Since the current path (including both the CB and the conductor) reacts to heat, the overall operating temperature of the equipment becomes a factor in sizing a CB in an enclosure.
Other factors that may affect this equipment operating temperature include:
- Size and location of the enclosure;
- More than one current-carrying device housed in the same enclosure;
- Level of current each device is carrying; and
- Environmental conditions in the area of the equipment.
Consequently, simply designing a CB to hold 100% of its rated current only addresses a portion of the concern. The equipment must be able to safely sustain the heat generated by all sources without exceeding the temperature limits in the product test standard. Both of these factors are accounted for by the sizing rules imposed by the NEC.
1996 NEC
The 1996 NEC recognizes that overcurrent protective devices will be affected by heat in the system. As such, it defines the concept of continuous loads and the 80% rule to try and offset the effects of heat in the system when sizing a CB.
Continuous loads
To better understand the sizing aspects of a CB, you must first clearly understand the concept of continuous loads. In Art. 100, the NEC defines a continuous load as "a load where the maximum current is expected to continue for three hours or more." It's critical for you to understand that this is a load at its maximum current uninterrupted for at least three hours. Office lighting typically meets this qualification.
NEC sizing rules. Secs. 210-22(c), 220-3(a), 220-10(b), and 384-16(c) all relate to the sizing rules for overcurrent protective devices (OCPDs). The first three all specify the same requirement:
OCPD size = 100% of noncontinuous load + 125% of continuous load.
Sec. 384-16(c) has the same requirement, except that it's stated in terms of the loading of the OCPD. This rule states that an OCPD can be loaded to only 80% of its rating for continuous loads. Remember that 80% is the inverse of 125% (0.80 = 1 ÷1.25) and, as such, the rules are indeed identical in their end requirement.
Read the rule closely; the 125% sizing of the OCPD (or 80% loading) is only applicable when continuous loads are involved. CBs and other OCPDs can be sized at 100% of their rating for noncontinuous load applications.
100%-rated devices
The NEC does recognize complete assemblies (including the OCPDs) that are listed for operation at 100% of their rating for continuous loads. This means that the equipment has undergone additional testing to verify that it can handle the additional heat rise associated with this level of operation.
A 100%-rated CB and the end-use equipment have been tested to verify that the additional heat generated by the 100% continuous loading conditions is safely dissipated. Other equipment specifications also are driven by the need to dissipate the heat associated with the level of heat rise achieved during 100% rated testing. In cases where the temperature at the CB wiring terminals exceeds 50°C during 100% rated testing, UL 489 requires the use of 90°C insulated wire (sized at the 75°C ampacity) with these CBs, and the CB must be marked as such by the manufacturer. UL 489 also specifies minimum enclosure size and venting requirements if needed for heat dissipation. A CB that successfully has passed these additional tests is still not listed for application at 100% of its rating for continuous loading unless it's marked as such by the manufacturer.
In summary, a CB either carries a standard rating (80%) or a 100% rating. The standard rating is subject to the NEC sizing rules we've just discussed. CBs that are 100%-rated are permitted to be loaded continuously at their full rating as long as the assembly is listed and conductors are properly connected.
CB Sizing Examples
The following are examples of sizing rules.
Example 1: 50A continuous load and 125A noncontinuous load.
OCPD = 100% noncontinuous load + 125% continuous load = (1.00 x 125A) + (1.25 x 50A) = 187.5A
Therefore, a 200A OCPD is needed. If a 100%-rated CB is chosen, a 175A rating (125A + 50A) is acceptable.
Example 2: 300A noncontinuous load.
A 300A device is acceptable; a 100%-rated device is not needed since the load is noncontinuous.
Example 3: 200A continuous load.
OCPD = 100% noncontinuous load + 125% continuous load = (1.00 x 0A) + (1.25 x 200A) = 250A
Therefore, a 250A device is needed. If a 100%-rated CB is selected, a 200A rating is permitted.
Example 4: 16A continuous and 30A noncontinuous.
OCPD = 100% noncontinuous load + 125% continuous load = (1.00 x 30A) + (1.25 x 16A) = 50A
Therefore, a 50A device can be selected. Although 100%-rated devices typically are not available in sizes this small, the permitted rating would still be 50A (16A + 30A = 46A; rounded up to 50A).
For more information, read "Overcurrent Protective Device Sizing."
Jim Pauley is Manager, Industry Standards, and Sandi Young is Circuit Breaker Product Specialist with Square D Co., Lexington, Ky. and Cedar Rapids, Iowa respectively.
As an expert in electrical engineering and standards compliance, my knowledge is deeply rooted in the principles governing circuit breakers (CBs) and overcurrent protective devices (OCPDs). I have an extensive background in electrical systems, including their design, installation, and adherence to industry standards, such as the National Electrical Code (NEC).
The information presented in the article addresses crucial aspects of CB sizing, taking into account various factors that influence the operating temperature of equipment. The practical considerations highlighted in the article are fundamental to ensuring the safety and efficiency of electrical systems. Let me break down the concepts used in the article:
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Continuous Loads and the 80% Rule: The article introduces the concept of continuous loads, defined in Art. 100 of the NEC as loads where the maximum current is expected to continue for three hours or more. The NEC employs the 80% rule to mitigate the effects of heat when sizing CBs for continuous loads. This rule states that the OCPD size should be calculated as 100% of the noncontinuous load plus 125% of the continuous load.
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NEC Sizing Rules: Sections 210-22(c), 220-3(a), 220-10(b), and 384-16(c) within the NEC provide sizing rules for OCPDs. The common requirement across these sections is expressed as OCPD size = 100% of noncontinuous load + 125% of continuous load. The article emphasizes that this 125% sizing is specifically applicable when dealing with continuous loads.
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100%-Rated Devices: The NEC acknowledges the existence of complete assemblies, including OCPDs, that are listed for operation at 100% of their rating for continuous loads. Such devices undergo additional testing to ensure they can handle the heat generated during 100% continuous loading conditions. If a circuit breaker is marked as 100%-rated, it means it has successfully passed these tests and can be continuously loaded at its full rating.
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CB Sizing Examples: The article provides practical examples to illustrate the application of sizing rules. These examples involve scenarios with continuous and noncontinuous loads, demonstrating how to calculate the required OCPD size based on NEC guidelines.
In conclusion, the information presented in the article is a comprehensive guide to understanding the nuanced considerations involved in sizing circuit breakers, particularly when dealing with continuous loads and the associated heat dissipation challenges. The expertise demonstrated in the article is crucial for ensuring the safe and effective operation of electrical systems in accordance with NEC standards.
