Derating Main Breaker - Residential - Does Anyone Do it?

[jmoles]

I'm in the process of installing solar and my plans derate the MSP main breaker from 200 to 175 because the backfed breaker is 60 amps.

Today I had a licensed electrician tell me I don't need to put in a 175 amp breaker and I'm fine leaving it. His argument was that no inspector ever looks at it and practically the bus bar would never be overloaded. He also said he'd need to pull a heavy up permit in order to replace the MSP main breaker.

I plan to call my POCO / AHJ / Inspector to get clarity on this but I'm curious to hear other people's experiences.

Is it common to skip derating breakers? Can the meter be pulled without the need for a heavy up permit? I though the electrical permit for the solar project covered the derating breaker (it's included in the electrical plans which were permitted for this project).

 

[tom baker]

I will allow this as the OP is asking for advice only. It’s a good question

 

[Carultch]

I'm in the process of installing solar and my plans derate the MSP main breaker from 200 to 175 because the backfed breaker is 60 amps.

Today I had a licensed electrician tell me I don't need to put in a 175 amp breaker and I'm fine leaving it. His argument was that no inspector ever looks at it and practically the bus bar would never be overloaded. He also said he'd need to pull a heavy up permit in order to replace the MSP main breaker.

Not surprising. One who doesn't typically do work involving Article 690 (solar PV) or 705 (interconnected power sources), might not be familiar with this rule. The run-of-the-mill panelboard typically has matching main breaker and busbar ratings, and an electrician or EE who specified it, likely wouldn't see any reason to do otherwise.

Sizing panels in general, uses a nuanced approach of adding up loads, based on expected user behavior. The main breaker is meant to be the failsafe, in the event that the load ever exceeds the original estimate.

Introducing another power source to the panel, creates a blindspot where the main breaker can't detect an overload. The 120% rule in 705.12(B), limits you to a total of 240A of sources feeding a 200A busbar, taking credit for properly locating the two sources within the equipment. Dropping the main breaker to 175A, expands your original limit of of the interconnected system size from 40A, to 65A. 120% is an industry compromise for feeding a panel from two sources, and is one of the options you have for justifying a panel's code compliance.

Estimating or calculating the load, doesn't make a difference in whether the main breaker can remain at 200A. It can justify reducing it to 175A, but it still is required to reduce it to 175A to connect this particular system.

 

[ggunn]

I'm in the process of installing solar and my plans derate the MSP main breaker from 200 to 175 because the backfed breaker is 60 amps.

Today I had a licensed electrician tell me I don't need to put in a 175 amp breaker and I'm fine leaving it. His argument was that no inspector ever looks at it and practically the bus bar would never be overloaded. He also said he'd need to pull a heavy up permit in order to replace the MSP main breaker.

I plan to call my POCO / AHJ / Inspector to get clarity on this but I'm curious to hear other people's experiences.

Is it common to skip derating breakers? Can the meter be pulled without the need for a heavy up permit? I though the electrical permit for the solar project covered the derating breaker (it's included in the electrical plans which were permitted for this project).

What matters is 125% of the maximum inverter current and the rating of the panel busbar. To comply with the NEC 120% rule, if the 200A main breaker rating plus 125% of the max inverter current exceeds 120% of the rating of the busbar, then by NEC rules you must derate the main breaker, move the interconnection to the line side of the main breaker, or find another provision in 705.12 with which you can comply. Whether or not an inspector will notice a 705.12 violation is immaterial; if anything untoward happens because of the code violation it is you who is on the hook for damages, not the inspector.

That said, some panels have busbars with a higher rating than the panel rating. For example Eaton CH panels rated from 150A to 225A all have busbars rated at 225A; do some research on the panel in question to see if that might be true in your case.

 

[jaggedben]

I'm in the process of installing solar and my plans derate the MSP main breaker from 200 to 175 because the backfed breaker is 60 amps.

Today I had a licensed electrician tell me I don't need to put in a 175 amp breaker and I'm fine leaving it. His argument was that no inspector ever looks at it and practically the bus bar would never be overloaded. He also said he'd need to pull a heavy up permit in order to replace the MSP main breaker.

I plan to call my POCO / AHJ / Inspector to get clarity on this but I'm curious to hear other people's experiences.

Is it common to skip derating breakers? Can the meter be pulled without the need for a heavy up permit? I though the electrical permit for the solar project covered the derating breaker (it's included in the electrical plans which were permitted for this project).

I've derated main breakers to meet the code dozens of times. I don't know what exactly a 'heavy up permit' is, sounds like some slang I don't know. Perhaps it refers to an energized work permit or a process with the utility. But I've never not done a main breaker downsize for that sort of reason. Around here there is a process to request a temporary utility disconnect or a meter pull to do it (although to be honest, many times we just pull and replace the meter ourselves and call for the temporary disconnect if the inspector wants the meter tagged again. Depends on the utility though, some really don't like it).

The bottom line is that if you don't ever increase load - or really, deliberately increase load when solar producing - then there is no actual safety issue. But on the other hand if you don't downsize the breaker then you don't have a code compliant install. There's also always the chance your electrician gets the one city inspector who knows the code and has to do it anyway.

 

[tthh]

At least in Dominion Virginia Power territory, heavy up was commonly used to describe a POCO service upgrade -- e.g. 100A to 200A.

 

[jaggedben]

At least in Dominion Virginia Power territory, heavy up was commonly used to describe a POCO service upgrade -- e.g. 100A to 200A.

That makes sense ... except it doesnt. A main breaker downsize is usually about 10 mins work with the meter pulled. It shouldn't require the same level of permit as a service upgrade. Of course I can't speak to what red tape they have in Virginia or Maryland, so maybe they make it that difficult. Around here it's a phone call to schedule the meter pull with the utility, and there's no extra permit or fees beyond the normal permit for the solar install.

 

[jaggedben]

I would try to find out from the utility if they have a separate process for this. If you do it right by the utility I don't think it's any of the building department's business how you did it. Of course that's just my opinion.

 

[Elect117]

I am personally not a fan of when contractors say, "inspectors rarely notice it anyway". I would start with asking the engineer on the plans. Maybe they can choose a different calculation method where you don't need to reduce the main breaker. After that, I would ask the city inspector.

 

[ggunn]

I am personally not a fan of when contractors say, "inspectors rarely notice it anyway".

As a professional engineer, I absolutely cannot take that position.

 

[solarken]

I'm in the process of installing solar and my plans derate the MSP main breaker from 200 to 175 because the backfed breaker is 60 amps.

Today I had a licensed electrician tell me I don't need to put in a 175 amp breaker and I'm fine leaving it. His argument was that no inspector ever looks at it and practically the bus bar would never be overloaded.

Is it common to skip derating breakers?

No, it's not common to skip downsizing the main breaker if the backfed inverter size causes the 120% busbar rule to be exceeded. Your electrician is wrong. What is common is to switch to a supply side connection, by tapping the mains in the load center, instead of back feeding a breaker. Down sizing the main breaker is fine too as long as you can find the breaker needed for your load center.