Derating for PV, When is it Required

[dofdear]

Hello,
Building a house with PV. The Main Service Panel (MSP) and service coming to the house is 400 Amps. The MSP has two 200 Amp main breakers, one that protects the CBs in the MSP and the other that services a 200 Amp Sub-Panel (SP) in the middle of the house. The busbar in the MSP is totally independent of the branch that feeds the SP. The MSP busbar is rated at 200 Amps. A majority of house circuits are feed into the SP as a cost savings measure (wire is expensive these days).

The PV is 12 Kw and has 3 strings each with a 20 Amp CB into the MSP, or 60 Amps total. The other circuits in the MSP sum is 120 Amps so the total value of the CBs in the MSP is 180 Amps.

Here is my question - Do we need to de-rate the 200 Amp CB that services the MSP since the max load in theory could only be 180 Amps? The Solar Company is asking for a 175 Amp CB in that position but I think they are just trying to sell me a CB. I don't get it?

Thanks in advance, BillyBob

 

[winnie]

You will need to investigate which version of the code applies to your home, because things have changed several times.

The general rule is commonly called the 120% rule, and would limit you to a 40A solar breaker unless you reduce the size of the main breaker.

See https://www.electricallicenserenewa...ation-Courses/NEC-Content.php?sectionID=972.0

In the grey box there are three options, a) b) and c). Option a) limits main + solar to 100% of bus rating, but permits breakers anywhere. Option b) is the 120% rule but limits the PV breaker to the opposite end of the bus from the main. Option c) may apply in your case.

Doing what the installer suggests might be the best approach, because option c) greatly limits what you can add to that panel in the future.

Ask your installer which code version is being followed (which depends on who issues the permits), and if 2017 or later, ask your installer if option c) would work for you.

Jon

 

[dofdear]

You will need to investigate which version of the code applies to your home, because things have changed several times.

The general rule is commonly called the 120% rule, and would limit you to a 40A solar breaker unless you reduce the size of the main breaker.

See https://www.electricallicenserenewa...ation-Courses/NEC-Content.php?sectionID=972.0

In the grey box there are three options, a) b) and c). Option a) limits main + solar to 100% of bus rating, but permits breakers anywhere. Option b) is the 120% rule but limits the PV breaker to the opposite end of the bus from the main. Option c) may apply in your case.

Doing what the installer suggests might be the best approach, because option c) greatly limits what you can add to that panel in the future.

Ask your installer which code version is being followed (which depends on who issues the permits), and if 2017 or later, ask your installer if option c) would work for you.

Jon

Thanks for responding Jon. The permit application identifies NEC 2020. So if I'm understanding your response and Option c), since the total of CBs in the panel, including the PV system, do not exceed the busbar rating then I should not need to de-rate the MSP 200 Amp CB. Do I understand correctly? BillyBob

 

[winnie]

Keep in mind that this is explicitly not a DIY forum, and I am prohibited from giving you a solid answer more than 'ask your electrician'.

The issue is that there may be aspects of the installation that don't match my assumptions and which you as a non professional don't even know to ask about.

This is why my answer is 'Option c appears to apply, discuss this with your professional installer to confirm. Additional option c severely limits future loads that can be installed in that panel board, and your installer might be pushing for option b as a good design decision even if option c is allowed. '

Jon

 

[jaggedben]

The PV is 12 Kw and has 3 strings each with a 20 Amp CB into the MSP, or 60 Amps total. The other circuits in the MSP sum is 120 Amps so the total value of the CBs in the MSP is 180 Amps.

This situation is allowed in the 2020 NEC without changing the 200A main breaker. The rule is 705.12(3)(3). You are required to put a label on the panel and it would not be allowed to add more than an additional 20A breaker. i.e the total which you say is 180A may not exceed the 200A busbar rating.

Here is my question - Do we need to de-rate the 200 Amp CB that services the MSP since the max load in theory could only be 180 Amps? The Solar Company is asking for a 175 Amp CB in that position but I think they are just trying to sell me a CB. I don't get it?

This allows the connection under a different rule, 705.12(3)(2). If you do this there is no limit to the load you can add to the busbar as long as the calculated load (which is usually much less than the sum of breakers) were not to exceed the 175A main breaker rating. This rule is the more common way to do things so I'm not surpised they're suggesting it.

The solar company may not be aware of the first rule: it's a newer and harder to understand rule, and I've met many solar installers and inspectors alike who don't know it. Or they may not be aware that you only have 180A of breakers and assume there will be more. Or, since you say you are 'building' the house (implying construction is underway), perhaps they don't trust that there won't be changes and the electrician won't add more breakers during construction, such that the first rule gets violated.

 

[PWDickerson]

There may be other options to consider. Lots of residential service gear is being made with 225A bus bars. Your should check to see if this particular unit is made that way. If so, problem solved. Another possibility is to consider a feeder tap. Sounds like you have a set of 200A conductors to a sub-panel that is fed from a 200A breaker straight from the meter. If this is the case, you could tap the feeders for the PV interconnection.