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90% of service cable and transformer rating cap

sleevemedia

Member
Jul 1, 2020
57
41
Orlando
Duke Energy Florida's interconnect process for systems with greater than 10kW includes a verification that the PV system does not produce more than 90% of the rating for the service cable and transformer. If it does, we pay for service cable and transformer upgrades as applicable.

We have a 150 amp main service disconnect outside next to the meter with other breakers for improvements made by the former owners. The indoor panel is full and has cheaters, that's likely why the breakers for the improvements are in the outdoor panel, it had room.

So if I math right, 150 amps, 120 volts, that's 18kW. I've ordered XL. It's not the 16.32 vs. 16.2 (90% of 18) that concerns me, it's that I want to add panels. I only have bills since we moved in April, 2300kWh used in May and again in June, and the current XL layout first year production estimate is 24MWh.

I have the roof to add 8 panels for a total of 19.04kWh. I might need to pay for at least a service cable upgrade and possibly a transformer upgrade, but I won't know for sure until permits are submitted and the Duke engineer reviews the interconnect request. Ideally I don't want to wait that long to find out.

I expect that the main breaker is 150 amps because that is what is specified to protect the service cable based on the cable's wire gauge. In 2003 construction, I would not expect that my service cable is oversized for my main breaker.

Are my math and assumptions right? Anything I'm missing? Main panel upgrade? Sounds like I need to hire an electrician to look at the service cable before I expand the system and run into fees from Duke. Should I be able to get Duke to come out and advise me before I proceed with a design?
 
Jun 20, 2014
115
213
Tulare
Just some assumptions but if Duke Energy is anything similar to SCE in California there is most likely multiple customers fed from the transformer so it would take many customers all having Solar to potentially overload or in your case have the transformer reach the 90% threshold. The service cable is a much more likely possibility to be in need of an upgrade but until you hear from the Engineer it will also be a guess as we don't know which size conductor was installed for your service.
 

sleevemedia

Member
Jul 1, 2020
57
41
Orlando
So they're not going to calculate transformer load as what all my neighbors are capable of drawing plus what I'm capable of sending? I don't understand transformer design, only that they convert distance-friendly high voltage to two-phase 240v that homes can use.

When you advise that the 90% would need many solar customers to be exceeded, I interpret that as the transformer has separate send and receive limits.
 

Patrick66

Member
Oct 27, 2019
74
42
Honolulu
Utility service lateral cables are generally sized at about 1/3 the service disconnect size, so you would only expect ~50-60A in your scenario. Your voltage is 120/240V though, so 12kW, or at 90% 10.8kW maximum PV before you need to look at upgrading the service lateral and transformer.
 

sleevemedia

Member
Jul 1, 2020
57
41
Orlando
OK. I guess I'm surprised. I would expect the service disconnect to protect the cable from overload, and to have 1/0 cable for a 150 amp service that will likely need to upgrade to 2/0 to carry 200 amp service. That would give me more than the needed 90% headroom for a 19kW install. Unless I've got this all wrong.
 

pdx_m3s

Active Member
May 18, 2019
1,356
1,184
Portland, OR
How are you tying in the AC power coming our of the solar inverters into your panel? Generation panel? Your 150 amp panel won't be able to accept that much solar due to the 120% rule... it wouldn't even be able to accept half of what you're considering.
 
Last edited:

Patrick66

Member
Oct 27, 2019
74
42
Honolulu
The utility uses different criteria in sizing things; they look at damage curves and are willing to let things run to the edge— where it does not pose a fire risk for occupied structures. Your 150A seevice will never actually draw 150A... except for maybe a few minutes over the course of a year. Their wires are sized for that reality to help keep service costs low.

Once you start generating a full day’s worth of power and dumping that back to the grid over a few hours, their math starts to break down.
 

sleevemedia

Member
Jul 1, 2020
57
41
Orlando
How are you tying in the AC power coming our of the solar inverters into your panel? Generation panel? Your 150 amp panel won't be able to accept that much solar due to the 120% rule... it wouldn't even be able to accept half of what you're considering.

I'm fine with installing a new panel to deal with the 120% busbar limits, I'm in Florida where an outdoor panel with the main service disconnect is right next to the meter, then what most people would call a main panel is in the garage and it does not have a subpanel breaker in it. The inverters will install outside next to the main. Replacing that panel is nowhere near the cost of a full MPU.

For the 120% rule, I just need a busbar able to handle the flow from the inverter breaker to the main breaker so that I can send to grid.

Where I get more concerned is with exceeding what the service lateral and transformer can handle.
 

sleevemedia

Member
Jul 1, 2020
57
41
Orlando
But wouldn't Duke want the service lateral gauged to handle my maximum unused production just in case the batteries are full? Or in case I switch to battery and send everything I produce to the grid?

I may end up having to get PWs anyway, Duke Florida appears to be on a trend toward TOU. They provoked me into reading their smart grid propaganda this morning because my daily email from USPS shows I have a promo piece from them on smart grid coming today. I already have a smart meter and get all the monitoring benefits without having to navigate TOU.

I was really hoping to let mass market battery technology mature a couple more years.
 

BrettS

Active Member
Mar 28, 2017
2,128
2,539
Orlando, FL
I’m with Duke in central Florida as well and I saw that 90% thing in the documentation as well. In my case, I already had 200A service, though. My 15.12kW system with 4 powerwalls was installed a little less than 3 weeks ago and I still don’t have my PTO. Once I get the PTO I’ll let you know how it went and if any service upgrades were needed. At this rate it may still be a few weeks though.

On a side note, given that I do have the powerwalls, I’d love to see a TOU plan become available in this area. As far as I know, though, they haven’t really been moving in that direction at all.
 

sleevemedia

Member
Jul 1, 2020
57
41
Orlando
BrettS, when did Tesla file your interconnect application, around the same time as the permit? I called Duke Renewable and managed to get a ticket submitted for an install engineer, but it was closed because I didn't have an application yet.
 

BrettS

Active Member
Mar 28, 2017
2,128
2,539
Orlando, FL
In my case Tesla submitted the permit request on 5/29 and my city approved it on 6/2. They called to schedule the install very soon after that. As I recall I think they called on 6/3 and the install was scheduled for 6/22. Then on 6/13 I got an email from duke energy asking me to sign the interconnection agreement. I don’t know if they actually submitted the request on 6/13 or if they submitted it earlier and it took duke a while to request my signature. I didn’t see any dates that told me when it was initially submitted.
 

sleevemedia

Member
Jul 1, 2020
57
41
Orlando
OK, so front side of the install, good. One of the Tesla advisors described Duke as a restrictive utility and explained that means they can't set foot on site until Duke approves. I'm going to guess Duke looked at your 200A and 15.12kW and didn't see a reason to come out or call you.

I'd hope Tesla wouldn't follow through with an install without knowing whether their customer requires service upgrades. With their agreement that provides a full refund up to 7 days after PTO, it'd be pretty expensive to install systems only to find out that the customer can't swing the cost of the required utility upgrades.

Hopes and guesses, let's see what reality brings. As it is, the advisors told me I won't know if they can install my inverters inside and get a subpanel for them until the crew lead walks through the site. I'd rather not have them outside on the south wall, but my indoor panel is full so I'd either need a sub with a separate run to the outdoor panel or the AC outputs from the inverters could run back to the outdoor panel. I haven't approved my roof layout yet, trying to add a few more panels, so they haven't done the electrical design or the single line diagram.
 

BrettS

Active Member
Mar 28, 2017
2,128
2,539
Orlando, FL
OK, so front side of the install, good. One of the Tesla advisors described Duke as a restrictive utility and explained that means they can't set foot on site until Duke approves.

I’m not sure if that’s the case. When I noticed that the city had approved my permit I called tesla and I said “Hey, they permit has been approved, can we schedule the install now?” He checked and saw that the permit was approved and then said “We need to wait for utility approval, then we can get your install scheduled”. I thanked him and hung up, but a short time later I got a message that said “I just checked and we actually don’t need preapproval from your utility and we can go ahead and schedule the install now.” Shortly after that I got a call from the scheduling team.
 

Alset2

Supporting Member
Supporting Member
Feb 20, 2020
112
32
Ocala. Fl
I’m not sure if that’s the case. When I noticed that the city had approved my permit I called tesla and I said “Hey, they permit has been approved, can we schedule the install now?” He checked and saw that the permit was approved and then said “We need to wait for utility approval, then we can get your install scheduled”. I thanked him and hung up, but a short time later I got a message that said “I just checked and we actually don’t need preapproval from your utility and we can go ahead and schedule the install now.” Shortly after that I got a call from the scheduling team.
Brett, was wondering how this turned out?
I am currently waiting on Duke after my 18kw install (200amp service) and was just curious if you needed any upgrades from Duke?

Thanks
 

Flick75

Member
Aug 11, 2020
74
38
Hawaii
So if I math right, 150 amps, 120 volts, that's 18kW. I've ordered XL. It's not the 16.32 vs. 16.2 (90% of 18) that concerns me, it's that I want to add panels.

Technically it's 18kW per leg and I'm assuming you have two phases of 120V so the total power for 150 amp service for two phases is 36kW. It would be 90% of that number which is 32.4kW or as you said 16.2kW x2. 200 amp service would be 48kW, 24kW per leg.
 

BrettS

Active Member
Mar 28, 2017
2,128
2,539
Orlando, FL
Brett, was wondering how this turned out?
I am currently waiting on Duke after my 18kw install (200amp service) and was just curious if you needed any upgrades from Duke?

Thanks

I did not need anything from Duke. It took forever for Tesla to request PTO, but once they did Duke came out and did the inspection and a week or two after that they came out again and replaced my meter and then I was good to go.
 

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