Welcome to Tesla Motors Club
Discuss Tesla's Model S, Model 3, Model X, Model Y, Cybertruck, Roadster and More.
Register

AWG 8 wire with 50amp breaker

I have AWG 8 wire running from 50amp breaker to my 14-50 plug in my garage.

Am I able to draw 40amps using my first gen mobile charger?
 

Attachments

  • 20180522_191831.jpg
    20180522_191831.jpg
    248.9 KB · Views: 220
WARNING: I'm not an expert or a licensed electrician.

TLDR: I think you are technically okay based on code and material ratings so long as your total cable length is less than 80-100 feet.

Detailed version:

So I could be wrong, but here's how I read it:
  • Your cable is 3+1 conductor of 8 AWG copper in an NMD90 cable sheath; NMD90 cable has a 90 degree C temp rating for the insulation.
  • Table 2 of the CEC 2015, says that the maximum allowable ampacity of AWG 8 cable with a 90 degree C rating is 55 Amps (note that the 60 degree C rating is 40 Amps, and the 75 degree C rating is 50 A). Note this table is also for a maximum of 3 copper conductors in a cable or a raceway (you have a "cable").
  • The maximum permitted load you can put on a 50 A breaker is 40 A (80%).
Here are the parts I am not clear on (with respect to complying with code), and my thoughts on them, as well as other factors to consider:
  • Since Table 2 lists the maximum ampacity, is that for the load (which would be 80% of the breaker), or is the maximum draw based on the breaker? From a safety perspective, I would not want the cable to ever carry more load than the breaker trips at (as it could melt before the breaker trips). As such, I would interpret the maximum ampacity of the cable as the maximum breaker size for the circuit the cable is used in.
  • Since this is a high-current, continuous load circuit, I would tend to lean towards the 60 or 75 degree C rating, even though NMD90 cable has a 90 degree C insulation rating. The maximums in Table for are based on an ambient temperature of 30 degrees C, and while most of the cable will usually be below that, the part in the garage could certainly get hotter on hot summer days, and thus the characteristics change. As such, I would err towards the 60 or 75 degree rating. As such, that would suggest a maximum ampacity of 40-50 A for 8 AWG. As with any material, it's engineered to a certain specification, but sometimes it's always best to leave safety margin.
  • I know that most of the electricians I have seen quote or comment on running a 100A sub panel want to use 3 AWG cable -- which at 60 degrees C is only permitted to carry 85 A -- which means they must be using the 90 degree C rating, which is 115 A for 3 AWG (75 degree C rating is 100A).
  • Depending on the length of your cable, you may experience voltage drop. For longer lengths, you need thicker cables to reduce the voltage drops. I haven't done the digging for how that affects thermal, but digging back to university physics, I would suggest the more current drop, the more heat dissipated in the cable, thus more temperature increase in the cable. Running really long cable (80-100 feet) on the limit might be bad idea. Also, if your cable is (incorrectly, and not code compliant) packed next to lots of other cables, you might also have thermal issues.
Anyway, those are the bits of info I have, and my thoughts on it. Hopefully that helps a little.
 
FWIW, my electrician installed the 240 volt 14-50 box in the garage using 8AWG cable (about 20' length) two years ago. He installed a 40A breaker on the panel as he was adamant that a 50A breaker would need the heavier 6AWG cable according to code at that time, which may be different now. He also said that the max current on the 40A breaker should be 32A. I'm happy with that, which will allow me to fully charge the Model 3 overnight with the UMC, and maybe upgrade to hard-wired HPWC shortly.
 
WARNING: I'm not an expert or a licensed electrician.

TLDR: I think you are technically okay based on code and material ratings so long as your total cable length is less than 80-100 feet.

Detailed version:

So I could be wrong, but here's how I read it:
  • Your cable is 3+1 conductor of 8 AWG copper in an NMD90 cable sheath; NMD90 cable has a 90 degree C temp rating for the insulation.
  • Table 2 of the CEC 2015, says that the maximum allowable ampacity of AWG 8 cable with a 90 degree C rating is 55 Amps (note that the 60 degree C rating is 40 Amps, and the 75 degree C rating is 50 A). Note this table is also for a maximum of 3 copper conductors in a cable or a raceway (you have a "cable").
  • The maximum permitted load you can put on a 50 A breaker is 40 A (80%).
Here are the parts I am not clear on (with respect to complying with code), and my thoughts on them, as well as other factors to consider:
  • Since Table 2 lists the maximum ampacity, is that for the load (which would be 80% of the breaker), or is the maximum draw based on the breaker? From a safety perspective, I would not want the cable to ever carry more load than the breaker trips at (as it could melt before the breaker trips). As such, I would interpret the maximum ampacity of the cable as the maximum breaker size for the circuit the cable is used in.
  • Since this is a high-current, continuous load circuit, I would tend to lean towards the 60 or 75 degree C rating, even though NMD90 cable has a 90 degree C insulation rating. The maximums in Table for are based on an ambient temperature of 30 degrees C, and while most of the cable will usually be below that, the part in the garage could certainly get hotter on hot summer days, and thus the characteristics change. As such, I would err towards the 60 or 75 degree rating. As such, that would suggest a maximum ampacity of 40-50 A for 8 AWG. As with any material, it's engineered to a certain specification, but sometimes it's always best to leave safety margin.
  • I know that most of the electricians I have seen quote or comment on running a 100A sub panel want to use 3 AWG cable -- which at 60 degrees C is only permitted to carry 85 A -- which means they must be using the 90 degree C rating, which is 115 A for 3 AWG (75 degree C rating is 100A).
  • Depending on the length of your cable, you may experience voltage drop. For longer lengths, you need thicker cables to reduce the voltage drops. I haven't done the digging for how that affects thermal, but digging back to university physics, I would suggest the more current drop, the more heat dissipated in the cable, thus more temperature increase in the cable. Running really long cable (80-100 feet) on the limit might be bad idea. Also, if your cable is (incorrectly, and not code compliant) packed next to lots of other cables, you might also have thermal issues.
Anyway, those are the bits of info I have, and my thoughts on it. Hopefully that helps a little.
So if the wire is rated at 45amps (which all my research seems to indicate), and I have it on the 50amp breaker, but only pulling 40amps, then I should be fine?
 
  • Disagree
Reactions: M3BlueGeorgia
So if the wire is rated at 45amps (which all my research seems to indicate), and I have it on the 50amp breaker, but only pulling 40amps, then I should be fine?
No, the breaker cannot exceed any component in the circuit, so if the wire is 45 A, the breaker must not be higher than that. And for safety margin, the wiring should exceed the breaker. Even if the load is lower, the circuit must comply.

I would strongly suggest not going with more than 40A breaker for the circuit.
 
No, the breaker cannot exceed any component in the circuit, so if the wire is 45 A, the breaker must not be higher than that. And for safety margin, the wiring should exceed the breaker. Even if the load is lower, the circuit must comply.

I would strongly suggest not going with more than 40A breaker for the circuit.
What happens when I use the Mobile Connector that will try to draw 40amps?
 
Ok, I am an electrician, in Ontario.

Breaker can only be loaded to 80%, but the frame size of the breaker is actually a 100 amp. Loading the breaker to a full 40 amp is ok.

Wire: listed in table 2 and 4 is 100% value and wire can be loaded up to 100%. Never overload the wire.

Plug: 14-50 is rated for 50 amps but the 80% rule applies so 40 amps maximum.
 
Ok, I am an electrician, in Ontario.

Breaker can only be loaded to 80%, but the frame size of the breaker is actually a 100 amp. Loading the breaker to a full 40 amp is ok.

Wire: listed in table 2 and 4 is 100% value and wire can be loaded up to 100%. Never overload the wire.

Plug: 14-50 is rated for 50 amps but the 80% rule applies so 40 amps maximum.
I think more clarification might be required (at least for me) around the breaker loading and frame size. If you wouldn't mind...
 
Not an electrician but recently had Nema 14-50 outlet installed by a certified electrician with 6AWG wire and 50A breaker. 8AWG wire is too thin for 50A and the 50A breaker will not be able to protect you if the 8 AWG wire burn. Whoever installed it has to change the breaker for safety reason.
That's the thing, the AWG 8/3 is rated for different temperatures (this one being rated at 90C), and it would never reach 50amps in my case as the mobile connector can only pull a maximum of 40amps (which is also 80% of the 50amp breaker).
 
That's the thing, the AWG 8/3 is rated for different temperatures (this one being rated at 90C), and it would never reach 50amps in my case as the mobile connector can only pull a maximum of 40amps (which is also 80% of the 50amp breaker).

So you are going to put a label there to make sure only the mobile connector can be used on that outlet? Some year later, you may sell the house and the new owner may not know this and cause a fire. The 50A breaker doesn't do your any good period.
 
  • Like
Reactions: akidesir
So you are going to put a label there to make sure only the mobile connector can be used on that outlet? Some year later, you may sell the house and the new owner may not know this and cause a fire. The 50A breaker doesn't do your any good period.
Hence why I am here, asking for help, as table 2 of CEC says that my 8/3 90C cable has an allowed 55 ampacity.

upload_2018-5-23_11-4-19.png


While I understand the basic "AWG 8 -> 40AMPS, AWG6 -> 50AMPS" it clearly isn't that simple, and other factors should be considered, like the temperature rating, length, etc.

I also am not an electrician, so that's why I was hoping an electrician on here could help since I have seen similar questions answered before.
 
I wired my HPWC myself and used 6 gauge 3 wire plus ground and it's about 30 feet from the breaker box.(my local box store only had 6/3 in stock). Used 60amp breaker. The model 3 charges at 48Amp which is the highest it will charge at from the Wall charger. I have no issues with the 6 gauge wire. I capped off the third wire for future use.
 

rypalmer

Active Member
Aug 22, 2014
1,537
1,783
Canada
I'm assuming you are asking about the Mobile Connector? Got it from someone who received it with their 2014 Model S.
No - specifically the part that plugs into the 14-50 outlet. The swappable adaptor. There are at least two versions - an early version that supported 40a, and a revised version that can only do 32. Tesla made a thing about not allowing the 40A models in Canada.

Are you absolutely certain yours will do 40a even?
 
No - specifically the part that plugs into the 14-50 outlet. The swappable adaptor. There are at least two versions - an early version that supported 40a, and a revised version that can only do 32. Tesla made a thing about not allowing the 40A models in Canada.

Are you absolutely certain yours will do 40a even?
99.9% sure, since this is the original adapter that came with the 2014 Model S. At that time Tesla only had the 40amp ones. I believe it was in 2016 that they came out with the 32amp chargers and adapters.
 

Products we're discussing on TMC...

About Us

Formed in 2006, Tesla Motors Club (TMC) was the first independent online Tesla community. Today it remains the largest and most dynamic community of Tesla enthusiasts. Learn more.

Do you value your experience at TMC? Consider becoming a Supporting Member of Tesla Motors Club. As a thank you for your contribution, you'll get nearly no ads in the Community and Groups sections. Additional perks are available depending on the level of contribution. Please visit the Account Upgrades page for more details.


SUPPORT TMC
Top