9 ms·

I believe Tesla has 48A and 72A chargers. An 80A circuit (wired and fused accordingly) can sustain this, since the wiring standards have some overhead in them f

I believe Tesla has 48A and 72A chargers. An 80A circuit (wired and fused accordingly) can sustain this, since the wiring standards have some overhead in them for this.

Waterluvian 9y ago
That sounds like trying to needlessly get away with the minimum.
- kurthr 9y ago
  It seems important to distinguish between wire rating (guage, junction, and length) vs fuse rating. Typically, you would fuse well below the wire rating (at least 25%) so unless you're worried about the breaker box flipping, ruining at 90% of the fuse rating is reasonable (if it is a stable 'non-inductive' load). Besides, Tesla (non-super) chargers are typically limited by the vehicle.
  - amluto 9y ago
    Please read the electric code before you touch or specify any wiring. 90% is neither reasonable nor legal, and non-inductive has little to do with it.
    - kurthr 9y ago
      I'm not modifying wiring. I am not an elecrrician. I am a homeowner. I have read California/County/City codes, and low fuse rating has little to do with safety. You can put a 10A breaker on a 100A #3 guage circuit less than 50ft long and it won't magically become a hazard at 7.5A, however you shouldn't put larger than a 80A breaker on it for safety. If someone is installing a 100A (or even 80A) breaker on #5 AWG, then they have a problem, and are likely violating code because the breaker won't protect them. That is too high a fuse rating and is unsafe. How would a normal homeowner even know if they were approaching 75% of their breaker rating, unless the breaker tripped at <75% of its stated limit? If someone has had 100A service installed with 100A breakers and it's not wired for at least 125A with #2, then they have been defrauded by their electrician and contracor. If you think that inductive power factor doesn't matter for power efficiency, rating, and safety then I also recommend that you continue your education. I can't imagine a licensed electrician saying that.
      - amluto 9y ago
        You said: > unless you're worried about the breaker box flipping, ruining at 90% of the fuse rating is reasonable (if it is a stable 'non-inductive' load). A continuous load (usually) requires a breaker rated at 125% of the load, as calculated by the electrical load. A 90A continuous load on a normal 100A breaker is not okay, full stop. It doesn’t matter if that load is literally a giant resistor or is some real-world device. > If someone has had 100A service installed with 100A breakers and it's not wired for at least 125A with #2, then they have been defrauded by their electrician and contracor. In the infamous words of Wikipedia, citation needed. In standard US residential usage, a “200A service” is 200A for non-continuous loads and needs to be wired for 200 Amps. Same for 100A, etc. The only common exception I’ve heard of is “class 320” service, which is genuinely rated for 320A continuously, and that’ll have 400A wiring and perhaps a 400A main breaker or a pair of 200A breakers. The latter seems fairly common, since you can buy the hardware at Home Depot.
        kurthr 9y ago
        Great, I'm glad for this new information. So when I go out to look at my breaker box which is billed as 200A (or 400 which they will at least tell me is really 320) service and it has two 100A (or 200A) breakers for a house/garage full of loads, I just hope it doesn't burn down? As a home owner how will I ever discover this in a non-catastrophic way? Will I put an ammeter clamp on at the breaker box with an alarm on it? Will I know the load of everything that is plugged into that service? The breaker won't blow, but the building will burn down... and it's not the fault of the electrician who installed it and code enforcement says it's fine? As examples of non-continuous loads that are likely to be continuous for 8 hours or more: a double oven during Thanksgiving, halogen flood lamps at night, or an electric space heater during winter. Oops, you burned your house down for Christmas. (edit) Discussing this with an electrician, he says that in fact the breaker will flip after extended/continuous load near the breaker limit. It would not be considered a hazard, because the home owner would experience the breaker flip each time before there was a problem. He also said that it's the difference between running at 75 vs 100% of rated current is about 15C of increased temp on the wires and also not a concern for copper.
    - Filligree 9y ago
      On an unrelated note... Given the skin effect, the ampere rating of an AC cable should be the square root of the material used, more or less, but that's per cable. Obviously you can't get around that by braiding cables, but does anyone ever try to save on material by running multiple independent cables? Or would that end up being more expensive overall?
icelancer 9y ago
80A circuit for 72A continuous load is absolutely in violation of code.
- tgtweak 9y ago
  This is correct, actually the reason the rating on the charger is 72A is to fit into a 90A circuit with 25% overhead for continuous loads.
ajuc 9y ago
> since the wiring standards have some overhead in them for this. And there's a good reason they do.
hinkley 9y ago
For those playing along at home, a 64 amp load requires an 80 amp circuit to meet the 125% rule. 72 amp loads would require 90 amp circuits.

I believe Tesla has 48A and 72A chargers. An 80A circuit (wired and fused accordingly) can sustain this, since the wiring standards have some overhead in them for this.

Waterluvian 9y ago

That sounds like trying to needlessly get away with the minimum.

kurthr 9y ago
It seems important to distinguish between wire rating (guage, junction, and length) vs fuse rating. Typically, you would fuse well below the wire rating (at least 25%) so unless you're worried about the breaker box flipping, ruining at 90% of the fuse rating is reasonable (if it is a stable 'non-inductive' load). Besides, Tesla (non-super) chargers are typically limited by the vehicle.
- amluto 9y ago
  Please read the electric code before you touch or specify any wiring. 90% is neither reasonable nor legal, and non-inductive has little to do with it.
  - kurthr 9y ago
    I'm not modifying wiring. I am not an elecrrician. I am a homeowner. I have read California/County/City codes, and low fuse rating has little to do with safety. You can put a 10A breaker on a 100A #3 guage circuit less than 50ft long and it won't magically become a hazard at 7.5A, however you shouldn't put larger than a 80A breaker on it for safety. If someone is installing a 100A (or even 80A) breaker on #5 AWG, then they have a problem, and are likely violating code because the breaker won't protect them. That is too high a fuse rating and is unsafe. How would a normal homeowner even know if they were approaching 75% of their breaker rating, unless the breaker tripped at <75% of its stated limit? If someone has had 100A service installed with 100A breakers and it's not wired for at least 125A with #2, then they have been defrauded by their electrician and contracor. If you think that inductive power factor doesn't matter for power efficiency, rating, and safety then I also recommend that you continue your education. I can't imagine a licensed electrician saying that.
    - amluto 9y ago
      You said: > unless you're worried about the breaker box flipping, ruining at 90% of the fuse rating is reasonable (if it is a stable 'non-inductive' load). A continuous load (usually) requires a breaker rated at 125% of the load, as calculated by the electrical load. A 90A continuous load on a normal 100A breaker is not okay, full stop. It doesn’t matter if that load is literally a giant resistor or is some real-world device. > If someone has had 100A service installed with 100A breakers and it's not wired for at least 125A with #2, then they have been defrauded by their electrician and contracor. In the infamous words of Wikipedia, citation needed. In standard US residential usage, a “200A service” is 200A for non-continuous loads and needs to be wired for 200 Amps. Same for 100A, etc. The only common exception I’ve heard of is “class 320” service, which is genuinely rated for 320A continuously, and that’ll have 400A wiring and perhaps a 400A main breaker or a pair of 200A breakers. The latter seems fairly common, since you can buy the hardware at Home Depot.
      - kurthr 9y ago
        Great, I'm glad for this new information. So when I go out to look at my breaker box which is billed as 200A (or 400 which they will at least tell me is really 320) service and it has two 100A (or 200A) breakers for a house/garage full of loads, I just hope it doesn't burn down? As a home owner how will I ever discover this in a non-catastrophic way? Will I put an ammeter clamp on at the breaker box with an alarm on it? Will I know the load of everything that is plugged into that service? The breaker won't blow, but the building will burn down... and it's not the fault of the electrician who installed it and code enforcement says it's fine? As examples of non-continuous loads that are likely to be continuous for 8 hours or more: a double oven during Thanksgiving, halogen flood lamps at night, or an electric space heater during winter. Oops, you burned your house down for Christmas. (edit) Discussing this with an electrician, he says that in fact the breaker will flip after extended/continuous load near the breaker limit. It would not be considered a hazard, because the home owner would experience the breaker flip each time before there was a problem. He also said that it's the difference between running at 75 vs 100% of rated current is about 15C of increased temp on the wires and also not a concern for copper.
  - Filligree 9y ago
    On an unrelated note... Given the skin effect, the ampere rating of an AC cable should be the square root of the material used, more or less, but that's per cable. Obviously you can't get around that by braiding cables, but does anyone ever try to save on material by running multiple independent cables? Or would that end up being more expensive overall?

icelancer 9y ago

80A circuit for 72A continuous load is absolutely in violation of code.

tgtweak 9y ago
This is correct, actually the reason the rating on the charger is 72A is to fit into a 90A circuit with 25% overhead for continuous loads.

ajuc 9y ago

> since the wiring standards have some overhead in them for this. And there's a good reason they do.

hinkley 9y ago

For those playing along at home, a 64 amp load requires an 80 amp circuit to meet the 125% rule. 72 amp loads would require 90 amp circuits.