Minimum Requirements of the NEC With PV Systems

Don’t let the title of this blog fool you. This is not about how to get by with minimum compliance to NEC when designing and installing PV systems. It is about the dangers of choosing that minimum over safety.

There are contractors who look harder at the financial bottom line than at safety. That is a dangerous mistake.

“The purpose of the National Electrical Code (NEC) is the practical safeguarding of persons and property from the hazards arising from the use of electricity.” The NEC is written by the National Fire Prevention Association (NFPA) with updated versions published every three years.

When you read articles and sections of the NEC that apply to the electrical construction work in which you are undertaking, first look at the code, then look at the intent of that code. If there is a potential for an unsafe practice, err to the side of safety not to the minimum standard.

The NEC is adopted by states and local jurisdictions and enforced as electrical construction law. It is up to each state to choose when to adopt later versions of the NEC; neighboring states may be using different code versions. It is the inspector’s job to know and understand the code and at times correctly interpret the meaning of the applicable code sections. When two sections of the code differ on the same topic, the good electrical contractor should choose the safer one.

I bring this up because of various situations where the code can be interpreted in more than one way and I want to illustrate one in particular; making electrical taps on the supply side of the main service disconnect. On residential services, the main service disconnect should be located adjacent to the utility meter. This has not been the requirement or standard in the past.

Older homes may have the meter on the side of the house and the main service disconnect/overcurrent device (OCD) somewhere in the middle of the house; that service disconnect/OCD is located in the top of the main electrical branch-circuit load-center. This means there is no protection for the conductors from the meter to the location of the service disconnect.

In more recent years, the max allowable unprotected service conductor length was shortened to 6 feet and the conductors were usually protected with PVC conduit. The latest NEC code requires the main service disconnect to be on the outside of the structure. The problem is that the old installations that would not meet today’s code are grandfathered in; they don’t have to be upgraded unless they are being completely changed. This can make for dangerous situations with older homes.

There are several code sections that apply to making a tap on the service conductors where the interactive inverter AC output connection can be made:

Article 240.21 Location in Circuit. This article specifies the location of the overcurrent device.

Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 240.21(A) through (H).

Article 240.21(B)(1) and (2) covers taps with residential and most small commercial PV installations.

Both (1) and (2) require the tap conductors to be placed in an approved raceway if they leave the enclosure where the tap is made. The tap conductors must be rated for the load they supply. (1) The conductors cannot be longer than 10 feet in length from the tap and they cannot be rated less than 10% of the OCD protecting the feeder conductors where the tap has been made. (2) The requirement is similar to (1) except the conductor cannot be longer than 25 feet and can be rated less than 33% of the OCD rating protecting the service conductors where the tap has been made.

NOTE: The big stand-out here is the reference to the OCD protecting the service conductor where the tap is made. If this is an older home with the service disconnect/OCD located other than outside the structure and it is also located in the top of the AC load center, the tap will be made to the conductor before the service OCD. This violates the tap rules. It is here where the inspector can set rules for safety such as requiring the tap to be made on the outside of the structure or allow the tap to be made at the AC load center located within the structure if the service cable is protected in a raceway.

There is another section of the code that also addresses connection to the supply side of the service disconnect/OCD.

That is Article 705.12(A) Interconnected Electrical Power Production Sources. Supply side.

It states that that an electrical power production source can be connected to the supply side of the service disconnecting means. It also states that the sum of the ratings of all overcurrent devices connected to power production sources cannot exceed the rating of the service.

A third section of the NEC addresses the OCD of the power source connected to the supply side of the service disconnect/OCD.

705.31 Location of Overcurrent Protection.

This article was added in the 2014 NEC and removed in the 2020 NEC; that means two versions of the NEC have this article, the 2014 and 2017 versions.  The article states that the overcurrent protection for electric power production source conductors that are connected to the supply side of the service disconnecting-means in accordance with 705.12(A), shall be located within 10 ft of the point where the electric power production source conductors are connected to the service.

Article 705.31 gives permission to make a tap to the supply side of the unprotected service conductors within the structure. If a short circuit occurs between any of these line conductors, the full force and power of the utility transformer is released. That is more than enough energy to create a fire ball that will ignite whatever is around it; a structure fire will occur. If this tap is made inside the main AC load center and the tap conductors are placed in an electrical raceway, it will be much safer.

In Summary, Article 240.21(B) tap rules should be followed even when the situation is for supply side taps without the service disconnect/OCD protecting the service conductors. Where Article 705.31 applies, protect the conductors in conduit; metal conduit will provide an extra element of protection. Or install overcurrent protection at the location of the tap; directly adjacent to the main load center where the tap was made.

Kelly Provence
NABCEP Certified Master PV Professional Installer
NABCEP Certified PV Inspector
IREC Certified Master PV Trainer

Solairgen
www.solairgen.com