HPWC instructions different than NEC rules?

[NeedToDrive]

Does anybody know whats going on with the HPWC installation instructions? I've got an installation with a short offset conduit directly attached to a surface mount main panel. I elected to use THHN #6 AWG wire connected to a dedicated breaker (GE THQL type). Only three wires are in the 3" conduit length, each with a length less than 36" from the HPWC junction. Since this is all outside in southern California the ambient temperature is certainly going to be less than 40 deg C. Therefore, I don't believe I need to derate for temperature or bundled cables. Based on this, my code check guide suggests 75A is achievable. Even if I decided to derate for the fuse connection (something I don't think should be necessary up to the fuse limit) or a more typical wiring, this installation should allow me 60A. The other variable is perhaps adjusting for continuous operation but I have not considered that.

However the HPWC installation guide doesn't seem to follow these standard NEC rules. It is has a table of maximum output current vs circuit breaker size. That's odd. I would think the installation instruction should be geared to wire size instead of fuse size. Why shouldn't that be the case? In any case, this table shows a 60A fuse limited to 48A, a 70A fuse limited to 56A, a 80A fuse limited to 64A and to get 80A one would need a 100A fuse.

Let's say I want to get a charge at 64A. The above suggests I need a 80A fuse which in turn would require #3 AWG wire. From NEC tables #3 AWG wire is rated to 110A max. This seems to be highly over designed and doesn't make sense to me.

As another example, consider an existing installation where one has #8 THHN AWG wire attached to a 40A fuse (the wire is rated to 55A but derated to 40A). If I attach a NEMA 14-50 receptacle to this I'd be free to draw up to 40 A (e.g. via a mobile connector). However if I followed similar rules to the HPWC installation I'd be limited to 32A. This would suggest the HPWC is not as sturdy as the mobile connector. However, we know the HPWC is actually designed for use up to 80A!

 

[brkaus]

I believe it's due to being a continuous load, so needs to to be sized 25% higher.

This is the starting point for all things charging related -

FAQ: Home Tesla charging infrastructure Q&A

 

[TexasEV]

You don't use a 40A circuit breaker for a NEMA 14-50, you use a 50A breaker. See Tesla's instructions:
https://www.tesla.com/sites/default/files/downloads/US/universalmobileconnector_nema_14-50.pdf

The circuit is not overdesigned as you claim. It's sized at 125% load for continuous draw. See the above mentioned FAQ or search the hundreds of posts on this topic.

 

[davewill]

... As another example, consider an existing installation where one has #8 THHN AWG wire attached to a 40A fuse (the wire is rated to 55A but derated to 40A). If I attach a NEMA 14-50 receptacle to this I'd be free to draw up to 40 A (e.g. via a mobile connector). ...

Incorrect. you can only charge up to 32a on this connection. I have this exact circuit at home, and my EVSE is limited to 30a (the cord and plug are only rated for 30a, if they were rated >= 32a, I could go that high). It's a problem using this kind of circuit with a UMC as there is no adapter that will limit it to 32a (except for the Canadian 14-50 adapter). As explained by others, these are the NEC rules for continuous loads.

 

[NeedToDrive]

Incorrect. you can only charge up to 32a on this connection. I have this exact circuit at home, and my EVSE is limited to 30a ...

I don't know where you got your electrical training but you are wrong. There is a reason for the 40A fuse. Your situation is limited by the appliance you choose to plug in not the receptacle rating. The circuit is capable of drawing 40A for the one appliance in this example as long as there are no other loads that would combine to exceed the 40A breaker rating (assuming the breaker is of good quality). Note that I specifically constructed the example with #8 wire that is derated per NEC sizing rules.

 

[NeedToDrive]

I believe it's due to being a continuous load, so needs to to be sized 25% higher.
This is the starting point for all things charging related -
FAQ: Home Tesla charging infrastructure Q&A

Thanks, I forgot about FlasherZ's FAQ. Actually, I was hoping to get his take on the situation -- always appreciate his insight. Although I spent a couple of hours searching for related information I didn't find a clear answer.

OK, so section 625.40/625.41 states that vehicle charging loads are always to be considered continuous. Therefore #6 AWG (75A max rating) ought to be limited to 60A, again assuming I don't need to derate for temperature or bundled cables. That is, if I had any other appliance attached I'd be OK to draw 60A and I'd pass an inspection for the circuit. The bottom line is that the Tesla HPWC that is the limitation in this scenario. I realize the NEC code requires following a manufactures instruction regarding installation. I'm not asking how to follow the Tesla instruction, I'm asking why Tesla is doing what they are doing. According to the Tesla table, a 60A breaker should be limited to 48A maximum current draw. Essentially one is told to dial back the HPWC capability based on breaker size. I'm trying to understand why this is the case. What would be the difference between running the HPWC at 56A (the next step up) vs maintaining the recommended 48A max and adding another appliance on the same circuit that draws an additional 8A? From the circuit breaker and wire perspective I'd think the load looks the same!

NEC has been around for a long time and one of it's nice features is that it protects uniques situations -- it recognizes all installations are not the same. This allows cost effective installations across the country while requiring more where it is needed. From what I can tell Tesla's table is base on a worst case scenario where you have all possible negatives associated with wiring -- whether is actually present or not. Perhaps I'm missing something subtle about what Tesla is doing, but it just does not make sense to me.

 

[davewill]

I don't know where you got your electrical training but you are wrong. There is a reason for the 40A fuse. Your situation is limited by the appliance you choose to plug in not the receptacle rating. The circuit is capable of drawing 40A for the one appliance in this example as long as there are no other loads that would combine to exceed the 40A breaker rating (assuming the breaker is of good quality). Note that I specifically constructed the example with #8 wire that is derated per NEC sizing rules.

All I can say is that you're wrong, wrong, wrong...as several people here can tell you. You must have a 50a circuit with a 50a breaker/fuse in order to charge at 40a per NEC rules for continuous loads.

 

[brkaus]

According to the Tesla table, a 60A breaker should be limited to 48A maximum current draw. Essentially one is told to dial back the HPWC capability based on breaker size. I'm trying to understand why this is the case. What would be the difference between running the HPWC at 56A (the next step up) vs maintaining the recommended 48A max and adding another appliance on the same circuit that draws an additional 8A? From the circuit breaker and wire perspective I'd think the load looks the same!

48A at 125% due to continuous load is 60A. Therefore, the 60A breaker is fully consumed.

 

[coolgj]

All I can say is that you're wrong, wrong, wrong...as several people here can tell you. You must have a 50a circuit with a 50a breaker/fuse in order to charge at 40a per NEC rules for continuous loads.

I think he is confusing derailing with continuous load. When they are two different scenarios.

 

[coolgj]

Thanks, I forgot about FlasherZ's FAQ. Actually, I was hoping to get his take on the situation -- always appreciate his insight. Although I spent a couple of hours searching for related information I didn't find a clear answer.

OK, so section 625.40/625.41 states that vehicle charging loads are always to be considered continuous. Therefore #6 AWG (75A max rating) ought to be limited to 60A, again assuming I don't need to derate for temperature or bundled cables. That is, if I had any other appliance attached I'd be OK to draw 60A and I'd pass an inspection for the circuit. The bottom line is that the Tesla HPWC that is the limitation in this scenario. I realize the NEC code requires following a manufactures instruction regarding installation. I'm not asking how to follow the Tesla instruction, I'm asking why Tesla is doing what they are doing. According to the Tesla table, a 60A breaker should be limited to 48A maximum current draw. Essentially one is told to dial back the HPWC capability based on breaker size. I'm trying to understand why this is the case. What would be the difference between running the HPWC at 56A (the next step up) vs maintaining the recommended 48A max and adding another appliance on the same circuit that draws an additional 8A? From the circuit breaker and wire perspective I'd think the load looks the same!

NEC has been around for a long time and one of it's nice features is that it protects uniques situations -- it recognizes all installations are not the same. This allows cost effective installations across the country while requiring more where it is needed. From what I can tell Tesla's table is base on a worst case scenario where you have all possible negatives associated with wiring -- whether is actually present or not. Perhaps I'm missing something subtle about what Tesla is doing, but it just does not make sense to me.

Ok I think I understand your confusion. Yes you are correct you have to dial it back to match what the charger in the car is capable of handling. It doesn't handle 60amps (75AMP breaker) but 64 AMPS (80amps Breaker). The charger in the car can only handle
output: 80A, 72A, 64A, 56A, 48A, 40A, 36A, 32A, 28A, 24A, 20A, 16A, 12A.

BTW you are looking at the wrong NEC rule. You need to be looking at the rule for dedicated circuit. For a dedicated circuit you are to size/match what the manufacture states the breaker size should be. It is the same rule that applies to AC units, ovens, etc...

 

[NeedToDrive]

All I can say is that you're wrong, wrong, wrong...as several people here can tell you.

No need to be abusive. I did not state it was a continuous load. In hindsight, I should not of added a parenthetical for an example type of load. My point was that, in the scenario I created, the NEMA 14-50 receptacle installation would be approved by an inspector and could be used up to 40A. A better response would be to have noted that, yes, I could draw 40A if the load was not continuous and then point out that section 625.40/625.41 states that vehicle charging loads are always to be considered continuous.

 

[NeedToDrive]

...You need to be looking at the rule for dedicated circuit. For a dedicated circuit you are to size/match what the manufacture states the breaker size should be. It is the same rule that applies to AC units, ovens, etc...

Thanks. This pointed me in the right direction.

What is so bizarre about the whole thing is that the continuous load is a condition the installer may not know about. If I walk up to a NEMA 14-50 receptacle at some random location it may or may not be connected to a 50A breaker. To plug in my mobile connector for a charge I'd have to know it had #6 wire and a 50A breaker -- something an average person would't know to look for. In fact, I'd bet there are many out there with 40A breaker and #8 wire. More frighting, for a standard home with a 20A breaker and #12 wire this says you can't plug in a continuous load. I'd bet there are countess examples where someone is pulling 20A continuously right now.

Before this discussion, I've always viewed a breakers role as protecting wire. And I alway relied on derating rules to determine wire size for desired current. It intuitvly makes a lot of sense to consider bundling or high ambient temperatures in installation since heat and heat dissipation is critical for a wire. I really hadn't given much thought to continuous loads since it's not typically considered with standard home wiring. I guess deep down I expected that a wires rating (e.g. THHN) was based on continuous loads and that the breaker matched to wire was sufficient.

In any case this all turned out to be moot for me. Since my car is 4 years old, it can only charge at 40A max. so I removed the HPWC, returned it, and installed a NEMA 14-50 receptacle instead. I left in the #6 wire so it is sized for continuous 40A loads. I saved $550 and now just plug in my mobile connector.

 

[clyjr]

I'm not an electrician, but the 50 in 14-50 is for the amp rating. Pretty sure if someone wired a 14-50 receptacle to a 40 amp breaker, then they screwed up and broke the rules...

 

[DarkMatter]

More frighting, for a standard home with a 20A breaker and #12 wire this says you can't plug in a continuous load. I'd bet there are countess examples where someone is pulling 20A continuously right now.

#12 wire is the correct size for a 20A circuit, and if you plug in to that with the 5-20 adapter you'll get the car charging at 16A, which is correct and meets code.

It's true that you can't depend on random 14-50s to be overcurrent protected at 50A. If they are they must be correctly wired with #6 wire to meet code. If you don't know the size of the overcurrent protection device it is safer to assume 32A maximum on the outlet.

 

[tga]

I'm not an electrician, but the 50 in 14-50 is for the amp rating. Pretty sure if someone wired a 14-50 receptacle to a 40 amp breaker, then they screwed up and broke the rules...

Actually, it is legal/NEC compliant to install a 14-50 on a 40A circuit (ie, 8ga and 40A breaker) if, and only if, you are plugging in a device with a nameplate rating of 40A (ie, the manufacturer says it's OK), since there is no 40A outlet (you can't install a 14-50 on a 30A breaker, since there is a 14-30 outlet). Many ranges only require 40A, so you often see builders cheap out nowadays and install 14-50 range outlets with 8ga and a 40A breaker. You can't use the UMC @ 50A on these outlets, because the manufacturer doesn't allow a connection to a 40A circuit (it's a 50A rated appliance)

My house (built '72) has a 10-50 range outlet, on a 50A breaker, wired with 6ga Al SE cable. That wire is rated for 50A (intermittent), but at 75C - I think the inspector now requires new installations to use the 60C column for all new NM-B and SE branch circuit/subpanel installs.

More frighting, for a standard home with a 20A breaker and #12 wire this says you can't plug in a continuous load. I'd bet there are countess examples where someone is pulling 20A continuously right now.

I'd doubt that (bolded assertion) - I can't think of any true continuous loads in residential circumstances (other than car charging). Heating elements cycle on and off during use.

 

[davewill]

No need to be abusive. I did not state it was a continuous load. In hindsight, I should not of added a parenthetical for an example type of load. My point was that, in the scenario I created, the NEMA 14-50 receptacle installation would be approved by an inspector and could be used up to 40A. A better response would be to have noted that, yes, I could draw 40A if the load was not continuous and then point out that section 625.40/625.41 states that vehicle charging loads are always to be considered continuous.

I only matched your own rhetoric to me. You were the one coming on here and stubbornly pitting your knowledge against years of posts, research, and manufacturer published information.

I don't know where you got your electrical training but you are wrong. ...

 

[Rocky_H]

More frighting, for a standard home with a 20A breaker and #12 wire this says you can't plug in a continuous load. I'd bet there are countess examples where someone is pulling 20A continuously right now.

No, this is exactly why all of the Tesla adapters for the UMC, and the Tesla wall connectors, and every J1772 AC charging unit take this derating factor into account. The Tesla 5-15 adapter will only pull 12A max. The 5-20 will only pull 16A. The 14-30 will only pull 24A. This is always accounted for, and it is exactly because of this U.S. NEC rule. They rate the wire for intermittent use of the maximum circuit rating. Continuous loads must build in that margin. However, I have seen mentioned that most of Europe does their electrical code standards differently. They do specify wire ratings for assuming continuous use at the full breaker rating so that is allowed.

 

[TexasEV]

More frighting, for a standard home with a 20A breaker and #12 wire this says you can't plug in a continuous load. I'd bet there are countess examples where someone is pulling 20A continuously right now.

How would they do that? The UMC 5-20 adapter draws 16A for that reason.

 

[clyjr]

Good thing I'm not an electrician, lol
Seems dumb to me that they allow the larger receptacle, since as mentioned some people would assume that it's a 50A circuit. Guess you just have to rely on the wiring matching the breaker, and the breaker will trip before the wire overheats?

Actually, it is legal/NEC compliant to install a 14-50 on a 40A circuit (ie, 8ga and 40A breaker) if, and only if, you are plugging in a device with a nameplate rating of 40A (ie, the manufacturer says it's OK), since there is no 40A outlet (you can't install a 14-50 on a 30A breaker, since there is a 14-30 outlet). Many ranges only require 40A, so you often see builders cheap out nowadays and install 14-50 range outlets with 8ga and a 40A breaker. You can't use the UMC @ 50A on these outlets, because the manufacturer doesn't allow a connection to a 40A circuit (it's a 50A rated appliance)

My house (built '72) has a 10-50 range outlet, on a 50A breaker, wired with 6ga Al SE cable. That wire is rated for 50A (intermittent), but at 75C - I think the inspector now requires new installations to use the 60C column for all new NM-B and SE branch circuit/subpanel installs.


I'd doubt that (bolded assertion) - I can't think of any true continuous loads in residential circumstances (other than car charging). Heating elements cycle on and off during use.

 

[SomeJoe7777]

Good thing I'm not an electrician, lol
Seems dumb to me that they allow the larger receptacle, since as mentioned some people would assume that it's a 50A circuit. Guess you just have to rely on the wiring matching the breaker, and the breaker will trip before the wire overheats?

This is done in residential homes usually only for the range in the kitchen. This specific instance (50A outlet on 40A breaker with 8 ga wire) is allowed because:

1. There is no such thing as a 40A outlet. Available NEMA outlets jump from 30A to 50A.
2. 8 ga wire is rated for 40A, and breaker is 40A, thus protecting the wire.
3. The appliance nameplate rating is 40A, so it will not attempt to draw more current than that.
4. The appliance is permanently installed, so it's highly unlikely someone would plug a different device into the 50A outlet.
5. The appliance is not classified as a "continuous load", so it's not subject to the 80% rating rule.

If any of those things are not true, then you cannot use the 50A outlet on a 40A breaker.