sunMAX - Install: End-Run to Mains AC Service Equipment

Once the Solar Gateway and Microinverters have been mounted, connected and provisioned, it is time to connect the solar energy system to the customer's electrical panel.

Instructions for End-Run Install:

WARNING:

This step should be completed by/with the help of a licensed electrical contractor. When working with any electrical system, safety should always be the highest priority.

Safety Guidelines

Prior to working with the electrical box at the home, first make sure that it is turned off at the main breaker. Make use of the current tester and voltage meter to make sure that there is no active current coming from any of the wires. Also make use of the grounding wrist strap when working inside the electrical box even though the power is turned off.

Considerations

Utility Point of Connection

There are many rules and design issues that apply to the utility point of connection, usually the main electrical service panel of a residence or the meter main. This is usually where the connection takes place in some manner, either through the addition of a circuit breaker, circuit breaker panel or a tap.

The final connection to the utility is usually on the load side (customer side) of the main electrical service disconnecting means (usually the main breaker in the service panel). This allows for the solar generated power to flow into the house first and reduce the net consumption from the utility.

The final connection however may be made on the supply side (utility side) of the service disconnecting means. The power is effectively owned by the utility first, and the utility is basically just using the roof space to hold their generator. This application is generally for commercial type of applications, as most residential situations do not allow for a supply side tap.

On a residential application, this is not possible today, as the manufacturers have not made a way to allow us to tie into their system without voiding the warranty.

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 Options for System Integration to the Utility

There are three scenarios that can be considered to allow for interconnection of a solar electric system to a residential service. These are reflected in the examples above.

1. Addition of Circuit Breaker

The simplest solution is if the existing main breaker is large enough to allow for the addition of a solar breaker, to comply with the 120% rule of NEC 690.64. Even if there is no physical space for a new solar breaker, if the main is large enough then mini-breakers can be installed on existing circuits to free up enough space for a new solar breaker. 

2. Downsize Main Breaker

If the main breaker is not large enough, then it may be possible to replace the existing main breaker with a slightly smaller main breaker that will allow for the desired solar breaker to fit. An electrician must calculate the typical load of the existing circuits to determine if a smaller main can be used. Reference the NEC 220.40 through 220.61. A smaller main breaker will allow for a large enough solar breaker, as shown in the example below.

Example of downsizing main breaker:

The existing main service panel has a 200 amp busbar and 200 amp main breaker. The maximum size solar breaker that can be installed is therefore 40 amps. (see 3.14 above)

 The installer wants to install 2 x 4000 watt SMA inverters (2 x 18 amps x 1.25% = 45 amps) which requires a 50 amp solar breaker. The installer replaces the 200 main breaker with a 150 amp main breaker. This allows enough room, per NEC 690.64, for the 50 amp solar breaker. 

3. Service Panel Upgrade

If the main breaker cannot be downsized, then the installer can consider a service panel upgrade. The entire service panel is removed and replaced with a larger service and main breaker. The existing service panel may be too old and undersized for the current or future load demand as well as the ability to interconnect the solar or any additional loads as this could compromise the bus of the panel and create an overheating condition. A new service panel may not only allow for a larger main breaker and the interconnection of the desired solar system, but may result in more breaker spaces for new equipment.

Size of Solar Circuit Breaker- Residential & Commercial

There is a limit on the electrical service as to the size of a photovoltaic system and how it interacts with the electrical infrastructure.

The sum of the ampere ratings of breakers supplying power to a service panel busbar shall not exceed 120% (residential) or 100% (commercial) of busbar rating (this did change in 2008 NEC to reflect the residential 120% rule). The breakers supplying power are the service panel main breaker and the solar breaker. NEC 690.64(B)

The current rating of the solar breaker in the service panel should be the same as the rating selected for the utility ac disconnect described earlier. Solar Breaker (amps) + Main Breaker (amps) = 1.2 Busbar rating (amps) Solar Breaker = (1.2 x Busbar) – Main Breaker

Residential Examples:

  1. What is the largest rating of solar breaker that can be installed in a 100 residential service panel with a 100 amp main breaker with a 100 amp bus bar rating?
    Solar Breaker = 1.2 x 100 amp busbar – 100 amp main breaker
    120 amps – 100 amps = 20 amps available for solar breaker
  2. What is the largest rating of solar breaker that can be installed in a 100 residential service panel with a 100 amp main breaker with a 125 amp bus bar rating?
    Solar Breaker = 1.2 x 125 amp busbar – 100 amp main breaker
    150 amps – 100 amps = 50 amps available for solar breaker
  3. What is the largest rating of solar breaker that can be installed in a 200 residential service panel with a 200 amp main breaker with a 200 amp bus bar rating?
    Solar Breaker = 1.2 x 200 amp busbar – 200 amp main breaker
    240 amps – 200 amps = 40 amps available for solar breaker
  4. What is the largest rating of solar breaker that can be installed in a 200 residential service panel with a 200 amp main breaker with a 225 amp bus bar rating?
    Solar Breaker = 1.2 x 225 amp busbar – 200 amp main breaker
    270 amps – 200 amps = 70 amps available for solar breaker 
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