He’s running lights, dishwasher, and a microwave. Other than AC the big consumers are gas. Wash dishes before 9pm and charge after 9pm. It doesn’t seem like a problem. If he pops a breaker then he knows he needs to reduce overall load. The car charging amps can be adjusted. Certainly a few months a year he can shut off the AC breaker altogether and not worry about that consumer.
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I live in a condo. The main breaker is only 70A. Now what? [Resolved]
- Thread starter andy92782
- Start date
How could this have passed code in 1989 when the condo was built? Have there been any significant changes to the way loads are calculated over the past 30 years?
juliusa
Active Member
@eprosenx
Help me understand this snippet of NEC. Suppose I have an accurate history of my electricity usage. I do, thanks to my utility's website.
Let's use July of this year as an example. I consumed more electricity that month than any other over the past year. The weekend of 7/7-7/8 was a hot one. The Weather Underground almanac says temps were in the upper 90s. I was definitely using my 30-year old central AC system:
Looking at July 7th in particular, the worst hour of the day was at 4PM where I consumed 4.53 kWh:
I know it's not possible to determine *peak* loads from this data but if my math is correct, I averaged 4.53 kW consumption for that one hour, which at 240V equates to an average current of 18.875A. In fact, over the past two years the most electricity I've ever consumed over a 1-hour period is about 4.6kWh. That could have been on a hot weekend or on a hot weeknight after getting home from work and firing up the AC (which we never run during business hours on weekdays while we're at work).
I'm sure this 4.6 kWh number isn't a worst case scenario. The AC may be running but other goodies that use a lot of juice probably aren't (ie the Keurig, the toaster, the Instant Pot, the sous vide, my SO's hairdryer, etc). They probably *do* include my desktop PC and a 50" old school plasma TV, as well as the kitchen fridge and a small wine fridge that cycle on and off.
I suppose if I wanted to get really pessimistic about this I could simply turn on my AC and bottom out the thermostat ensuring it stays on steadily for over an hour, fire up a few appliances and then go back and look at the utility website again.
Can any of this data be used to determine actual loads that can be used to satisfy NEC?
juliusa
Active Member
You can show that to the electrician that is doing the load calc. IIRC, the electrician said you could add a 30a charging circuit to your panel.
You can see the impact car charging has on your home load, and the grid. Charging your car at 24a would change your charts dramatically.
Confirms to me that adding the 30a charging circuit that your electrician suggested would be fine. If you set your car to charge between midnight and 8am in the hot months (and probably the rest of the year too) you will be doing your panel and the grid a favor. It will also change your bill dramatically. You currently use 17kWh daily in a peak month. That is somewhere around 60 miles of driving in a M3.
But to answer your question, I don't think that will affect the load calc. It is driven primarily by the charging (24a) and the HVAC when it is running (24A).
You can see the impact car charging has on your home load, and the grid. Charging your car at 24a would change your charts dramatically.
Confirms to me that adding the 30a charging circuit that your electrician suggested would be fine. If you set your car to charge between midnight and 8am in the hot months (and probably the rest of the year too) you will be doing your panel and the grid a favor. It will also change your bill dramatically. You currently use 17kWh daily in a peak month. That is somewhere around 60 miles of driving in a M3.
But to answer your question, I don't think that will affect the load calc. It is driven primarily by the charging (24a) and the HVAC when it is running (24A).
dhrivnak
Active Member
The panel also appears full with all the 120v circuits already doubled up. So I do think you need to replace or upgrade if you want to do anything above 20 amp 120v.
eprosenx
Active Member
I concur with your analysis. Clearly you have not used anywhere near the 70a sustained for any hour period of time.
The question is: Would this granularity of consumption be enough to convince the “Authority Having Jurisdiction” (AHJ) that you could add a 30a circuit safely for car charging?
It is technically possible that of that 4.6kWH used in an hour that it all was in one 15 minute interval and the rest of the hour you used nothing, but that is exceedingly unlikely.
This just goes to show how vastly off the NEC load calculations can be. They have to be very pessimistic since you never know what a homeowner might do.
Based on this data, I would personally not have a problem installing a 30a charging receptacle (or wall connector) by putting a quad tandem breaker in the 30a AC unit position, but it all depends on finding an electrician willing to do it and an inspector willing to sign off. Note that I would research that panel a bit more to see if it is a hazard and I would replace it if it was a hazard.
davewill
Active Member
This. I never understood why the OP didn't simply go with the original electrician that said this would work. As long as he's properly permitting the install, there should be no issue. If it won't work, he won't be able to get the permit and you'll then KNOW you need to do more. It seems lately that every install thread had devolved into a convoluted debate about code intricacies. Let the professional hammer that out for you.
If I was in that building I would not want my safety compromised. If it can meet the NEC load calculations and pass an inspection that is one thing but to install it because it should work is not an option.
What is someone else moves in with different equipment. It has to be sized for the maximum load possible or it is unsafe and a fire hazard. .
You can't pull 70 amps on a 70 amp circuit. The 80% rule only allows for 56 amps of continuous load.
Thank goodness for circuit breakers. Also, at moving out he can cap off the wires and turn the breaker off.
eprosenx
Active Member
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)
Just this weekend I reviewed an electrical system at a friends vacation home that was installed in recent years. I found numerous obvious errors. It was fully permitted and signed off by inspectors. He has three subpanels off the main panel and all of them fail to have a ground wire run separately from the neutral. They are improperly bonded at every panel. When the inspectors miss simple stuff like that, it makes you wonder what else they are missing. The install also had at least two breakers of improper size (one too small for the cooktop, and one too large for a wall heater).
Take a look at NEC 220.87 (below). It allows you to use actual data (collected for at least a year - which the OP has two years of data) to determine how heavily loaded a feeder / service is and then use that to determine if adding more load is allowed. 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 if that level of data and granularity is sufficient for the AHJ, it sounds like he could add a 20a or 30a charging circuit and still not overload that feeder.
This is clearly allowed by code if I am interpreting NEC 220.87 correctly.
I agree with you in principal, but this does not appear to be how NEC code is written. Load calculations are done off the equipment intended to be used on that circuit.
Regardless though, in this use case we are suggesting a 20a or 30a circuit be installed and then with the proper UMC adapter (or Wall Connector setting) only 16a or 24a would be drawn. Future owners would not be able to plug larger loads in without modifications of some kind.
My apologies, my wording there I think was poor. I was trying to convey that the OP's data was only of one hour granularity and so we don't know within the hour period what the peak load was (we only know the average). I was not trying to suggest that the service is good for 70a continuous. If all loads on the feeder were continuous loads then we should never push that feeder over 56 amps (though it is OK for the feeder to go over 56 amps at times with non-continuous loads). Which actually begs a question of how to properly apply 220.87 (above)... Obviously the conservative thing would be to keep it under 56 amps.
All right, this does answer one of my two questions then. 220.14 (A) does specify that load calculations are only going by the load rating of the appliance, and not the breaker. That surprises me just a little, but that's clear in the code then. So if a load calculation would allow extra 15A and 20A and 30A appliances, you could put every one of those on additional 100A circuits, even if your main feed was only 125A. Just seems weird.
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?
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.
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