Incoming utility cable capacity - A cautionary tale

[ModelS1079]

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Yikes! I'm sorry about your scare, and I'm glad everyone is safe and sound. I would be very tempted to have everything your installing electrician touched checked by a qualified professional. There should always be a breaker on every circuit, including the one from the street, that shuts off when current exceeds what the lines are rated for. Period. Anything else is a violation of code that should result in an investigation.

I'm surprised you have your Tesla on a 40A circuit. Are you only charging at 32A? You must have a 50A circuit for 40A charging, as no sustained load is ever supposed to exceed 80% of the rated current for those lines.

Thanks much.
I have had a brigade of electricians in and out of the house. I may scalp tickets.
50A service to Elwood (my Model S), yes, so he can draw 40A when thirsty.
Post above just updated with picture of circuit board.

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Any pics of this carnage?

No carnage pics - just cold kids:

 

[mknox]

Then your neighbours should have no complaint with you, but they should be rather circumspect about their shared utility and town governance! I certainly hope that the car does not get any blame in this! As a matter of fact, the same thing could have happened if you had installed a new electric oven, rather than an EV!

Not really. The issue here is that traditional electricity loads are intermittent (even heating systems). Ovens cycle on and off, and are usually not on for many hours on end. What we have in this example are two very large continuous loads: the thermal storage heating system and the car - both "re-charging". The heating effect likely triggered the burn-off at a weak connection point. I can tell you that electric utilities are starting to get concerned about EV charging due to the long duration of time that full current is drawn on these circuits. Several EVs on a street can lead to this type of thing happening at the transformer.

 

[ModelS1079]

...There should always be a breaker on every circuit, including the one from the street, that shuts off when current exceeds what the lines are rated for. Period. Anything else is a violation of code that should result in an investigation...

Wondering: the breakers installed in my house are downstream of the source of the problem - the wires that burned are outside the house. Should the in-house breakers have tripped?

 

[FlasherZ]

Wondering: the breakers installed in my house are downstream of the source of the problem - the wires that burned are outside the house. Should the in-house breakers have tripped?

No. You weren't using current greater than your main breakers, but the power company's service cable was undersized due to oversubscription rules.

It is likely that if the fire progressed any more, it would have short-circuited the service entrance cable and the transformer would have blown its fuse (or blown up). That would have stopped the electricity, but by then the dripping insulation from the cables may have ignited your home anyway.

As adelman said, there are different rules for the service provided prior to your meter -- oversubscription is permitted and very typical. Transformers are protected on the primary side by a fuse (and I'm surprised adelman's test didn't blow that fuse that before it spewed oil), but the secondary side of that transformer is not protected until your main breaker.

Just as another example, prior to me calling the PoCo, I had a 200A service, served by a 15 kVA (roughly 60A) transformer (dedicated to my home) and 2/0 AL SE cable, which is good to 135-150 amps. They told me that prior to the Tesla arriving, I was at times drawing 197% of the transformer's rated load (120A or so). Adding the Tesla could have done the same thing, as it would have driven my 2/0 service cable well over rating. After I made them aware of the HPWC's load, we upgraded to 400A (320A) service and 350 kcmil SE cable, rated at 250-280 amps.

Bottom line, as mknox said, you really need to tell your power company when you add significant continuous loads.

In non-smart-meter areas, the Power Company (PoCo) knows what size service you have (because they generally have an inventory of the meter base at a particular service location), but doesn't really know how much of that you are using on an instantaneous base. They have your average load over the month, and models they use based on time-of-day and temperature for an average home, but the details aren't available.

I'm glad to hear that everyone is safe. My Tesla *would* be back in my garage tonight, but today it received a special flatbed ride to Chicago where some of the queued up items are going to be addressed.

As a side note, this is why I am concerned about those who want to use 14-30 or 10-30 to 14-50 FrankenCables. You have a circuit breaker which is intended to be secondary protection, but if it doesn't blow (like the transformer's primary fuse) -- this is what tends to happen (except inside your walls instead of outside your home).

 

[GeekGirls]

Wondering: the breakers installed in my house are downstream of the source of the problem - the wires that burned are outside the house. Should the in-house breakers have tripped?

It's not really my field, but I spent a lot of time grilling the electrician who did my install. We moved into the house relatively recently and are still getting used to its quirks (we have five electrical panels – one outside, two primary panels, and two sub panels scattered across four distinct locations in the house.) He was adamant that there was a breaker at the house on each of our 100A primary lines from the street that would trip if we exceeded their rated capacity. It makes sense since the breakers within each of the two primaries add up to more than 100A apiece, it's just assumed that not everything will be at peak load simultaneously. If it did happen, a breaker should blow before we staged a repeat of your experience.

 

[FlasherZ]

PS: I'm working on a charging/electrical infrastructure FAQ for the charging subforum, and this topic (ensuring your PoCo knows you're going to add 40A or 80A continuous load) is one of the items I have penciled in.

 

[rolosrevenge]

Not really. The issue here is that traditional electricity loads are intermittent (even heating systems). Ovens cycle on and off, and are usually not on for many hours on end. What we have in this example are two very large continuous loads: the thermal storage heating system and the car - both "re-charging". The heating effect likely triggered the burn-off at a weak connection point. I can tell you that electric utilities are starting to get concerned about EV charging due to the long duration of time that full current is drawn on these circuits. Several EVs on a street can lead to this type of thing happening at the transformer.

Another interesting thing is that it burned in freezing weather when the maximum current limit should have been higher than normal. Those must have been seriously undersized conductors.

As adelman said, there are different rules for the service provided prior to your meter -- oversubscription is permitted and very typical. Transformers are protected on the primary side by a fuse (and I'm surprised adelman's test didn't blow that fuse that before it spewed oil), but the secondary side of that transformer is not protected until your main breaker.

The fuse only protects against fault currents which are several times the rating. A mild yet persistent overload wouldn't blow the fuses ever, even though the transformer would burn up.

 

[FlasherZ]

Another interesting thing is that it burned in freezing weather when the maximum current limit should have been higher than normal. Those must have been seriously undersized conductors.

Indeed. Older 200A services are supplied by 2/0 AL cable, as mine were, rated for only 130-150 amps.

What should scare ModelS1079 here is that he has been on the edge since he moved into this home -- an additional 40A load shouldn't have that type of result, and I'm surprised the wires weren't giving off steam prior to the Tesla being installed in the cold evenings.

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The fuse only protects against fault currents which are several times the rating. A mild yet persistent overload wouldn't blow the fuses ever, even though the transformer would burn up.

It is generally 2-3x rated load of the transformer, at least for the transformers my PoCo uses.

 

[adelman]

I believe that PG&E has an "overrating" on their transformers, based on climate, that allows them to go significantly over the rated capacity (2-3x). I suspect that if they fuse them at all, they're fused at this "overrated" capacity. Mine has no visible fuse; I'm not sure if there is something internal to the transformer. The 21kV line is fused about two miles away.

 

[brianman]

PS: I'm working on a charging/electrical infrastructure FAQ for the charging subforum

Thanks in advance for doing this. Your expertise is definitely valued.

 

[gg_got_a_tesla]

This is scary stuff indeed; thanks, Jim, for living to tell the tale in your own unique and humorous way! Glad that there was no damage too.

This is indeed a wake-up call. I've been lax about giving my local PoCo (PG&E) a heads-up about what's coming when my S starts juicing up in a couple of weeks. I've been assuming that the 14-50 installer (SolarCity) and the inspector from the City of Newark had it all covered when they signed off on the 14-50 install back in August.

And, I have a SolarCity-installed PV system as well and whatever goes along with it (including the limited edition E-7 rate schedule) so, PG&E should be "familiar" with my house.

Will call PG&E the first thing tomorrow to check if any Service Upgrades or even System Upgrades might be needed in the run to my house:

http://pge.com/myhome/environment/whatyoucando/electricdrivevehicles/installationprocess/

http://pge.com/includes/docs/pdfs/shared/environment/pge/cleanair/electricdrivevehicles/pev_home_installation.pdf

Since I'm not getting an HPWC and am planning on juicing up only after 10 pm or so (and trickle-charging at work as well to reduce the charging time at home), I may not have issues, I hope.

 

[MikeK]

Wow, what a story! I'm glad to hear that everybody is fine, there was no fault in the Model S or its accessories, and I also am fascinated to learn of this type of heating system!

I had the main supply lines to my house replaced a few years ago when I phoned up PG&E because I was seeing significant voltage sags (indicated by dimmed lights and the occasional protest from a UPS) whenever my air conditioning would start up. They came to investigate and found that the supply wires had rubbed against a tree and completely worn through the neutral! The two hot wires were fine, but the neutral was completely severed. The nice man from PG&E told me that I clearly had a very good house ground.

The lines were replaced and protected to prevent a repeat. I wonder whether they are adequately sized. My Model S has a single charger so I won't be pulling more than 10kW, but I am putting in a HPWC so conceivably a visitor with dual chargers might draw more... I will check with my electrician.

 

[dtich]

It's not really my field, but I spent a lot of time grilling the electrician who did my install. We moved into the house relatively recently and are still getting used to its quirks (we have five electrical panels – one outside, two primary panels, and two sub panels scattered across four distinct locations in the house.) He was adamant that there was a breaker at the house on each of our 100A primary lines from the street that would trip if we exceeded their rated capacity. It makes sense since the breakers within each of the two primaries add up to more than 100A apiece, it's just assumed that not everything will be at peak load simultaneously. If it did happen, a breaker should blow before we staged a repeat of your experience.

there are several more experienced 'electricians' on this board than i, but i'll say to this that while i may be misunderstanding the layout of your electric, geek, you should be very careful to have the power util take a look at your setup. while it's true that it is common and within code to have subpanel main breakers add up to more than your service rating, your main panel service entrance breaker should actually equal your service rating (ie. the gauge of wire coming off street). just as you can have more than 200A worth of branch circuits in a 200A panel with a 200A main breaker, you can have, say a 100A sub and a 50A sub on a 100A service to the house... but all the subs have to wire back to your main panel, the one directly after your meter block/service entrance, that's why it's called a main panel, and that panel's main breaker needs to equal your service entrance rating. in this way the system works in a daisy chain with the limiting factor being the properly sized main panel breaker, so that anything downstream of that (i.e. in your house) that pulls current is subject to the limit of that main breaker which is sized to protect not only your house, but in fact mainly the wires coming off the street, your service entrance.

if this is not how your system is wired, you should take heed. what happened to models1079 could very well happen to you. as there is no 'breaker' between your meter and the pole to catch this problem.. if your house draws more than the wires can handle, they will overheat. period. so if you have, as you indicate, 2 or more 100A panels on, presumably, 100A service, but wired in parallel, not series as i laid out above... this sounds very wrong. this allows for the potential draw of more than double your service rating current. and if you were adding an outdoor lighting circuit or a basement sump pump, i'd say fine, no issue, but a 40A continuous draw circuit for the tesla is no joke. i'd go so far as to say that the electricians in models1079's town were fairly remiss in spec'ing out his system knowing the overall load on his house. it seems they just added main panels (connected to separate meter blocks but to the pole over the same service entrance wires) for each type of metering without much thought to overall load on the house, ie the wires from the pole. which given that they were talking about a house with a 200A heating system... is insane. that house needs 400A service no matter what, 40A tesla not withstanding. very glad that nothing more serious happened. (but, aside from all the danger and stuff, a fun story to read knowing it turned out fine!)


thx for sharing all.

 

[NigelM]

Good to hear that everyone is Ok and thanks for the clarification.

Mod note: title updated to reflect the story; also search tags added. We'll move the thread to a better location in a couple of days, leaving it here for now will allow any worried members to easily find it.

 

[mknox]

Another interesting thing is that it burned in freezing weather when the maximum current limit should have been higher than normal. Those must have been seriously undersized conductors.

...or a loose/bad connection. As I mentioned, electrical "burn-offs" are more common than most people realize. I had one on the underground cable that feeds my house a few years ago, and the only difference was that the "fireworks" were underground and out of sight. It was quite a mess when dug up, and faulted right at a splice that had been made (by the utility) many years prior.

The fuse only protects against fault currents which are several times the rating. A mild yet persistent overload wouldn't blow the fuses ever, even though the transformer would burn up.

Fuses and breakers will trip almost instantly on fault currents significantly over the rating. They will (or are supposed to) trip on currents at or just slightly over the rating, but not instantly. It takes time for the heating effects to melt the fuse link or trip the breaker.

I suspect in this situation, the connection at the top of the weatherhead was the "fuse".

 

[FlasherZ]

It's not really my field, but I spent a lot of time grilling the electrician who did my install. We moved into the house relatively recently and are still getting used to its quirks (we have five electrical panels – one outside, two primary panels, and two sub panels scattered across four distinct locations in the house.) He was adamant that there was a breaker at the house on each of our 100A primary lines from the street that would trip if we exceeded their rated capacity. It makes sense since the breakers within each of the two primaries add up to more than 100A apiece, it's just assumed that not everything will be at peak load simultaneously. If it did happen, a breaker should blow before we staged a repeat of your experience.

Northern California (at least where I've lived) requires an outside disconnect to be available, and this is typically where all the power is initially protected. Breakers in that panel will feed other panels throughout the house -- so simply add them up and that's the maximum current you could draw across your service cable at full peak, everything loaded up to the gills. No matter what the size of breakers for panels inside, you should be ok.

Since we are talking about overcurrent protection... anyone reading this thread whose home was built or updated between 1940 and 1990 should look at your circuit breaker panels and ensure that you do not have a panel made by Federal-Pacific Electric (look for orange "tips" on the breaker handles, the term "Stab-Lok", "FPE" or "Federal-Pacific Electric") or a panel made by Zinsco (later Sylvania-Zinsco). If you have either of these, you may as well not even have a breaker panel because trip failure rates are above 50% in many of these panels.

 

[mknox]

I believe that PG&E has an "overrating" on their transformers, based on climate, that allows them to go significantly over the rated capacity (2-3x). I suspect that if they fuse them at all, they're fused at this "overrated" capacity. Mine has no visible fuse; I'm not sure if there is something internal to the transformer. The 21kV line is fused about two miles away.

It is quite common for utilities to overload transformers, but only for a certain period of time. There is an assumtion that a cool down period will follow, and the transformers are designed for this type of duty cycling. Problems arise, for example, in heat waves when air conditioners don't cycle back at night, and the transformer doesn't get its cooling off time. Continuous loads like EV charging are going to present similar problems for utilities.

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Since we are talking about overcurrent protection... anyone reading this thread whose home was built or updated between 1940 and 1990 should look at your circuit breaker panels and ensure that you do not have a panel made by Federal-Pacific Electric (look for orange "tips" on the breaker handles, the term "Stab-Lok", "FPE" or "Federal-Pacific Electric") or a panel made by Zinsco (later Sylvania-Zinsco). If you have either of these, you may as well not even have a breaker panel because trip failure rates are above 50% in many of these panels.

"Stab-Loc" is branded Federal Pioneer Electric (FPE) in Canada.

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As adelman said, there are different rules for the service provided prior to your meter -- oversubscription is permitted and very typical. Transformers are protected on the primary side by a fuse (and I'm surprised adelman's test didn't blow that fuse that before it spewed oil), but the secondary side of that transformer is not protected until your main breaker.

Distribution transformers are protected by a fuse on the primary side, but I can tell you from first hand experience, they don't always blow. We had a situation where, when doing a meter change, a lug in the meter socket broke and the conductor "welded" itself to the grounded metal meter base. Lots of fireworks, and this is why it is important for utility workers to wear proper PPE (clothing, face shields, rubber gloves etc.). At this point the service lines and the overhead conductors on the street started to "gallop" and smoke. Lines on the street caught fire and came flaming down on to traffic. Neighbors came out of their homes saying smoke was coming out of switches and receptacles. All this time, the transformer's primary fuse didn't blow. The transformer was ruined along with a few spans of overhead line, not to mention property damage to a few homes on the street.

Had ModelS1079's fault not burned clear, a similar scenario could have unfolded. Typically, most of these kinds of faults do burn clear.

I mention this not to scare, but to point out that there is a lot of energy on tap, and that if released in an unsafe manner can be quite dangerous. People get that gasoline or natural gas are flammable and explosive, but seem to take a much more lax approach to electricity, I presume because you can't see or smell it.

 

[GeekGirls]

i may be misunderstanding the layout of your electric, geek, you should be very careful to have the power util take a look at your setup. while it's true that it is common and within code to have subpanel main breakers add up to more than your service rating, your main panel service entrance breaker should actually equal your service rating

I'm sorry if I wasn't clear, but that's absolutely the case. We have two lines coming from the street, both protected with 100A breakers on the outdoor panel that feeds the two primary indoor panels. The primaries each have a breaker covering the corresponding sub-panel, and wire gauges have been verified as within spec. It all looks like it was done correctly to me, and we've had more than one electrician out to make sure we're in good shape. The house was built in 1989 in a newer community, which helps some but is no guarantee of anything.

I've been negligent about calling PG&E regarding the additional load, but it's on my immediate future list. I'm less concerned since we replaced an existing equivalent load, but it is admittedly one that wouldn't have been in use nearly as often. I've been stalling on the call while I investigate the E-9B option.

 

[Robert.Boston]

Jim, I'm glad to hear that everyone is safe and sound. I hope you didn't take my moderator's remarks the wrong way!

One of the first things I did when I ordered my Model S was to call the electric utilities where I charge to make sure their equipment could take the load, and I had the electricians installing the NEMA 14-50s to check that the service wires were sufficient. It was a relief when I got thumbs-up on all fronts. So, let me reiterate what others have said: call your utility, and keep talking to people until you get someone who can affirmatively answer that your transformer and related gear are up to the task.

 

[mknox]

I've been negligent about calling PG&E regarding the additional load, but it's on my immediate future list. I'm less concerned since we replaced an existing equivalent load, but it is admittedly one that wouldn't have been in use nearly as often. I've been stalling on the call while I investigate the E-9B option.

In Ontario, Canada anyone who takes advantage of the provincial EV rebates has their postal code (zip code) provided to their local electric utility. I understand that the EV owner can opt to have their exact municipal address provided, but for privacy concerns, it defaults to just the postal code. This is to allow the utility to "see" if pockets of EVs are starting to pop up and take preventative measures. One EV in a neighborhood is not a problem... multiple EVs in a neighborhood could be.