I live in a condo. The main breaker is only 70A. Now what? [Resolved]

[eprosenx]

Update: I had an electrician do an on-site inspection. He says the wire in the conduit between the main breaker and the subpanel in my garage is #4 THHN copper. He says it's good for 100A, so the only necessary hardware change is to replace the 70A main breaker with 100A. I submitted the request to my HOA. If they say yes, I should be good to go. Crossing fingers.

Awesome! That jives with my understanding of code when you factor in the 83% rule which you are allowed to do on 100a service feeds.

So have you determined that your panel is not one of the hazardous ones and so just adding a new quad tandem breaker will be safe?

What size circuit will you add then for your car? Are you going to do just a receptacle for the UMC or are you going to do a Wall Connector? Since you will be getting another 30 amps of capacity you could go larger than a 30a circuit potentially. Perhaps a 14-50 which would need 32a of continuous capacity (40a) if you are using a UMC Gen 2. I would still wire it with wire for a 50a circuit regardless (6awg instead of 8awg).

 

[Petrlol]

Awesome! That jives with my understanding of code when you factor in the 83% rule which you are allowed to do on 100a service feeds.

So have you determined that your panel is not one of the hazardous ones and so just adding a new quad tandem breaker will be safe?

What size circuit will you add then for your car? Are you going to do just a receptacle for the UMC or are you going to do a Wall Connector? Since you will be getting another 30 amps of capacity you could go larger than a 30a circuit potentially. Perhaps a 14-50 which would need 32a of continuous capacity (40a) if you are using a UMC Gen 2. I would still wire it with wire for a 50a circuit regardless (6awg instead of 8awg).

What am I missing here? How is #4THHN good for 100amp. I'm showing it good at 70amp. 80amp requires #3, 100amp requires #1 assuming a length of ~50ft. Wire Size Calculator

 

[animorph]

Yeah, the HPWC manual had me going with #2 THHN for 100A, and I think some get by with #3.

 

[eprosenx]

What am I missing here? How is #4THHN good for 100amp. I'm showing it good at 70amp. 80amp requires #3, 100amp requires #1 assuming a length of ~50ft. Wire Size Calculator

Yeah, the HPWC manual had me going with #2 THHN for 100A, and I think some get by with #3.

You are both correct to a degree. @Petrlol Assuming the wire is in conduit the entire way (which THHN would have to be) then you can use the 75c rating which is 85 amps.

@animorph You are correct, except for this clause in 310.15(B)(7). Basically for whatever reason code allows you to undersize the wire in these special cases by up to 17%. I don't really know the full logic behind this - I presume it is because they were finding that the traditional load calculations were overshooting on the wire size needed by too much and so they put this in as a compensating factor (rather than re-factoring the load calcs themselves). I really don't get it. But it is allowed by the code...

 

[eprosenx]

This is the part that still seems odd to me. It sounds like trying to have your cake and eat it too. You said you are using the dial to set the nameplate rating. OK. That would mean if you are setting it to "be" a 20A device, then it is not a 100A device. Let's say this were a hardwired air conditioner. If it has a nameplate rating that says 240V 30A circuit, could you really put that on a 100A or 125A breaker? I thought you would have to go with what its nameplate rating says, and if you're setting the nameplate rating on the wall connector for 20A, does it really allow you to oversize the breaker like that?

This is a great example actually! So here is the nameplate on an AC I was looking at this weekend. It specifies how much power it draws (I am not sure if this is raw amps, or if it includes the 125% over-rating as I suspect AC is a continuous load - I think it may already include the 125%), but it also specifies the maximum circuit ampacity you can put it on. So you would use the motor specs to determine the load calc for this device but then you would be allowed to put it on up to a 50a circuit (which is what this unit was being fed by).

I wish Tesla would provide better specs on their stuff, but from reading the nameplate and the install manual it is clear to me that it is fine to install a Wall Connector on up to a 100a circuit as long as the wire is sufficient for that ampacity (matching or exceeding the breaker rating).

 

[andy92782]

Update: I had an electrician do an on-site inspection. He says the wire in the conduit between the main breaker and the subpanel in my garage is #4 THHN copper. He says it's good for 100A, so the only necessary hardware change is to replace the 70A main breaker with 100A. I submitted the request to my HOA. If they say yes, I should be good to go. Crossing fingers.

Update #2: Electrician #2 (aka the second opinion) says 4 gauge THHN copper wire *isn't* capable of 100A and he immediately quoted me the specific section of NEC that says so. So... It looks like I'm back to pursuing the DCC route. The good news is Electrician #2 is familiar with the DCC and has installed them before, so the only hurdle left to clear is the city inspector.

(the specific limitation of the THHN wire is the temperature rating. It has to be de-rated to the value used by the sub-panel where it connects, which is 75C. At that temp, 4-gauge THHN is only good for 85A, not 100A. The first electrician either didn't know that or skipped over it).

 

[andy92782]

FYI:

I found this thread on Stack Exchange. It explains why I can't use #4 THHN to carry 100A. Specifically, it has to do with the definition of service-entrance conductors and service lateral conductors and the fact that the wire between my main panel and the sub-panel in my garage are neither. Therefore I have to de-rate the current capacity to the 75C spec, which is 85A, not 100A.

I was impressed that Electrician #2 was able to rattle this specific section of NEC off *immediately*, without hesitation. I think he's the guy I'm gonna hire.

What wire gauge do I need for a 100 Amp subpanel at the end of a 60' wire run?

 

[juliusa]

Electricians and code inspectors make mistakes CONSTANTLY.

Based on OP's data, it sounds like he has never used over 18.875A in a one hour period over the last two years.

So are you saying the electrician was wrong to say he could put in a 30a charging circuit, or putting in a 30a charging circuit is ok?

It looks like I'm back to pursuing the DCC route.

Why not simply add a 30a charging circuit to the existing panel? In two years you've never averaged more than 19A in a peak hour, and peak hour is a time you will probably not be charging your car anyway.

 

[andy92782]

Why not simply add a 30a charging circuit to the existing panel? In two years you've never averaged more than 19A in a peak hour, and peak hour is a time you will probably not be charging your car anyway.

If I didn't care about meeting code I probably would.... but I do, so I'm not.

 

[davewill]

Electricians and code inspectors make mistakes CONSTANTLY. Just because one electrician out of several was willing to do the work one way and an inspector signed off on it does not necessarily mean it is correct or safe. (though obviously it is better than no permit)

...

Your concern is duly noted.

 

[eprosenx]

FYI:

I found this thread on Stack Exchange. It explains why I can't use #4 THHN to carry 100A. Specifically, it has to do with the definition of service-entrance conductors and service lateral conductors and the fact that the wire between my main panel and the sub-panel in my garage are neither. Therefore I have to de-rate the current capacity to the 75C spec, which is 85A, not 100A.

I was impressed that Electrician #2 was able to rattle this specific section of NEC off *immediately*, without hesitation. I think he's the guy I'm gonna hire.

What wire gauge do I need for a 100 Amp subpanel at the end of a 60' wire run?

Please see this post above where I posted NEC 310.15(B)(7) - you are not trying to use the 90c rating of the wire, you are using the 75c rating of the wire, but this code snippit allows you have a 100a service or feeder (that serves the entire load of a single family residence) on wire only rated for 83 amps (which is less than the 85 amps #4 copper is rated to at 75c).
I live in a condo. The main breaker is only 70A. Now what?

In the web site linked above they are referencing 2014 code. They changed this section in the 2017 code to read differently, but it still I think for the most part has the same effect. That StackExchange article *IS* correct for a subpanel in the garage, HOWEVER, it is not correct to apply it to your situation. The subpanel in the garage does not handle the entire load of a single family residence. Since your feeder from the main service entrance panel to the subpanel in your garage DOES handle the full load of a single family residence you qualify for the 83% rule.

So I would say Electrician #1 is not wrong.

 

[eprosenx]

So are you saying the electrician was wrong to say he could put in a 30a charging circuit, or putting in a 30a charging circuit is ok?

I am saying that if the OP's two years of actual usage data (with hourly granularity) are acceptable as evidence of actual load then yes, he could install a 30a charging circuit on that existing panel with the 70a feed per NEC 220.87. I have never worked with an inspector on leveraging this code section before so I am not sure if they would accept this evidence or not, but the code exists.

 

[juliusa]

If I didn't care about meeting code I probably would.... but I do, so I'm not.

Meets NEC code. If you have a local code that is different, perhaps inquire about a variance?

The 100amp minimum is sort of silly. A 70a panel could accommodate a L2 charger where a 100a panel could not. That is why NEC does a load analysis.

You could offer to put in a new 70a main breaker and/or have the electrician test the breaker to make sure. Combined with the load analysis, actual usage, and 4gauge THHN that seems very reasonable.

 

[andy92782]

The subpanel in the garage does not handle the entire load of a single family residence. Since your feeder from the main service entrance panel to the subpanel in your garage DOES handle the full load of a single family residence you qualify for the 83% rule.

So I would say Electrician #1 is not wrong.

I haven't seen the updated code. At the end of the day I have to go with what my gut tells me, and right now it's telling me the DCC option is the better one. There are a few reasons why I think this:
- It's the more conservative option of the two. It prevents me from ever drawing more than the 70A current allowable by the equipment installed by the builder. There is no need to argue over which NEC spec to follow.
- It requires zero approval from my HOA. The DCC doesn't affect any part of the property's common area electrical infrastructure (main panel, breaker and conduits). The HOA already confirmed that anything in my garage is my property and I don't need to ask them for permission to do anything. As I may have mentioned earlier, working with my HOA is a time consuming and frustrating process that I would strongly prefer to avoid.
- Because I wouldn't have to touch any part of the common area infrastructure, there is zero chance I will need to pull a $1M umbrella liability policy specified by CA SB209 (reported earlier in this thread).

The downside to the DCC installation is increased cost, but the good news is Tesla literally just lowered the price of my new car by the exact same amount as the price of the DCC. I'm covered there.

I really wish I didn't live in a condo. This process would be so much easier if I was in an SFR and had full control of my destiny.

 

[Dana1]

If they work on an old overloaded panel like that.

I’d probably invest in a new panel and breakers just because I’m keen on maintenance.

 

[eprosenx]

I haven't seen the updated code. At the end of the day I have to go with what my gut tells me, and right now it's telling me the DCC option is the better one. There are a few reasons why I think this:
- It's the more conservative option of the two. It prevents me from ever drawing more than the 70A current allowable by the equipment installed by the builder. There is no need to argue over which NEC spec to follow.
- It requires zero approval from my HOA. The DCC doesn't affect any part of the property's common area electrical infrastructure (main panel, breaker and conduits). The HOA already confirmed that anything in my garage is my property and I don't need to ask them for permission to do anything. As I may have mentioned earlier, working with my HOA is a time consuming and frustrating process that I would strongly prefer to avoid.
- Because I wouldn't have to touch any part of the common area infrastructure, there is zero chance I will need to pull a $1M umbrella liability policy specified by CA SB209 (reported earlier in this thread).

The downside to the DCC installation is increased cost, but the good news is Tesla literally just lowered the price of my new car by the exact same amount as the price of the DCC. I'm covered there.

I really wish I didn't live in a condo. This process would be so much easier if I was in an SFR and had full control of my destiny.

I totally get it, the DCC should be pretty low friction to get installed. My guess is it will never engage the load shedding.

But FWIW, I did post the exact relevant 2017 code snippet in the thread above. There should be no need to argue with the inspectors over it. Pretty much all residences built these days exercise this clause.

I totally get not wanting to deal with the HOA at all though.

 

[juliusa]

It prevents me from ever drawing more than the 70A current allowable by the equipment installed by the builder.

The 70a circuit breaker does that.

It's your money. Seems like a waste to put in that fancy switching gear when in two years the highest average hourly draw on your 70a service was 18a.

 

[andy92782]

The 70a circuit breaker does that.

It's your money. Seems like a waste to put in that fancy switching gear when in two years the highest average hourly draw on your 70a service was 18a.

I think you missed the part about not having to deal with my HOA. Did you read my story earlier about the egress replacement windows that didn't meet code? I would pay the $1000 price for the DCC all day long and twice on Sunday rather than deal with them...

 

[Sophias_dad]

I think you missed the part about not having to deal with my HOA. Did you read my story earlier about the egress replacement windows that didn't meet code? I would pay the $1000 price for the DCC all day long and twice on Sunday rather than deal with them...

I don't think you understood the previous poster's comment. He's suggesting to leave your main breaker 70 amps, and just put in a double tandem breaker in place of the current AC breaker. I don't think your HOA needs to be involved in that. If it were me, I'd probably just do that and wait until the first nuisance trip of the main breaker before dropping another 1000 on a DCC that's probably not needed. Hopefully that questionable panel won't burst into flame before the main breaker flips.

 

[juliusa]

He's suggesting to leave your main breaker 70 amps, and just put in a double tandem breaker in place of the current AC breaker.

Yep.

Hopefully that questionable panel won't burst into flame before the main breaker flips.

.
Charging at night when the HVAC is not running would solve that. For the summer months could dial back the charging current in an abundance of caution. Could also measure the actual current for the HVAC - could be below the 24a on the plate.

I'd probably just do that and wait until the first nuisance trip of the main breaker before dropping another 1000 on a DCC

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There would have to be car charging, HVAC running, and another 22a load for the main breaker to trip. I don't think there is another 22a in the house. Well, maybe running microwave, hair dryer, dish washer, washing machine, dryer, and both fridges at the same time.

Loved the graph showing 2a average usage midnight to 8am during peak day.
Peak hour below 20a - means if charging peak hour (why would you do that??) would be 44a average. That does not sound like it should stress a 70a panel. If the panel is defective, it should be replaced.