Beatrice Lang
Derating an electrical panel refers to reducing the power rating of the main breaker. Each electrical panel has a maximum solar capacity, and derating can help avoid the cost of upgrading the panel to a higher rating. The process involves lowering the potential draw of the entire set-up to allow room for a new power source from batteries. For example, if you have a 125A electrical service, the total amperage feeding the panel must not be higher than 120% of that, according to the 120% rule in the 2011 National Electric Code. This means that a 125A service with a 125A breaker only has 25A left for a solar system. However, if you want to install a 50A solar system, you can derate the panel by reducing the power rating of the main breaker to 100A, thus making 50A available for solar.
| Characteristics | Values |
|---|---|
| Definition | "Keep the more-capable thing, but treat it as if it were the less-capable thing" |
| Purpose | To avoid the cost and time of a main panel upgrade |
| Application | Applicable when installing solar |
| 120% Rule | The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120% of the rating of the busbar or conductor |
| Example | If you have a 125A electrical service, the total amperage feeding the panel must not be higher than 120% of that (i.e., 150A). If you want to add a 50A solar breaker, you can reduce the power rating of the main breaker to 100A, leaving 50A for solar. |
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What You'll Learn
- Derating is lowering the potential draw to allow room for a new power source
- The 120% rule limits the sum of the main and PV breakers to 120% of the busbar ampacity
- A derate can be a cheaper alternative to a main panel upgrade
- A derate is not always possible, especially if the main panel is outdated or needs to be moved
- The rating of the bus bar needs to be considered when installing a PV supply
Derating is lowering the potential draw to allow room for a new power source
Derating an electrical panel is a process that helps to create more capacity for a new power source, such as solar, without the need for a full upgrade.
Each electrical panel has a maximum solar capacity, and an amperage rating, which indicates how many appliances or electrical units can be run simultaneously. When installing solar, the electrical panel must have enough amp space to handle the load. If not, it usually needs to be upgraded to a higher rating. However, derating can be a simpler and less expensive solution.
The process of derating involves lowering the potential draw of the electrical panel, creating room for the new power source. For example, a 200A panel with a 200A main and a 40A breaker for PV interconnection would be at maximum capacity. By derating to 175A, there is now room for 65A of generation, allowing for the addition of a new power source. This is known as the 120% rule, where the sum of the PV breaker and the main breaker must not exceed 120% of the busbar ampacity.
It is important to note that derating a main breaker is not always a solution, especially if the main panel is no longer up to code requirements or needs to be relocated. In some cases, it may be necessary to upgrade the entire main panel to accommodate a new power source.
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The 120% rule limits the sum of the main and PV breakers to 120% of the busbar ampacity
The 120% rule is a concept in electrical engineering that pertains to the safe operation of electrical panels, also known as panelboards or busbars. It is a safety measure designed to prevent overloading, overheating, and potential failure of the electrical system.
The rule states that the sum of the currents from the main breaker and the solar (PV) breaker must not exceed 120% of the ampacity of the busbar, or in other words, the maximum current capacity of the electrical panel. Ampacity refers to the maximum amount of electric current a conductor can carry continuously without exceeding its temperature rating.
For example, consider an electrical panel with a 200-amp bus rating. In this case, the 120% rule would limit the combined current from the main breaker and the PV breaker to 240 amps (200 amps x 120% = 240 amps). This means that if the main breaker is drawing 200 amps, the PV breaker can only contribute up to 40 amps (240 amps - 200 amps = 40 amps) to stay within the safe limits.
The rule is particularly important when dealing with solar circuits or any back-fed circuits, which are considered sources rather than loads. If the main breaker and the PV breaker are located at opposite ends of the busbar, the total current at any point on the busbar will be the sum of the currents from both sources. This is known as Kirchhoff's current law. By ensuring that the combined current does not exceed 120% of the busbar's ampacity, the 120% rule helps prevent overloading and potential damage to the electrical system.
It is important to note that the specific code and implementation of the 120% rule may vary depending on local regulations and standards. Electrical work should always be performed by qualified professionals who can ensure compliance with the relevant codes and safety standards.
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A derate can be a cheaper alternative to a main panel upgrade
Each main electrical panel has a maximum solar capacity. To exceed this capacity, the panel usually needs to be upgraded to a higher rating. However, in some cases, a derate can be a cheaper alternative to a main panel upgrade.
A derate involves reducing the power rating of the main breaker, or in other words, lowering the potential draw of the electrical panel. This allows room for a new power source, such as solar power. For example, if you have a 125A electrical service, the total amperage feeding the panel must not be higher than 120% of that, or 150A. This means that a 125A service with a 125A breaker only has 25A left for a solar system. By reducing the size of the main breaker to 100A, you now have 50A available for solar.
The 120% rule, as outlined in the 2011 National Electric Code, states that "the sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120% of the rating of the busbar or conductor". This means that the amperage rating of power supplied to the main panel must not exceed 120% of the service's rating.
It is important to note that a derate is not always a solution, especially if the main panel is no longer up to current code requirements or needs to be relocated. Additionally, a derate for solar cannot be undone without a full main panel upgrade. Nevertheless, a derate can be a cheaper and less time-consuming alternative to a main panel upgrade, bringing quicker solar savings.
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A derate is not always possible, especially if the main panel is outdated or needs to be moved
The process of derating a main electrical panel involves reducing the power rating of the main breaker. This can be done to avoid the cost and time of a full main panel upgrade, especially when installing solar.
Each main electrical panel has a maximum solar capacity and an amperage rating. The amperage rating indicates how many appliances or electrical units can be run simultaneously. When installing solar, some electrical panels may not have enough amp space for the system, and the panel needs to be upgraded to handle the excess load.
However, a derate is not always possible, especially if the main panel is outdated or needs to be moved. If the main panel is no longer up to current code requirements, a derate may not be a viable solution. In such cases, a full main panel upgrade may be necessary to accommodate the solar system.
Additionally, the rating of the bus bar needs to be considered when installing a solar supply. The 120% rule, outlined in the 2011 NEC (National Electric Code), states that the sum of the ampere ratings of overcurrent devices supplying power to a busbar or conductor must not exceed 120% of the rating of the busbar or conductor. This rule must be considered when determining whether a derate is possible.
It is important to consult with electrical professionals to determine the best course of action for your specific situation. They can advise on whether a derate is possible and necessary or if a full main panel upgrade is required.
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The rating of the bus bar needs to be considered when installing a PV supply
When installing a PV supply, it is essential to consider the rating of the bus bar to ensure the system functions effectively and safely. The bus bar is a critical component in the distribution of electricity, and its rating plays a vital role in determining the overall performance of the PV supply.
The bus bar is a conductor that connects various electrical components within a system. It provides a common path for electrical current to flow between different devices, such as the main breaker, generators, and the main panelboard. The rating of the bus bar, typically measured in amperes (amps), indicates its maximum current-carrying capacity. This rating is crucial because it sets a limit on the total amount of electrical power that can be safely distributed through the system.
When installing a PV supply, such as solar panels, the bus bar's rating becomes even more critical. This is because the PV system generates additional electrical power, which needs to be accommodated within the overall electrical setup. The National Electric Code (NEC) specifies the well-known "120% rule," which states that the sum of the ratings of the devices supplying power to the bus bar should not exceed 120% of the bus bar's rating. In other words, the combined amperage of the main breaker and the PV breaker must not surpass 120% of the bus bar's ampacity.
For example, consider a 200-amp bus bar with a 200-amp main breaker. Without any PV system, this setup can safely handle a maximum of 240 amps (200 amps x 120%) of power. If you now want to add a PV system with a 40-amp breaker, the total amps become 240 (200 + 40), which is still within the allowable limit. However, if you want to add a larger PV system with, say, a 60-amp breaker, the total amps become 260 (200 + 60), exceeding the 120% limit. In such cases, derating the main breaker becomes necessary.
By derating the main breaker, you intentionally reduce its power rating to create more capacity for the PV system. In the previous example, if you derate the 200-amp main breaker to 175 amps, you can now add the 60-amp PV breaker, resulting in a total of 235 amps (175 + 60), which complies with the 120% rule. This approach allows for the integration of a larger PV system without upgrading the entire main panelboard, saving time and costs.
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Frequently asked questions
Derating an electrical panel means reducing the power rating of the main breaker. This allows for the addition of a solar system without upgrading the entire panel.
Derating can be a cheaper and quicker solution than upgrading the entire panel. It also allows for the addition of a larger solar system.
Each electrical panel has a maximum solar capacity, which is determined by its amperage rating. Derating the panel reduces the potential draw, creating more capacity for the solar system.
