The OBC Type AFCI and NFPA 70
This article focuses on the origin and the journey of the permissions for use of the Outlet Branch-Circuit (OBC) type Arc-Fault Circuit Interrupter (AFCI) found in NFPA 70 210.12. Together we will review and completely understand how the lengths specified in 210.12(A)(3)(b) and 210.12(A)(4)(b) of NFPA 70 were arrived upon.
Background
NFPA 70-1999 was the first edition to include 210.12 provisions for AFCI protection. At that time there were no inclusions of exceptions or recognition of methods to achieve AFCI protection other than a circuit breaker branch-feeder type AFCI located at the source of a branch circuit.
The options of protection methods in 210.12 originated during the NFPA 70-2005 revision cycle. An exception permitted the location of the AFCI to be at other than the origination of the branch circuit if two conditions were met:
- The AFCI was installed within 1.8 m (6 ft) of the branch circuit overcurrent protective device as measured along the branch circuit conductors.
- The circuit conductors between the branch circuit overcurrent protective device and the AFCI installed in a metal raceway or a cable with a metallic sheath.
The public inputs sought permission for an OBC AFCI installed downstream of a molded-case circuit breaker (MCCB), but the technical panel selected generic language to permit any type of AFCI to be in this location.
NFPA 70-2011 specifically called out the OBC AFCI removing the generic AFCI provision. Two new exceptions were introduced with the intent to permit the use of an OBC AFCI device yet ensure protection of the entire branch circuit and connected cords.
The new exceptions for NFPA 70-2011 included the following:
- The home run circuit physically protected using RMC, IMC, EMT, Type MC or steel armored type AC cables meeting requirements of 250.118. Metal outlet and junction boxes were also required. When met, an OBC AFCI was permitted to be installed at the first outlet providing protection of the remaining portion of the branch circuit.
- A listed metallic or nonmetallic conduit or tubing encased in not less than two inches of concrete for the home run circuit. If this was met the OBC AFCI was permitted to be installed at the first outlet to provide protection for the remaining portion of the branch circuit.
The home run circuit is that portion of the branch circuit that begins at the branch-circuit overcurrent protective device and ends at the first outlet. When the OBC is the only source of protection, the home run circuit is not afforded complete AFCI protection hence the need for additional physical measures for protection.
During the NFPA 70-2014 cycle, research was conducted by Underwriters Laboratories (Table 1) to determine if a standard MCCB could provide protection of the home run circuit from arcing faults. It was thought that the instantaneous trip function of a MCCB could clear an arcing fault if the arcing fault current was high enough. Each of these reports included revisions and corrections and expanded data and information to previous editions of the same report.
Table 1
Sept 30, 2011
“Effectiveness of Circuit Breakers in Mitigating Parallel Arcing Faults in the Home Run”
Oct 4, 2011
“Effectiveness of Circuit Breakers in Mitigating Parallel Arcing Faults in the Home Run”
Jan 11, 2012
“Effectiveness of Circuit Breakers in Mitigating Parallel Arcing Faults in the Home Run”
Jul 11, 2012
Sep 12, 2012
“Evaluation of Run Length and Available Current on Breaker Ability to Mitigate Parallel Arcing Faults Part II: Effect of Run Length with 500A Available at the Panelboard”
The final UL report was issued on September 12, 2012, just before the NFPA 70-2014 second draft meetings which occurred November 28 – December 1 of that same year. CMP 2 first draft meetings for NFPA 70-2014 occurred on January 16 – 21 2012 in Hilton Head, SC.
AFCI Technology Options of 210.12(A)
NFPA 70-2014, Section 210.12 introduced 6 options for AFCI protection. Three of these options were new to NFPA 70. The following 6 options included in NFPA 70-2023 that have remained since 2014:
Option 1:
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Placing a listed combination-type AFCI to protect the entire branch circuit. This option has been a part of 210.12 since its introduction outside of the fact that the branch-feeder AFCI was permitted up until NFPA 70-2005.
Option 2:
———————————————————————–
A listed branch/feeder-type AFCI at the origin of the branch circuit in combination with a listed OBC AFCI at the first outlet. The first outlet must be marked as such. This option recognizes that the Branch-Feeder AFCI provides both a high energy (parallel) and low energy (series) arc protection for NMB wire. It also provides high energy arc protection for connected SPT-2 cords, but it does not provide low energy arc protection for these same SPT-2 cords. The OBC AFCI at the first outlet, regardless of the length of the home run circuit, provides the additional protection of low energy (series) arc protection of SPT-2 type conductors. SPT-2 conductors are common for appliances like lamps and similar equipment. These conductors do not have a grounding conductor, they only include a hot and neutral conductor.
Option 3:
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A listed supplemental arc protection (SAP) circuit breaker at the origin of the branch circuit in combination with a listed OBC type AFCI at the first outlet. Three conditions must be met including the following:
Condition 1: The branch circuit must be continuous from the SAP circuit breaker to the OBC AFCI
Condition 2: The length of home run circuit cannot exceed the following:
14 AWG conductor – 15.2 m (50 ft.)
12 AWG conductor – 21.3 m (70 ft.)
Condition 3: the first outlet must be marked as such
Option 4:
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A listed MCCB at the origin of the branch circuit in combination with a listed OBC AFCI at the first outlet. Four conditions must be met including the following.
Condition 1: The branch circuit must be continuous from the SAP circuit breaker to the OBC AFCI
Condition 2: The length of home run circuit cannot exceed the following:
14 AWG conductor – 15.2 m (50 ft.)
12 AWG conductor – 21.3 m (70 ft.)
Condition 3: the first outlet must be marked as such
Condition 4: The MCCB and the OBC AFCI have to be identified as meeting the requirements for a system combination-type AFCI and listed as such.
Option 5:
———————————————————————–
A listed MCCB at the origin of the branch circuit in combination with a listed OBC AFCI at the first outlet. The home run must be protected through use of the following:
- RMC
- IMC
- EMT
- Type MC, or steel-armored Type AC cables meeting the requirements of 250.118
- Metal wireways
- Metal auxiliary gutters
Metal outlet and junction boxes must be used. The thought process is that the home run circuit is afforded an additional level of protection to prevent damage that could lead to an arcing fault.
Option 6:
———————————————————————– A listed MCCB at the origin of the branch circuit in combination with a listed OBC AFCI at the first outlet. The home run must be protected through use of the one of the following
- Listed metal or nonmetallic conduit, or tubing
- Type MC cable encased in not less than 50 mm (2 in.) of concrete.
The thought process is that the home run circuit is afforded an additional level of protection to prevent damage that could lead to an arcing fault.
Home run circuit lengths [210.12(A)(3) & 210.12(A)(4)]
The conductor lengths identified in 210.12(A)(3)(b) and 210.12(A)(4)(b) were based on the premise that the high energy (parallel) arcing fault would trip a standard thermal magnetic circuit breaker in the instantaneous region. The low energy (series) arcing fault due to damage in the home run circuit is not detected nor protected by the circuit breaker at the origin of the branch circuit.
Low energy (series) arc detection of the home run circuit is achieved by the listed OBC AFCI at the first outlet. When/if a low energy (series) arc occurs in the home run circuit, the OBC AFCI at the end of the home run circuit will detect it and open. This removes most of the load current flowing through the damaged conductor. The only current continuing to flow through the damaged conductor is that which is required to light the indicating lights on the OBC AFCI device itself to provide trip indication.
To determine if the instantaneous trip of a MCCB will provide protection of the home run circuit, three key items are needed beyond a basic formula for the calculation.
- Available fault current at the panel
- Length of conductor
- Instantaneous pickup of the MCCB
The equation to determine the length of conductor was the focus of the work conducted by UL research published in a series of reports identified in Table 1.
Protection criteria:
The basic recipe to provide protection of the home run circuit from arcing and sparking dangerous arcs includes the following:
Instantaneous trip threshold:
The instantaneous pickup of the circuit breaker is the current value beyond which the instantaneous trip will occur for the circuit breaker. The values are obtained from the UL research reports (Table 1). The UL standard for a MCCB is UL 489 and does not include a requirement for a MCCB to have an instantaneous trip capability, hence the need for an independent study to determine the values for this parameter across all MCCB manufacturers.
Arcing fault current magnitude:
This is the amount of arcing current flowing in the circuit during the arcing fault. To determine the arcing current magnitude, the following information is needed:
- Available fault current at the source:
The available fault current at the origin of the branch circuit is established by the UL 1699 standard to be 500A. The origin of the branch circuit may not be at the service equipment for the structure. - Circuit impedance:
The impedance of the home run circuit determines the total current to flow in the circuit. The impedance of the home run is determined by the length and impedance/ft. of the conductor.
The calculated arcing fault current must exceed the instantaneous trip threshold of the MCCB located at the origin of the branch circuit.
The equations and instantaneous pickup values found in Table 2 demonstrate how the information of these reports progressed over time and how the key parameters changed as research revealed more information.
Table 2
Edition
Equation Version
circuit breaker instantaneous and document comments
September 30, 2011
212A – 300A
“The circuit breaker age did have a significant effect on magnetic trip level, with “new” breakers showing a normal distribution with mean of 212A and 99% of all breakers having a magnetic trip current at or below 300A.”
October 4, 2011
213A – 300A
“New circuit breakers show an average magnetic trip level of 213A, with a standard deviation of 33.2A. This suggests that 99% of all circuit breakers will possess a magnetic trip level at or below 300A. This is true for all brands of circuit breakers investigated in this work.”
January 11, 2012
15A MCCB: 278A – 299A
20A MCCB: 202A – 314A
“New circuit breakers show magnetic trip levels that are normally distributed around an average value of 213 A, and a standard deviation of 33 A. This suggests that 95% of all 15 A residential breakers will instantaneously trip at or above 278 A, and 99% of all breakers will magnetically trip at or above 299 A. 20A circuit breakers showed a mean value of 202 A, with 95% of all 20 A residential breakers instantaneously tripping at or above 314 A, and 99% of all breakers magnetically tripping at or above 349 A.”
July 11, 2012
350A – 400A
“These revised values show that the magnetic trip level of circuit breakers is not as well controlled as was previously found in a previous study. The new data suggest that the 99th percentile upper bound may need to increase to at least 350A, perhaps as high as 400A, though this may suggest that for the application discussed in this work, circuit breakers with a known magnetic trip level may be required.”
September 12, 2012
400A – 450A
Too much variation to specify
“The initial set of circuit breakers, which were sampled from four North American manufacturers and included circuit breakers of different designs for each manufacturer, suggested that 99% of all circuit breakers would magnetically trip at or below 300A for 15A breakers and 350A for 20A breakers. However, follow-up testing one year later negated these findings, with circuit breakers of the same model number but of a different batch had significantly different magnetic trip levels, varying by 50A or more for some manufacturers, yet unvarying for others. These results showed that magnetic trip levels could conceivably be controlled, but were not in all cases. The revised data showed that panelboard current and run length would need to be set assuming magnetic trip thresholds as high as 400-450A would be needed, which makes arc mitigating using magnetic trip levels not specifically calibrated for this application impractical as well as potentially unreliable.”
calculating circuit lengths - equation parameters
Throughout the revisions of the UL studies (Ref. Table 1), the equations continued to change seeing three major revisions. The final equation is a part of the September 12, 2012, edition of the report titled “Evaluation of Run Length and Available Current on Breaker Ability to Mitigate Parallel Arcing Faults Part II: Effect of Run Length with 500A Available at the Panelboard.” The formula from this UL research paper is as follows:
ρL = Resistance per foot of Type NMB cable size in AWG @ 25°C (range of 20°C to 30°C)
L = Length of the “home run” in feet
Imag = Magnetic / instantaneous trip current pickup value for the circuit breaker
Ipssc = Short-circuit current at the beginning of the branch circuit (500 Amps from UL 1699)
Vrms = Supply voltage (generally 120 Vrms)
RC = Series contact resistance at point of contact arcing (experimentally determined to be 30 milliohms [mΩ] from the UL research paper)
The above equation will be used to calculate the lengths found in 210.12.
Available Fault Current (Ipssc)
The available fault current at the beginning of the branch circuit protected with an AFCI circuit breaker, is established at 500A as per how AFCIs are tested and listed to UL 1699.
Resistance (ρl)
The resistance per foot of type NMB cable is calculated using the following equation:
ρ = Resistivity value: 10.37 cm ohm foot
L = Length in feet (1000) or converted from meters (304.8)
A = Area in circular mils or mm2. The area in circular mils for these conductors are taken from NFPA 70-2020 Chapter 9 Table 8. The (circular mils / mm2) for 14 AWG and 12 AWG conductors respectively are (4110 / 2.08) and (6530 / 3.31).
The resistance values of a 14 AWG and 12 AWG from the UL research paper at room temperature (20°C to 30°C) are 2.525 ohms/1000 feet for 14 AWG and 1.588 ohms/1000 feet for 12 AWG.
For these calculations, the resistance for each conductor material per 1000 feet (304.8 meters) were determined by applying base unit resistivity values from recognized standards, such as NIST (National Institute of Standards and Technology) and ASTM (American Society for Testing and Materials), at 20°C (allowed range of 20°C to 30°C from UL paper).
The calculated Resistance – ρL (Ω / foot) of conductor are as follows:
ρL 14AWG = [10.37 * (1000 / 4110)] = 2.5231(Ω/1000 ft)
ρL 12AWG = [10.37 * (1000 / 6530)] = 1.5880(Ω/1000 ft)
ρL 14AWG = (2.5231/1000) = 0.0025231 (Ω/ft)
ρL 2AWG = (1.5880/1000) = 0.0015880 (Ω/ft)
Circuit Breaker Magnetic Trip Levels (Imag)
Magnetic trip levels (instantaneous trip) values for MCCBs are not governed and are not required to be a part of a MCCB as per UL 489, “UL Standard for Safety Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures”. UL 489 is the standard to which all MCCBs are listed. Because of this, the magnetic trip levels will vary from manufacturer to manufacturer and from circuit breaker to circuit breaker within the same manufacturer’s product offering. The UL reports of Table 1 investigated this parameter and found that the magnetic trip levels of MCCBs for use in residential applications have a variance in pickup values. This study did not investigate other types of MCCBs that are more prevalent in commercial applications. The instantaneous pickup values throughout the various editions of these UL reports identified in Table 1 were documented anywhere from 212A through 450A. This information is identified and tabulated as part of Table 2.
The information found in Table 3 are those magnetic trip levels measured as part of the UL research report dated July 11, 2012. This is the most comprehensive table available that includes both 15A and 20A circuit breakers
As part of the first draft of NFPA 70-2014, the code making panel chose to accept a 300A circuit breaker instantaneous trip level stating the following as part of the acceptance of public input 2-92, “The panel accepted the principle of a circuit breaker having an instantaneous trip current of 300A or less.”
Calculating Circuit lengths - Let's do the math
This calculation will use the latest edition of the equation developed through the research of UL.
ρL = 0.001588 for 12AWG copper conductor
ρL = 0.0025231 for 14AWG copper conductor
Vrms = 120 Vrms
RC = 0.03
Ipssc = 500 Amps
Imag = 300 Amps
Supplementing the values for the parameters of this equation, the length of protected conductor is calculated as follows:
The calculated value of 15.5 ft does not equate to the expected 70 ft of conductor.
After research of public inputs and public comments and the UL research reports themselves, the lengths found in NFPA 70 are found to be based on the equations included in the first and second report published by UL.
This calculation will use the equation developed as part of the 1st edition of the UL reports and not the latest edition of the reports.
The values for each of the parameters:
Vrms = 120V
Imag = 300A
ρL = 0.002575
The value obtained of 0.161mW, as per the UL study, was adjusted from 90oC to 25oC via the following equation:
Substituting this new resistance value into the above equation:
The January 11, 2012 edition of this report stated the following:
“For 20 A circuit breakers, a resistance of 110 mOhm at 25°C is calculated. Assuming 12 AWG wire (pL = 1.588 m /foot at 25°C), this would suggest a maximum home run length L of 69 feet using 12 AWG NM cable.”
The 50 ft and 70 ft numbers calculated based on the original equation assumes an infinite amount of fault current available at the origin of the branch circuit as identified by UL in the second report issued January 11, 2012.
calculating circuit lengths - using latest equations
Table 4 includes the accurate lengths of protected home run conductors for both a 14 AWG and 12 AWG copper conductor home run circuit. These are based on the correct equations and accepted parameters by the Code Making Panel. The instantaneous pickup value used below is an area where one could argue should be a higher value than 300A based on these reports. The value used here is considered a compromise and one that is documented as being supported by the Code Making Panel.
12 AWG
Ipssc = 500A
Imag = 300A
Vrms = 120V
Rc = 0.03
Length = 15 ft.
14 AWG
Ipssc = 500A
Imag = 300A
Vrms = 120V
Rc = 0.03