Wiktor Wise
The 80% rule is a term used by electricians to refer to the National Electrical Code (NEC) rule that limits the continuous current allowed on a thermal magnetic circuit breaker to 80% of its rating. This rule is in place because a thermal magnetic circuit breaker will not reliably hold 100% of its rated current continuously and is likely to trip under those conditions. The 80% rule is also used as a quick way to calculate the sizing of conductors.
| Characteristics | Values |
|---|---|
| Current rating | 20-80A |
| Max current | 80A |
| EV charging | 80A |
| Continuous loads | 80% |
| Circuit breaker rating | 80% |
| Max rated current | Imax |
| Basic current | Ib |
| Rated maximum current | Imax |
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What You'll Learn
The 80% rule is actually the 125% rule
The 80% rule is a term used by electricians to refer to the sizing of conductors in relation to continuous loads. It is a simplified way of explaining a more complicated concept. In reality, the rule is the 125% rule, as it requires continuous loads to be rated at 125% of the actual draw for the purposes of sizing a branch circuit.
The 80% rule is a quick way of explaining that, if a circuit has a continuous load, then the ampacity of the entire circuit is limited to 80% of the breaker's labelled ampacity. This is because a thermal magnetic circuit breaker will not reliably hold 100% of its rated current continuously and is likely to trip.
For example, if a coffee maker draws 29.1 amps, then a 40 amp breaker is needed. This is because 80% of 40 amps is 32 amps, which is the closest amp rating to 29.1 amps that is higher than the required amount.
The 80% rule is also used to refer to the sizing of circuit breakers to protect the wire from overheating and catching on fire. A 1000-amp breaker should, in theory, hold 1000 amps indefinitely, but in reality, the trip curve begins earlier at 800 amps.
Therefore, the 80% rule is actually the 125% rule, as it requires continuous loads to be rated at 125% of the actual draw, and the 80% value is used as a quick way of calculating the required amp rating for a continuous load.
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It applies to conductors, not breakers
In the context of electrical things, the number 80 is often associated with the 80% rule. This rule is related to the ampacity or the amount of current that a device can draw. According to the 80% rule, you can use up to 80% of a breaker's labelled ampacity. For example, if a breaker is labelled as 30 amps, you can use up to 24 amps (80% of 30) before needing to move to a higher ampacity breaker. This rule is important for ensuring that electrical circuits do not overload and cause fires.
It is important to note that the 80% rule applies to conductors and not breakers. A conductor is a material or substance that allows electricity to flow through it. Metals, such as copper and aluminium, are common electrical conductors due to their high conductivity and low resistance. Conductors have free electrons on their surface that can easily pass through, allowing the flow of electric current. The resistance of a conductor depends on the material it is made of and its dimensions; for example, a thick copper wire has lower resistance than a thin copper wire.
Conductors play a crucial role in electrical circuits, as they enable the transmission of electricity, heat, or light from one source to another. They are used in various real-life applications, such as electrical wiring, circuit boards, and high-voltage systems. Conductors also have important safety considerations, as they can overheat and catch on fire if too much current is passed through them. Therefore, it is essential to follow guidelines, such as the 80% rule, to ensure that conductors are used within their safe limits.
While the 80% rule applies to conductors, it is important to understand how it relates to breakers. A breaker, or circuit breaker, is a device that protects an electrical circuit from damage caused by overload or short circuit. Breakers are designed to interrupt the flow of electricity when it exceeds a certain threshold, typically 80% of their labelled ampacity. This interruption is achieved through trip curves, which cause the breaker to trip and stop the flow of electricity when the current reaches a certain level.
It is important to distinguish between the application of the 80% rule to conductors and breakers. While the rule provides a guideline for the safe use of conductors, it does not dictate the same for breakers. Breakers are designed to handle higher levels of current, typically up to 100% of their ampacity, and the 80% threshold is a conservative estimate to ensure the safe operation of the electrical system as a whole. Therefore, while the 80% rule is a useful guideline for conductors, it does not impose the same restrictions on breakers.
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Breakers can be purchased as 100% rated
The 80% rule, also known as the 125% rule, applies to continuous loads such as motors, lighting, or any load expected to be on for 3 hours or more. A breaker is rated for 100% of the non-continuous load, which may include outlets or other small appliances. Breakers can be purchased as 100% rated, but they come at a higher cost. The 80% rule is a good practice to follow to avoid nuisance tripping, but it is not a requirement.
The 80% rule is not explicitly mentioned in the National Electrical Code (NEC), but it is a widely accepted practice among electricians and contractors. The NEC states that a circuit breaker for a branch circuit must be rated to handle the non-continuous load plus 125% of the continuous load. This means that a breaker rated at 80% can be applied at 100% of its rating if the load will last for less than 3 hours.
The difference between an 80% rated breaker and a 100% rated breaker lies in their continuous load capacity. Both types of breakers have the same amp rating and trip curve, but they are applied differently depending on the continuous load. The 80% rated breaker can only be applied continuously at 80% of its continuous current rating, while the 100% rated breaker can be applied at up to 100% of its current rating continuously.
It is important to note that the purpose of circuit breakers is to protect the wire from overheating and catching on fire. Therefore, it is crucial to choose the appropriate breaker size and rating for the specific application. An electrical engineer should calculate the required breaker size and rating based on the expected load and duration.
In summary, breakers can be purchased as 100% rated, but they are more expensive and may not be necessary for all applications. The 80% rule is a good practice to follow to avoid nuisance tripping, and it is important to select the appropriate breaker size and rating to ensure the safety of the electrical system.
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The 80% rule does not apply to residential circuits
The 80% rule is a term used by electricians to refer to the National Electrical Code (NEC) rule that continuous loads should be rated at 125% of the actual draw for the purposes of sizing a branch circuit. This rule is mentioned in the NEC in 210.19 and 210.23.
However, it is worth noting that some electricians consider it a good practice to load branch circuits to a fraction of their rating. For example, a 20-amp lighting circuit should only be loaded to a maximum of 16 amps. This is because a continuous lighting load of 5 amps would be treated as 6 amps for circuit calculation, and the remaining 14 amps could be non-continuous load.
Additionally, for fixed utilization equipment (e.g. a dishwasher) on a circuit that also supplies lighting and receptacles, the equipment cannot exceed 50% of the branch circuit rating.
While the 80% rule does not apply to residential circuits, it is important for homeowners to understand basic electrical terminology and how power works in their homes to maintain safety and avoid hazards.
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The rule helps prevent nuisance tripping
In the context of electrical things, 80 A likely refers to the 80% rule, which is related to circuit breakers and load calculations. According to this rule, you can use up to 80% of a breaker's labelled ampacity. For example, if a breaker is labelled as 30 amps, you can use up to 24 amps (80% of 30) before needing to move to a higher amp breaker. This rule is important for sizing circuit breakers to protect wires from overheating and catching fire.
Now, let's discuss how the 80% rule helps prevent nuisance tripping:
The 80% rule is a crucial guideline for electricians and electrical engineers to ensure the safe and efficient operation of electrical systems. By following this rule, electricians can prevent nuisance tripping, which occurs when a circuit breaker trips unnecessarily or due to non-electrical reasons. Nuisance tripping can have significant consequences, including interrupting the power supply to customers and causing monetary losses for utility companies.
One of the main causes of nuisance tripping is overload, which happens when the system draws more current than it can handle. By adhering to the 80% rule, electricians can ensure that circuits are not overloaded, reducing the likelihood of nuisance tripping due to overload conditions.
Additionally, the 80% rule helps to prevent under-voltage issues, which can also lead to nuisance tripping. Under-voltage occurs when the voltage supplied to a circuit is lower than expected, causing the frequency of connected machines or motor loads to decrease, resulting in large current draws that can trip breakers. By sizing circuit breakers according to the 80% rule, electricians can help maintain voltage stability and reduce the occurrence of under-voltage-related nuisance trips.
Furthermore, the 80% rule contributes to the overall reliability of the power supply system. Electric power regulators impose penalties on utility companies if the reliability of the power supply deviates from compliant standards, as measured by indices such as SAIFI and MAIFI. Nuisance trips can contribute to an increased trip frequency, leading to penalties for the utility company. By minimizing nuisance trips through the application of the 80% rule, electricians help maintain the reliability of the power supply and avoid these penalties.
In conclusion, the 80% rule is a critical guideline in the electrical industry that helps prevent nuisance tripping. By ensuring circuits are not overloaded, maintaining voltage stability, and improving the overall reliability of the power supply system, the 80% rule contributes to the safe and efficient operation of electrical systems.
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Frequently asked questions
This is a type of electric energy meter called a 4 times meter. The specifications of the meter apply to the range between 20A and 80A. Below or above this range, the meter is less accurate.
The 80% rule is a rule of thumb in electrical engineering that states that no individual load shall exceed 80% of the branch circuit current rating. This rule is in place to reduce the probability of nuisance circuit breaker tripping.
ALL circuit breakers are tested and listed to carry 100% of their rated load, but this is in free air, meaning all sides of the breaker can dissipate heat. When designing a PANELBOARD or SWITCHBOARD to house the breakers, the breakers are designed to work assuming no more than 80% continuous loading, as there is no free air to dissipate heat in the breaker.
The 80% rule is in the form of a 125% multiplier on continuous current for the purposes of breaker sizing. As a result, the wire size must also increase by 125%. This is because while wire is rated to carry 100% of its ampacity continuously, upsizing only the breaker would cause the wire to no longer be protected at its ampacity.
