp is by default, but you are allowed to derate the wire based on its max temp rating (typically 90c) so that might give you some wiggle room. The issue is that I don't know the proper deration application when it is the panel and/or the end device (receptacle) that is exposed to the high heat. They are typically only rated to 75c (though I think my receptacle is cheap and only rated to 60c).... I don't know about the situation where NM cable is rated to 90c, the breaker terminals and the receptacle terminals are rated to 75c, but the NM wire is artificially limited to 60c by code. Basically what I am getting at is I am unsure if you are allowed to use the 90c or 75c rating of the insulation which has higher limits to then derate back down to allow temps of 100f.
.

Exactly right on how to treat NM-B; deration happens from the 90C but not to exceed the 60C rating (NEC 334.80). In this case the 60C rating is the lesser of the two so it stays at 40A.

Deration of the branch devices depends on how the manufacturer has certified the device. They will supply a table with ambient deration values. Example: http://www2.schneider-electric.com/...-catalogue/abted201147en/EN/pdfs/page_166.pdf

This is an important part of branch design and is often overlooked in my experience. It is a common cause of nuisance trips for high ambient installations such as industrial spaces and enclosures.

 

[Rocky_H]

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.

4) Replace it with a 14-30 outlet.

 

[KyleDay]

4) Replace it with a 14-30 outlet.

That may be the route to go. Thanks all for the advice.

 

[eprosenx]

4) Replace it with a 14-30 outlet.

Yes! I meant to mention that also but got distracted. This would clearly work and be totally safe. Make sure to get the corresponding Tesla UMC adapter. (I assume they make one for the UMC gen 1?)

I will try and respond more on the circuit derating stuff later. You may be able to eek a little more capacity within NEC code, but you might find that a 14-30 is sufficient capacity or you may decide to upgrade the wire anyway.

@kdday can you respond with details on what kind of wire it is? Basically is it in conduit or not? I am guessing not and it is Romex (NM cable)?

 

[Rocky_H]

You may be able to eek a little more

By the way, just to help society's use of homonyms:
"eek" is what you shout when something startles you.
"eke" is to get a little bit more, as you are meaning here.

 

[KyleDay]

Yes! I meant to mention that also but got distracted. This would clearly work and be totally safe. Make sure to get the corresponding Tesla UMC adapter. (I assume they make one for the UMC gen 1?)

I will try and respond more on the circuit derating stuff later. You may be able to eek a little more capacity within NEC code, but you might find that a 14-30 is sufficient capacity or you may decide to upgrade the wire anyway.

@kdday can you respond with details on what kind of wire it is? Basically is it in conduit or not? I am guessing not and it is Romex (NM cable)?

It is not in conduit

 

[eprosenx]

8awg wire is only good if everything in the branch is rated for 75degC and your ambient rating doesn't exceed 30degC (86F).

Ok, so here is my math on this circuit:

I am going to assume that we are talking about NM cable here (romex). I am going to assume it has a labeled insulation rating at 90c (as all the modern Romex I have seen has). It is limited by Article 334 to the 60c rating, HOWEVER you are still allowed to do the temperature derating calculation from the listed insulation rating which is 90c.

It is 8 AWG copper which table 310.15(B)(16) tells us is good to 55 amps at the 90c rating (only good to 40 amps at 60c rating). Now these ratings are at 30c (max of 86 Fahrenheit)

So we use table 310.15(B)(2)(a) to adjust for other ambient temperatures. Checking the options it would appear that the 55c (131 Fahrenheit) limit for the 90c rated conductor column requires a .76 deration. This results 41.8 amps of capacity. Of course, we are still limited by the 60c limit of 40 amps.

So by this math it would appear the NEC would allow putting 8 AWG Romex (NM cable) on a 40 amp breaker even in temperatures up to 131 Fahrenheit would meet code. Since there is no 40 amp NEMA receptacle you can use the 14-50. But you still can't plug any load in that needs more than 40a (32a continuous). So in this configuration a UMC Gen 1 would NOT be allowed (even if you set it down in the car - I don't think that counts). But a UMC Gen 2 would be fine since that will max out at 32 amps.

Check my math, but this is my reading of the code.

 

[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.

Finally went and ran the math tonight on the following scenario:

8 AWG Copper THHN in conduit with 90c rated insulation. (for say a 14-50 receptacle)

The 90c rating gives us 55a of capacity, but the 75c rating only gives us 50a. The breaker and receptacle on the end are 75c rated only so that is our limiting factor to start with.

If we need to derate due to ambient temp being over 86F then we do it from the 90c ampacity since that is what the wire is rated to. 55 * .91 = 50.05 amps of capacity. So the .91 derating allows the ambient temp to be up to 104F.

So we in the forums here need to warn folks that if they are considering using 8 AWG in conduit for a 50a circuit, that no part of the circuit can be in an ambient temp of over 104F.

At least that is my reading of how the ambient temp deration works. (note I am not a certified professional in this field) Anyone have any feedback or want to chime in here?

 

[GWord]

Ok, so here is my math on this circuit:

I am going to assume that we are talking about NM cable here (romex). I am going to assume it has a labeled insulation rating at 90c (as all the modern Romex I have seen has). It is limited by Article 334 to the 60c rating, HOWEVER you are still allowed to do the temperature derating calculation from the listed insulation rating which is 90c.

It is 8 AWG copper which table 310.15(B)(16) tells us is good to 55 amps at the 90c rating (only good to 40 amps at 60c rating). Now these ratings are at 30c (max of 86 Fahrenheit)

So we use table 310.15(B)(2)(a) to adjust for other ambient temperatures. Checking the options it would appear that the 55c (131 Fahrenheit) limit for the 90c rated conductor column requires a .76 deration. This results 41.8 amps of capacity. Of course, we are still limited by the 60c limit of 40 amps.

So by this math it would appear the NEC would allow putting 8 AWG Romex (NM cable) on a 40 amp breaker even in temperatures up to 131 Fahrenheit would meet code. Since there is no 40 amp NEMA receptacle you can use the 14-50. But you still can't plug any load in that needs more than 40a (32a continuous). So in this configuration a UMC Gen 1 would NOT be allowed (even if you set it down in the car - I don't think that counts). But a UMC Gen 2 would be fine since that will max out at 32 amps.

Check my math, but this is my reading of the code.

100% agree with your math and your conclusions. I think there's something in the NEC about needing to label the 14-50 as 40A max in your example (I can't recall where). Your example is a great illustration of why a Romex 8awg installation would be questionable, but not outright contraindicated, for a Tesla charging system.

Just as an aside, in case anyone was wondering, my previous comment on the 30C ambient temp assumed one was using an approved wire insulation that could get to the 8awg, 75C column for the 50A one needs to have for a non derated 14-50. Although, as eprosenx pointed out, you might be able to squeeze out a little more with 90C wire.

 

[GWord]

Finally went and ran the math tonight on the following scenario:

8 AWG Copper THHN in conduit with 90c rated insulation. (for say a 14-50 receptacle)

The 90c rating gives us 55a of capacity, but the 75c rating only gives us 50a. The breaker and receptacle on the end are 75c rated only so that is our limiting factor to start with.

If we need to derate due to ambient temp being over 86F then we do it from the 90c ampacity since that is what the wire is rated to. 55 * .91 = 50.05 amps of capacity. So the .91 derating allows the ambient temp to be up to 104F.

So we in the forums here need to warn folks that if they are considering using 8 AWG in conduit for a 50a circuit, that no part of the circuit can be in an ambient temp of over 104F.

At least that is my reading of how the ambient temp deration works. (note I am not a certified professional in this field) Anyone have any feedback or want to chime in here?

This is how I use it professionally (or did when I was actually practicing in ye olden days). I'll be honest, I'd be a little hesitant to go that close to the edge case with a design in your example, but that's personal discretion. I would be concerned about nuisance trips from the breaker.

 

[eprosenx]

100% agree with your math and your conclusions. I think there's something in the NEC about needing to label the 14-50 as 40A max in your example (I can't recall where). Your example is a great illustration of why a Romex 8awg installation would be questionable, but not outright contraindicated, for a Tesla charging system.

Just as an aside, in case anyone was wondering, my previous comment on the 30C ambient temp assumed one was using an approved wire insulation that could get to the 8awg, 75C column for the 50A one needs to have for a non derated 14-50. Although, as eprosenx pointed out, you might be able to squeeze out a little more with 90C wire.

I have never found anything about a labeling requirement, but from a practical standpoint I absolutely would label it if I did this. I have read all the code sections I could find on receptacles and branch circuits pretty thoughly (but that does not mean I have not missed the labeling requirement).

 

[Rocky_H]

The 90c rating gives us 55a of capacity, but the 75c rating only gives us 50a. The breaker and receptacle on the end are 75c rated only so that is our limiting factor to start with.

If we need to derate due to ambient temp being over 86F then we do it from the 90c ampacity since that is what the wire is rated to. 55 * .91 = 50.05 amps of capacity. So the .91 derating allows the ambient temp to be up to 104F.

Thank you for checking into this, and you have provided the key piece of information that un-confuses me and makes it make sense. I would have thought the categorical 75C rating (like a coarse adjust) was the baseline, and then the temperature derating would come off of that (fine adjust). But that makes more sense if you temperature derate from the wire insulation type, and then go with whichever is the more restrictive outcome.

 

[AreV]

Well my question is my 50 amp charger for my model Y has 50 amp breakers and 8 AWG Romex with 10AWG ground wire. I get only 30 miles an hour charging it, at, according to my car, about 30 amps.

In preparation for getting our Tesla, I had the house's 100 Amp service switched over to 200 Amp, and had the breaker box ground rods, ground wire all replaced and the Tesla charger

I have the NEMA 14-50 outlet. I thought the Romex was wrong as I could not read it all and it turns out to be 8 AWG with a 10 AWG ground wire. I checked and that appears to be OK.

I am having my new electrician add outlets and rewire the existing on the wall conduit circuits to the outside motion security lights and my little workbench there. Would it be foolish to have him pull 6 AWG wire? The run is right at 36 feet. If so I'll also get the 60 Amp breaker set. Opinions, comments?

 

[nwdiver]

I am having my new electrician add outlets and rewire the existing on the wall conduit circuits to the outside motion security lights and my little workbench there. Would it be foolish to have him pull 6 AWG wire? The run is right at 36 feet. If so I'll also get the 60 Amp breaker set. Opinions, comments?

Depends on your personal circumstances. If you have access to a nearby supercharger you can use in a pinch on the rare occasion you need to charge faster I would just go with a 40A circuit that #8 wire supports and charge at 32A. Not sure what the cost would be to upgrade the wire but IMHO it's probably not worth it. If you want to be 4.0 with the code you can swap the outlet to a NEMA 14-30 and get an adapter for $35.

 

[Dave EV]

If you have a 50A breaker on your circuit, all the wiring must be 50A minimum rated. 8 GA THWN wire routed in conduit is rated for 50A. 8 GA Romex (aka NM-B) is NOT - it's rated for 40A.

It is fine (yes, up to code) to have a 14-50 outlet on a 40A circuit, but it sounds like what you have now is not up to code.

Personally, I have found it rare that I really need faster than 32A charging (the maximum rate on a 40A circuit). That said, I did upgrade to a 50A circuit when I installed my HPWC, anyway. Maybe I should have gone all the way to a 60A circuit, that would have been nice to have when adding a second HPWC with circuit sharing, but even then, 32A all night is fast enough even with two cars unless for some reason they both get home almost empty and require a full charge.

 

[Rocky_H]

Well my question is my 50 amp charger for my model Y has 50 amp breakers and 8 AWG Romex with 10AWG ground wire. I get only 30 miles an hour charging it, at, according to my car, about 30 amps.

I have the NEMA 14-50 outlet. I thought the Romex was wrong as I could not read it all and it turns out to be 8 AWG with a 10 AWG ground wire. I checked and that appears to be OK.

Nope, not OK.

If you have a 50A breaker on your circuit, all the wiring must be 50A minimum rated. 8 GA THWN wire routed in conduit is rated for 50A. 8 GA Romex (aka NM-B) is NOT - it's rated for 40A.

It is fine (yes, up to code) to have a 14-50 outlet on a 40A circuit, but it sounds like what you have now is not up to code.

I'm just chucking my two cents in to confirm this as well. That 8 gauge Romex would be good for a 40A circuit, so you could change the breaker to get in compliance.