Main panel sizing and city permit load calcs question (Socal)

[mrwassen]

Hi guys,

I was hoping to get a little feedback on my planned solar installation as it relates to load calcs and service panel sizing. I plan to install a 7.200 kW system (18 panels x 400 watts) with microinverters and have 2 questions:

1) Is my main panel adequate to support the planned PV output?
2) Why is the city asking me to add 40 amps to my residential load calc?

1) Is my main panel adequate to support the PV output?
====================================
Main service panel is an Eaton "solar ready" MBED2040PV125BF
bus = 225 amps
main breaker = 125 amps

Per the "120% rule" the maximum PV output the panel can support is:

((Busbar Rating (A) x 1.2) - Main Breaker Rating (A) )/1.25 = Max PV (A)

Max PV (A) = ((225 amp x 1.2) – 125 amp) / 1.25 = 116 amp
Max PV (kW) = 116 amp x 240 / 1000 = 27.84 kW

Since this is well beyond the planned system of 7.2 kW, I believe this is more than adequate and no derating is required etc. - could this be confirmed please?


2) Why is the city asking me to add 40 amps to my residential load calc?
==============================================
As part of the project I also plan to install EV charger. When applying for a city permit, the city's load calculation form shows all of the standard entries by square footage, appliances max. of non-coincident loads etc. per NEC standards.

But at the very end of the form they have included a line labelled "40 amp back-feed protection for future PV system (solar panels) +40 Amps".

So they basically require 40 amps to be added to the standard load calculation to account for solar.

Does this really make sense? I did a fair amount of googling and nowhere else (other municipalities etc.) do I see any versions of the NEC load calcs that include this. I just don't get why the solar output (which is supply) needs to be counted as load?? I have a feeling this is not correct. Any thoughts/feedback would be appreciated.

Thanks

Dennis

 

[Think About It]

1) Is my main panel adequate to support the planned PV output?

Your bus bar can support the additional solar which is 30A @ 240 VAC.

2) Why is the city asking me to add 40 amps to my residential load calc?

Good question. Ask them them to site the NEC code.

The NEC specifies how size a residential service. Basically you use so much power per square foot plus heating, cooling and appliance loads. (This is a very simplified explanation.) Since you are adding a substantial appliance load (EV Charger) they may be asking you to recalculate and resize your service. I would simply ask your inspector for clarification. Most of them love when you ask them questions as it demonstrates that you want to be code compliant.

 

[mrwassen]

Your bus bar can support the additional solar which is 30A @ 240 VAC.

Good news - thanks!

Good question. Ask them them to site the NEC code.

Thanks for the input - I have emailed the head of our building dept. and will update the post once I hear back.

 

[zanydroid]

Forcing you to do a load calculation is a favor. You likely would have trouble fitting in an EVSE without load management on 125A service, all electric appliances. And it is good to surface this early before you get in trouble.

For the odd solar line item: It’s possible your city wants to ensure that all houses are solar ready, but has chosen to express it in a super awkward way.

 

[mrwassen]

Actually my load calculation comes in quite comfortably - grand total per NEC standards is 95 amps with EV charger - if the city insists on adding the 40 amps, I will instead have an RMA installed which eliminates the 40 amp issue. However I feel is is a complete waste of $1362 (cost of the RMA installed by SDGE), so my preference is to connect the solar to the bus/load side as is the usual method.

 

[zanydroid]

Hmm sounds like a couple of gas appliances helping out. I’m at 90A with load management deleting the EVSE.

The 40A makes no sense. Except as reserving space on the bus bar for solar as I surmised. But that doesn’t require a service upgrade. I’m in Northern California with PG&E and I can backfeed 30A with 100A service on my current setup. I’m working on upping this to 60A with my current project.

What is an RMA?

 

[mrwassen]

Yes I do still have some gas appliances.

With my 125 amp / 225 amp bus panel I can backfeed 3 x my actual PV output so its a bummer that I might have to install the RMA. Agree that the 40 amp add-on makes no sense. Just as a sanity check I reviewed around 10-12 other Socal cities load calc forms (including Irvine which is Socal's ground zero for EV charger permits / EV's etc.) and none of these have the add-on. So most likely this was implemented by somebody who doesnt have a good understanding of these things.

 

[zanydroid]

You can connect up to 100A (125A MBK * 80%) of AC output. The bottleneck is not the busbar.

EDIT: assuming SDGE is OK with provisioning that much backfeed potential to you.

 

[zanydroid]

Unless your city has amended California code you should be able to eventually win on this.

I would suggest sending in the NEM2 interconnection paperwork while you haggle with the city on electrical (or set a time box on when you’re willing to wait). Assuming your roof mechanical and fire clearances are OK, this is important bc it maps to how many panels you have.

 

[mrwassen]

You can connect up to 100A (125A MBK * 80%) of AC output. The bottleneck is not the busbar.

Actually I believe it is 116 amps per the 120% rule (see my calcs in the original post), but no reason to haggle about this, in any case the allowed PV output is many times what I actually need (30 amps) so I should be good to go.

 

[zanydroid]

That would overload the MBK past 80%. Solar is supposed to be calculated as a continuous load.

 

[mrwassen]

Here is a great explanation of the 120% rule:

https://www.planetplansets.com/required-information/

Scroll down approx. 2 pages.

 

[zanydroid]

I’ve gone through load side connections portion of the code with a fine tooth comb, I’m fairly comfortable with my understanding of 120% rule.

I’m saying there are additional constraints beyond 120% rule.

Slightly simpler case. Suppose you only have a 100A service or feeder. 225A busbar. 120% rule says you can do 136A. The service/feeder generally needs OCPD, so that is a 100A breaker. You run the risk of tripping the main breaker.

Overall conductors and breakers are not to carry more than 80% ampacity if subject to a continuous load. This goes beyond solar.

(also 120% isn’t the only way you can get a panel config approved, you can go well over what 120% rule says with carefully crafted setup and sum of breakers rule)

 

[mrwassen]

OK getting a little silly here - but I have checked against a multitude of sources, and have always reached the same result.

Let's just leave it there...

 

[HarryN]

CA eventually wants everyone to turn off the natural gas and run in all electric mode.

I wonder if that is why they are pushing on you to increase the amps rating?

 

[mrwassen]

Hard to imagine that - no other city here in Socal including major ones such as Anaheim, San Diego and Irvine have that provision, so most likely its more of a misunderstanding/knowledge gap, but who knows.

By the way the major gas appliance remaining is the old gas furnace. We plan to replace this and the old AC unit with a heat pump system for heating and cooling. This will not change much on our load since the AC goes away.

 

[mrwassen]

You likely would have trouble fitting in an EVSE without load management on 125A service, all electric appliances

Although adding an EV charger would be feasible on my 125 amp main panel (but just barely), I would admittedly have issues adding other applicance upgrades such as heat pump dryer, heat pump hot water heater etc.

Enter load management (thanks to zanydroid for pointing this out).

After researching this (to me new) option, I found this device:

DCC-12 EV load management device

which should allow the installation of a Tesla Wall Connector configured in highest delivery mode (11.5 kW / 48 amps) without impacting the load calc on the 125 amp main panel (in other words the EV charger would not consume any main panel capacity).

Has anybody had any experience using this device?
Are there other better alternatives for load management?