Properly Applying the "Rounding Up" Rule
Jeffrey Sargent
Overcurrent protection of conductors is a critical element of electrical system safety. The general approach is to use a protective device having a continuous current rating or setting that does not exceed the allowable ampacity of the conductor. Using the standard ratings or settings for fuses and circuit breakers found in 240.6 and aligning those ratings and settings with the allowable conductor ampacities typically selected from Table 310.16 provides the protection against overcurrent required by the NEC®. However, the allowable conductor ampacities and the ratings or settings of standard overcurrent protective devices do not always perfectly align. In those cases, the provisions of 240.4(B)—often referred to as the “rounding up” rule— can be used. However, it needs to be said that this rule does not change the conductor ampacity to a higher value; it only allows for some latitude in using standard conductor sizes with standard ratings and settings of overcurrent protective devices.
The Basics
The first paragraph of 240.4 provides one of the most fundamental electrical safety requirements in the entire NEC. The use of insulated conductors and provision of overcurrent protection for a conductor based on its ampacity are long-standing requirements, having been in the Code since its first edition in 1897. Rule 17 in the 1897 NEC contained the following general requirements on overcurrent protection:
The use of fuses and circuit breakers to provide overcurrent protection in systems operating at 600 volts and less has been and continues to be the preferred method of achieving compliance with the conductor overcurrent protection requirements in the NEC. Based on the Article 100 definition of overcurrent, this condition can result from excessive continuous current (overload) and from excessive short-time current (short circuits and ground faults). The objective of the overcurrent protection rules for conductors is multi-faceted. The rules provide protection against overload by using a protective device with a continuous current rating or setting not exceeding the allowable ampacity of a conductor. Further, the objective of maintaining the integrity of the conductor insulation is accomplished by limiting the current from reaching temperatures along the conductor that will damage its insulation.
When responding to a short-circuit or ground-fault condition, the overcurrent protective device accomplishes the objective of limiting the time and magnitude of a failure of conductor insulation. The presumption in this case is that insulation failure of one or more of the circuit conductors has occurred and that damage has resulted in a ground-fault or short-circuit condition. By responding to this failure in the system of electrical insulation, the protective device limits the damage from extending beyond the electrical equipment and posing a fire and/or shock hazard. There is likely to be damage to the electrical system that will have to be repaired, but the building or structure has been protected against the electrical failure.
To sum this up, the objective of the NEC is to protect conductors against overload (thus helping to minimize the potential for insulation failure and subsequent ground faults or short circuits) and to protect the rest of the electrical system and the occupancy in which the electrical system is installed by reacting to overcurrent resulting from ground faults and short circuits. Properly rated overload protection and protection of the electrical system against physical damage through the use of wiring methods appropriate for the location serve to maintain the requisite level of insulation on electrical conductors. Another factor that contributes to electrical insulation protection is grounding in accordance with Article 250 to limit voltages on electrical systems.
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Rounding Up
Section 240.4 allows for alternative approaches to protecting branch-circuit, feeder, and service conductors with protective devices having ratings or settings not exceeding the allowable ampacity of the conductor. Section 240.4(B) specifies that where the ampacity of a conductor does not correspond to a standard overcurrent protective device rating or setting as specified in 240.6, the use of the next higher standard rating or setting is permitted. However, this provision is not an unlimited permission to simply jump up to the next standard rating or setting. There are limitations on applying this permissive rule.
Conditional Approach
First, the use of this rounding up provision is limited to overcurrent protective devices rated or set at 800 amperes and below. From 1 to 800 amperes, the largest increment between the standard ratings or settings is 100 amperes. Above 800 amperes, the difference grows as large as 1000 amperes. When we consider that the main objective of 240.4 has to be met, in that the overcurrent protective device has to afford overload, short-circuit, and ground-fault protection, the reason for limiting this provision to circuits 800 amperes and less becomes evident.
The second limitation on this rule is that the conductors cannot supply a branch circuit in which there are multiple receptacle outlets and those receptacles are used to supply portable cord-and-plug-connected loads. This limitation addresses the fact that the load connected to these receptacles is only limited by the number of receptacles and is not a fixed load. Think of the receptacles in your home: not all are being used at any given time, but the possibility of such use exists.
The third limitation of this section is that the ampacity of the conductors to be used does not correspond to the rating or setting of a fuse or circuit breaker. If the overload function of the overcurrent protective device can be adjusted (i.e. adjustable continuous current) above its marked rating or setting, the rounding up provision cannot be used.
Now, considering all three conditions specified in 240.4(B) can be met, there is one more, very basic requirement that has to be complied with—that is, the requirements in 210.19(A), 215.2(A)(1), and 230.42(A) for branch-circuit, feeder, and service conductors respectively. All three of these sections require that the circuit conductors have an ampacity not less than the load to be served. In other words, even though the Code permits a differential between the allowable ampacity of a conductor and the rating or setting of an overcurrent protective device, it does not allow for the conductors to have an ampacity less than the load to be served.
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Applying the Provision to Round Up
Four 2-wire branch circuits, installed in rigid metal conduit using 12 AWG copper conductors with 75° C insulation, supply multiple receptacles for cord-and-plug-connected portable loads. After applying the adjustment factor (25 amperes x 0.7) for the number of conductors in the raceway, the conductor ampacity is 17.5 amperes. The next standard rating or setting of an overcurrent protective device per 240.6 is 20 amperes. However, because of the type of load supplied by these conductors, the provision to round up to the 20-ampere overcurrent protective device cannot be used. The default in this case is to use a 15-ampere device or a nonstandard size having a rating or setting not greater than 17.5 amperes. There is a very likely possibility that a load greater than the adjusted ampacity will be imposed on these conductors. As an aside, the use of 90° C conductors yields a conductor ampacity of 21 amperes after ampacity adjustment, and a 20-ampere overcurrent protective device is compliant.
Let’s take the same raceway scenario, but instead of multiple receptacles, the load is fixed luminaires in a commercial occupancy. Because of the continuous nature of the luminaires, the load connected to a 20-ampere overcurrent protective device is limited to 16 amperes. In applying the continuous load rule to the conductors, the ampacity has to be not less than 125 percent of the continuous load before the application of ampacity adjustment or correction factors. The 75° C ampacity is 25 amperes and the 60° C conductor ampacity is 20 amperes. This conductor meets the requirement of 210.19(A) for continuous loading. For the portion of the circuit in the conduit, the adjusted ampacity is 17.5 amperes, which is sufficient to supply the actual 16-ampere load permitted to be supplied by the 20-ampere overcurrent protective device. Therefore, the 12 AWG copper conductor with an adjusted ampacity of 17.5 amperes is permitted to be connected to the next standard size overcurrent protective device, which is a 20-ampere device.
Feeder Example
Two sets of 500 kcmil copper conductors in separate conduits are connected in parallel. Based on the limitations of 110.14(C), the allowable load ampacity of this parallel arrangement is
380 amperes x 2 = 760 amperes.
The ability to use an 800-ampere fuse or circuit breaker is entirely dependent on the calculated load. If the calculated load is 760 amperes or less, the use of an 800-ampere overcurrent protective device is permitted. If the load is greater than 760 amperes, the conductor ampacity is not sufficient to carry the load in accordance with 215.2(A)(1). The next standard conductor size from Table 310.16 that would be sufficient to carry the load and be connected to an 800-ampere overcurrent protective device is 600 kcmil.
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No Rounding Up per 240.21(B) and (C)
A revision in the 2005 NEC clearly precluded the use of the “next standard size” approach permitted by 240.4(B) for feeder tap conductor applications and in 240.21(C) for transformer secondary conductor applications.
Let’s first look at the text that was added in the 2005 NEC. Section 240.21(B), which covers feeder tap applications, now has a last sentence in the opening paragraph stating that “the provisions of 240.4(B) shall not be permitted for tap conductors.” Similarly, 240.21(C) now has a last sentence in its opening paragraph stating that “the provisions of 240.4(B) shall not be permitted for transformer secondary conductors.” These two sentences added to the 2005 NEC clarify that conductors tapped to a feeder or connected to a transformer secondary must have an ampacity that is not less than the rating or setting of an overcurrent protective device to which the tap or secondary conductors connect at their load end. It is important to understand that the provisions of 240.4(B) do not change the ampacity of a conductor by permitting it to be connected to a higher rated device. The allowable ampacity remains as shown in the applicable table from Article 310 (typically Table 310.16), and the calculated load cannot exceed the allowable ampacity of the conductor.
For instance, 500 kcmil copper and 750 kcmil aluminum conductors are frequently used in circuits protected by protective devices rated or set at 400 amperes. From the 75° C column in Table 310.16 for the respective conductor materials, the 500 kcmil copper has an allowable ampacity of 380 amperes while the 750 kcmil aluminum conductor has an allowable ampacity of 385 amperes. Neither of these conductors is fully rated at 400 amperes. However, 240.4(B) permits the use of a 400 ampere device with these conductors under the conditions specified by that section. The circuit conductors cannot be part of a multioutlet branch circuit supplying receptacles from which cord-and-plug-connected loads are supplied (they cannot control the load connected to such a circuit), the ampacity does not correspond to a standard rating or setting of a protective device that does not have overload trip adjustments above its rating, and the rating of the next higher device cannot exceed 800 amperes (after 800 amperes, the increments between standard sizes grows substantially. See 240.6).
The various rules in 240.21(B) and (C) contain provisions requiring conductor ampacities to be not less than device ratings or settings and to be directly proportional to the size of the protective device installed on their line side. The rules of 240.21(B) and (C) provide an alternative to having complete overcurrent protection located at the point where a conductor receives its supply, and in these provisions the device on the supply side of the conductors provides only short-circuit and ground-fault protection. In order to ensure that protective devices are going to open under fault conditions, it is necessary to ensure that the feeder tap and transformer secondary conductors are large enough to sustain the current required to open the line-side overcurrent protective device—which in the case of the ten foot rule can have a rating up to 10 times the ampacity of the tap or secondary conductor. Use of the next standard size rule in these applications compromises the relationship between the tap and secondary conductors and the overcurrent protective device on their line side. The text added in 2005 to these sections reinforces the importance of the conductor size/overcurrent protective device size relationship.
The ampacity of a conductor is that determined by 310.15 and is typically selected from one of the allowable ampacity tables based on the load to be served. Section 240.4(B) is a conductor overcurrent protection requirement only and does not increase the allowable ampacity of a conductor.
One other point in this example highlights new wording for 2005 in 240.21(C) regarding the number of secondary connections that can be made to a transformer secondary. The example indicates that two separate 400-ampere taps have been made to the 225 kVA transformer. This had not been prohibited by previous editions of the NEC, but new 2005 text in the first paragraph clarified that the rules in 240.21(C)(1) through (6) can be applied to separate sets of secondary conductors supplied from a single transformer. This wording does not specify that the same set of provisions has to be used for each set, either. In other words, a set of secondary conductors installed using the provisions of 240.21(C)(2) and a set of secondary conductors installed using the provisions of 240.21(C)(6) can be connected to a single transformer, provided the feeder circuit has sufficient capacity for the loads to be supplied.
Summary
The rounding up provisions allow for a practical solution where the ampacity of a conductor and the rating or setting of an overcurrent protective device do not correspond. However, the basic fundamental of overload, short-circuit, and ground-fault protection are still the overriding consideration. Where used in accordance with the limitations set forth in 240.6 and with the conductor sizing rules in Articles 210, 215, and 230, the conductors will not be overloaded, and there is sufficient conductor circular mil area to permit the overcurrent protective device to function under short-time, high-current events. The next standard size provision is a slightly different approach to the total overcurrent protection package, but its conditions of use allow the overall safety objective of the NEC to be met.