hmmm - as I always suspected - the grid can handle EV's

David Bennett

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Interesting. My breaker is 40A. The man who did the survey in advance of fitting told me that sometimes the current can go up as high as 38A as the car comes off charge or at some point of the charge process. I didn’t really understand why that would be and if your 32A circuit has never tripped then maybe he was wrong. But 32A is a pretty fine margin on a 7.1kW EVSE
There are circumstances where a 40A for a 7KW EVSE would be right for the UK. Typically this would be if you where you had an additional circuit installed in an already well populated consumer unit. The combined heating effect would lead to the kind of de rating we were discussing earlier (it's in the standards and the wiring regs). If you are installing an additional feed direct from the meter serving only the ESVE then no derating is required (unless the ambient temperature is very high) and you will only need 32A for a 7KW ESVE. I don't see how a 7KW load will draw 38A unless I am missing something. If it was a very short term event (few minutes) it still won't trip the breaker but you don't want to be relying on that.

 
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daveo4EV

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It's interesting how local regulations / habits can impact how equipment is installed. All breakers are designed to carry their rated current on a permanent basis so this de rating to 80% isn't really required. Perhaps the fact that the 50A normally costs more than the 40A so the installer can make a bit more is also at play?
I think the breakers are fine and can handle the continuous the load - but the accumulated "heat" in the wire for "continuous use" is what is being managed with the 80% rule - and the "rule" is also probably conservative for safety - if you are running XX amps - there is an associated "minimum capacity" gauge wire that will be expected to be installed

so if you have…
  • a breaker of XX amp size
  • the minimum "spec" wire gauge associated with that XX breaker size
  • an electrical device "pulling" _MAX_ XX amps
  • pulling XX amps _CONTINUOUSLY_
the question becomes…

are we comfortable with the amount of accumulated "heat/energy" that will be deposited into the wire during the extended usage period?

because what the break is protecting _IS_ the wire - and in particular the breaker and wire-gauge rating supposedly guarantees the wire will not fail/overheat/melt/short/fire…
  • wire length is also a factor - longer runs are more resistance which is more heat
  • voltage variations are a factor
  • and wire composition/materials are a factor (copper/aluminum/alloy) - stranded or solid - insulation type
  • ambient temperatures are a factor
  • age of materials is a factor
  • quality of installation is a factor
  • actual draw/behavior of the electrical device in question is a factor
  • other potential "loads" on the circuit at the same time are a factor
    • you're EVSE been's cooking along for 6 hours at max XX amps (not 80% reduced load) and boom your electric dryer comes online - the wire is already "close to max heat" - so now it WILL overheat if your breaker doesn't kick in quickly…
  • building inspector quality/attention/integrity to detail is a factor (sometime paid to look the other way)
    • also sometimes not everyone interprets laws/codes correctly
  • there are humans in this equation so there are unforeseeable variables in the equation
So I'm betting the North American building code is conservative with their 80% rule for continous use and they are simply building in a healthy engineering margin to cover the wide wide wide range of potential on-site-installation variables. I know for a fact that not all 120V/15 amp circuits are equal - some older homes have problems with even low loads of 120V @ 10 amps on some of the really crappy install choices made by builders for these homes…

I'm willing to bet for a proper/well-done/excellent installation with proper materials you could run at 100% breaker rating for more than 3 hours with no actual failures (melting insulation leading to shorts leading to fires) - but that means you believe everything involved in that circuit has to be "right on spec"…and at least in North America from what i've seen - what your builder decides to do behind the walls of your building/home is sometime questionable…

I think the building code in this case is a case of "healthy" safety margin to cover the wide number of variables which are harder to control so that even the "worse-case" install meeting the minimum requirements and done poorly to boot still shouldn't fail when running for 3+ hours…

but all this is simply speculation on my part.
 
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daveo4EV

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you need look NO further than Porsche's own PMC+/PMCC with their NEMA 14-50/6-50 supply cables.

It's well documented that the North American PMC+/PMCC supply cables become uncomfortably "hot/warm" during normal usage even in modest ambient temperature conditions.

The root cause of this problem is a design/manufacturing choice/specification by VW/Audi/Porsche to only use 10-gauge wire in the manufacturing of this 12" long supply cable…

10-gauge wire _IS_ electrically safe and rated to handled 40 amp loads along with the insulation choices Porsche has made in the design/specification/manufacturing process - it will never fail nor reach the 90C rated thermal limits in any circumstances for nearly any usage scenario.

this supply cable will most likely _NEVER_ fail in normal use and there is no _ACTUAL_ problem…

but it can reach surface temperatures of 162F in normal use - which some people (myself included) find to be an unexpected result and in some conditions can be even worse for surface termperature causing some customers to consider it "too hot"

even in the worse case the 14-50/6-50 supply cables will not reach the 90C temperature rating of the cable's insulation (for which there is also a healthy engineering safety margin) - so it won't fail.

but people are conditioned to believe "heat is bad" and most of their other appliances they are familiar with never get that hot even after continuous use…

the behavior/temperature profile of the PMC+/PMCC is not meeting peoples expectations of a well designed and well behaved electrical appliance - the supply cable is getting to hot to handle in normal use - it's still mechanically/electrically safe - but it's not a good "look" for a premium product.

the fix for this perceived problem? change the design spec of the supply cables (14-50/6-50) from 10-gauge wire to 8-gauge wire increasing the cost of the supply by $0.3787652 total incremental cost…porsche decided to cheap out and ship something that meets the minimum spec, will never actually fail, and is 100% "safe"

meeting minimum spec for maximum usage can be safe and effective, but often times is insufficient for intangible reasons beyond a pure succeed/fail criteria.

8-gauge wire is the "minimum" spec for 50 amp circuits in North America - it would proably do fine 98.99999999% of the time for a full 50 amp load continously - but it may have some residual behaviors when used at those limits that are either intangible or installations variables might leave a latent failure scenario in place…

so by restricting it to 80% of rated continuous load you build in a health margin of error and also probably managed some of the intangible aspects of expectations - people might be uncomfortable with their wall getting warm every time they charge their EV since the 8 gauge wire running behind is being run at 100% rated capacity - even if it's perfectly safe…just look at everyone's response to the PMC+/PMCC…it's perfectly fine, but a lot of customers don't like it.

Porsche should've spec's 8/6 gauge wire for the supply cables - it would be a better "look" on a premium product at a premium price.

I suspect the North American building code 80% rule is purely a matter of overly conservative and health engineereing safety margins with the "theory" that even in the worse case these specifications will never fail even when the actual installation isn't the best or 100% correct…
 
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David Bennett

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I think the breakers are fine and can handle the continuous the load - but the accumulated "heat" in the wire for "continuous use" is what is being managed with the 80% rule - and the "rule" is also probably conservative for safety - if you are running XX amps - there is an associated "minimum capacity" gauge wire that will be expected to be installed

so if you have…
  • a breaker of XX amp size
  • the minimum "spec" wire gauge associated with that XX breaker size
  • an electrical device "pulling" _MAX_ XX amps
  • pulling XX amps _CONTINUOUSLY_
the question becomes…

are we comfortable with the amount of accumulated "heat/energy" that will be deposited into the wire during the extended usage period?

because what the break is protecting _IS_ the wire - and in particular the breaker and wire-gauge rating supposedly guarantees the wire will not fail/overheat/melt/short/fire…
  • wire length is also a factor - longer runs are more resistance which is more heat
  • voltage variations are a factor
  • and wire composition/materials are a factor (copper/aluminum/alloy) - stranded or solid - insulation type
  • ambient temperatures are a factor
  • age of materials is a factor
  • quality of installation is a factor
  • actual draw/behavior of the electrical device in question is a factor
  • other potential "loads" on the circuit at the same time are a factor
    • you're EVSE been's cooking along for 6 hours at max XX amps (not 80% reduced load) and boom your electric dryer comes online - the wire is already "close to max heat" - so now it WILL overheat if your breaker doesn't kick in quickly…
  • building inspector quality/attention/integrity to detail is a factor (sometime paid to look the other way)
    • also sometimes not everyone interprets laws/codes correctly
  • there are humans in this equation so there are unforeseeable variables in the equation
So I'm betting the North American building code is conservative with their 80% rule for continous use and they are simply building in a healthy engineering margin to cover the wide wide wide range of potential on-site-installation variables. I know for a fact that not all 120V/15 amp circuits are equal - some older homes have problems with even low loads of 120V @ 10 amps on some of the really crappy install choices made by builders for these homes…

I'm willing to bet for a proper/well-done/excellent installation with proper materials you could run at 100% breaker rating for more than 3 hours with no actual failures (melting insulation leading to shorts leading to fires) - but that means you believe everything involved in that circuit has to be "right on spec"…and at least in North America from what i've seen - what your builder decides to do behind the walls of your building/home is sometime questionable…

I think the building code in this case is a case of "healthy" safety margin to cover the wide number of variables which are harder to control so that even the "worse-case" install meeting the minimum requirements and done poorly to boot still shouldn't fail when running for 3+ hours…

but all this is simply speculation on my part.

"So I'm betting the North American building code is conservative"
This is very much the view Europe takes of North American standards and at the root of why I described the case as interesting in my earlier post.

I won't comment further, some of your post I agree with, some of it I don't :)
 
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daveo4EV

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"So I'm betting the North American building code is conservative"
This is very much the view Europe takes of North American standards and at the root of why I described the case as interesting in my earlier post.
I don't know much about European or UK building codes, but in the US, the codes are designed to prevent house fires. Much of the research underlying the electrical codes was performed over the last 100+ years by Underwriters Laboratories, a US company organized to improve the safety in America homes.
Here's an example of a code rule which affects anyone who is installing an EVSE.
8 gauge wire comes in several variants, but I'll show you just two of them. NMB is a package of three or four THHN wires in a plastic shell. THHN individually are wires which normally need to be run in a conduit for protection.
8 gauge NMB is rated to 40 amps.
8 gauge THHN (the individual wires inside an NMB cable) is rated to 55 amps.
If NMB has the same physical THHN wires inside the NMB shell, why would the amperage rating be different?
Because the NMB shell causes the THHN wires to heat up when run at the full THHN capacity of 55 amps and at full amperage can melt the NMB casing. So the building code requires the 8 gauge NMB wire to be derated to avoid overheating.
Individual THHN wires in a conduit are cooled by the ambient air between the individual wires and therefore can be run at full rated amperage.

So, you can say the North American building codes are guesses and are conservative. I prefer to think North American building codes are based on science, testing and history.
 
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rb33gl

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Other than the charger, the appliance that pulls the most in our house is the induction hob. This is rated at 7.5kw but this would require all 5 hobs to be on at the same time (unusual even at Christmas!). It is also not pulling a constant current, wheras the charger runs at a full 7.2 for 7 hours.

Our dryer is a condenser/heat pump model, so is pretty efficient. The dishwasher runs close to 3kw for some of the time on high heat cycles.
 

David Bennett

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I don't know much about European or UK building codes, but in the US, the codes are designed to prevent house fires. Much of the research underlying the electrical codes was performed over the last 100+ years by Underwriters Laboratories, a US company organized to improve the safety in America homes.
Here's an example of a code rule which affects anyone who is installing an EVSE.
8 gauge wire comes in several variants, but I'll show you just two of them. NMB is a package of three or four THHN wires in a plastic shell. THHN are individual wires which normally need to be run in a conduit for protection.
8 gauge NMB is rated to 40 amps.
8 gauge THHN (the individual wires inside an NMB cable) is rated to 55 amps.
If NMB has the same physical THHN wires inside the NMB shell, why would the amperage rating be different?
Because the NMB shell causes the THHN wires to heat up when run at the full THHN capacity of 55 amps and at full amperage can melt the NMB casing. So the building code requires the 8 gauge NMB wire to be derated to avoid overheating.
Individual THHN wires in a conduit are cooled by the ambient air between the individual wires and therefore can be run at full rated amperage.

So, you can say the North American building codes are guesses and are conservative. I prefer to think North American building codes are based on science, testing and history.
I'm not suggesting anyone has guessed anything :)

The discussion stated as one about MCB derating but has morphed into one about cables which I fully get. Cable have all sorts of requirements, many listed out by daveo4EV, where derating applies. But breaker selection can also affect cable size and vice versa.

To continue the MCB theme. I know you didn't refer to this but for those who are interested, the main reason they are derated in group mounting situations is not for safety as such. There is very little evidence of damage or worse occurring from group mounted fully rated breakers. Fires and serious overheating are normally the result of high impedance connections not the devices themselves (unless they are faulty). No, the main issue would be nuisance tripping. This is where the high ambient temperate, caused by group mounting or the environment, fools the circuit breaker into "thinking" more current is flowing than is actually the case and the breaker trips below its rated value.

For a 10A breaker this might look like:
30C = 10A
35C = 9.4A
40C = 8.8A (hence derate to 80% since 40C is a typical group mounted situation)
45C = 8A
50C = 7.5A
55C = 7A
60C = 6.4A
 


Jhenson29

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All breakers are designed to carry their rated current on a permanent basis
That not true. Breakers have trip curves and some trip after some period of time below their rated current. You have to look at the trip curve.

Edit; this was incorrect. My apologies to David.
 
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David Bennett

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That not true. Breakers have trip curves and some trip after some period of time below their rated current. You have to look at the trip curve.
Please share an example, I’ve not come across that in any electrical distribution circuit breaker. Normally the trip curve is asymptotic to the rated current
 

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Please share an example, I’ve not come across that in any electrical distribution circuit breaker. Normally the trip curve is asymptotic to the rated current
I'll see if I can find an example. There was a siemens one I was looking at last year in particular that I remember the margin of error dipped below the rated current after some number of hours. I don't remember if it was a branch circuit (UL489)or supplementary (UL1077) rated breaker though.
 

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"The grid" doesn't mean anything inside of your meter.

Most poletop transformers (in the Western US) transform 12.5kV to 110/220 and are rated anywhere from 10kva (lets call that 10kw and avoid a lot of confusion) and 170kva (170kw).

EVs are not randomly distributed. They turn up in nicer neighborhoods almost exclusively. It wouldnt take many EVs for one low-end poletop xformer to overheat and pop. Then it would require the grid operator/utility/muni to recognize what happened and upgrade it (or it might happen a second and third time).

Utilities are replacing a *lot* of poletops as they figure out that 4+ of the connected homes are charging EVs in the evening after they get home (and are still running AC). Very (very) few people time their EV charging to low tariff hours, even when they are on a time-based plan.

As for if the grid is ready for EV's to be distributed energy resources (the utility can pay you to borrow power form your EV when they need peak-time help), that's a whole different thing, but is a planned feature of future advanced distribution management.

Whether your grid can "handle" EVs is largely a very local answer and should not be assumed :)
 

David Bennett

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I'll see if I can find an example. There was a siemens one I was looking at last year in particular that I remember the margin of error dipped below the rated current after some number of hours. I don't remember if it was a branch circuit (UL489)or supplementary (UL1077) rated breaker though.
Thanks, that would be of interest. I suspect you will only find examples where it is possible due to tolerance, not design.

Here is an extract from some guidance notes around the EN60898 standard which might help clear things up

3.4.4. In Rated Current
The current that the circuit-breaker will carry continuously under specified conditions and
on which the time/current characteristics are based.
Unless otherwise stated In is based on a reference ambient temperature of 30ÂşC.
 
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David Bennett

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i went into the office today and took a look at the Ul489 standard. It said

“7.1.2.4.1 A circuit breaker shall be capable of carrying 100 percent of its rated current without tripping.”
 

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i went into the office today and took a look at the Ul489 standard. It said

“7.1.2.4.1 A circuit breaker shall be capable of carrying 100 percent of its rated current without tripping.”
And I think UL 1077 may be the same. I’ll see if I can find those breakers I saw before though.
 

 
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