Article 240 in the National Electrical Code (NEC) is titled “Overcurrent Protection.” While this term is not defined, Article 100 defines overcurrent. Overcurrent is any current in excess of the rated current of equipment or the ampacity of a conductor. The definition also states overcurrent may result from overload, short circuit or ground fault. Therefore, overcurrent protection is safeguarding against current in excess of a conductor’s ampacity or the current rating of equipment.


An overcurrent device, such as a circuit breaker or fuse, usually provides overcurrent protection. For the overcurrent device to safeguard against current in excess of the ampacity of a conductor, the conductor must be sized in accordance with all applicable provisions. The provision in 240.4 states conductors, other than flexible cords, flexible cables and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G). This general rule states that a conductor must be protected at its ampacity unless applying one of the alternative rules in 240.4(A) through (G).


Without the alternative rules, it would be necessary for a conductor’s ampacity to be more than the ampere rating of the overcurrent device protecting it. Section 240.4(E) shows various sections in the Code covering tap conductors. Rules covering the location and sizing requirements of overcurrent protection for tap conductors are in 240.21. While 240.21 has eight subsections (A through H), most of the rules pertain to feeder taps and transformer secondary conductors.


Previous Code in Focus articles have covered the feeder tap rules in 240.21(B). This month begins the coverage of rules for transformer secondary conductors.


In accordance with 240.21(C), a set of transformer secondary conductors feeding a single load, or each set of transformer secondary conductors feeding separate loads, can be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). Conductors connected to the secondary side of a transformer are not protected with an overcurrent device and, therefore, are not protected against overcurrent in accordance with their ampacities specified in 310.15. This section states that transformer secondary conductors can be installed without overcurrent protection at the secondary where installed in accordance with one of the transformer secondary conductor rules 240.21(C)(1) through (C)(6) (see Figure 1).


Section 240.21(C) also mentions a type of installation that is not well known in the electrical industry. This section states that more than one set of transformer secondary conductors can be installed to supply power to separate loads. With most transformer installations, the secondary conductors supply power to a single load, as illustrated in Figure 1. Therefore, transformer secondary conductors usually terminate at one circuit breaker or set of fuses. But in accordance with 240.21(C), more than one set of transformer secondary conductors can be installed to supply power to separate loads. Although not stated in this section, there is a maximum number of secondary overcurrent devices that one transformer can supply. As stated in Note 2 under Tables 450.3(A) and (B), where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. This is like the requirement for the maximum number of disconnects for a service. The maximum number of disconnects for a service is also six [230.71(A)] (see Figure 2).


If a single overcurrent device is installed on the secondary side of a transformer, the overcurrent protection for the transformer shall comply with 450.3, which includes Tables 450.3(A) and (B). Compliance with this section and one of these tables is also required where multiple overcurrent devices are installed on the secondary side of a transformer. In accordance with the second note below Tables 450.3(A) and (B), if a transformer is used to supply more than one overcurrent device, the total combined rating of all the overcurrent devices shall not exceed the allowed value of a single overcurrent device. If installing a transformer that will be used to supply more than one overcurrent device, first determine the maximum ampere rating for a single overcurrent device on the secondary side of the transformer. Then, ensure the total of the device ratings does not exceed the rating permitted for a single overcurrent device.


For example, a single transformer will be used to supply power to six fused disconnects. If a single overcurrent device were to be installed on the secondary side of this transformer, the maximum rating would be 600 amperes (A). If six fused disconnects will be installed, each disconnect could be rated 100A. Since the total combined ratings for these disconnects do not exceed 600A (100 + 100 + 100 + 100 + 100 + 100 = 600), this installation is permitted (see Figure 3).


In the previous example, the ampere rating of each disconnect was the same, but it is not required that each overcurrent device have the same rating. The second note under Tables 450.3(A) and (B) does not specify that the ratings of the overcurrent devices must be rated the same. The note just states, where multiple overcurrent devices are used, the total combined rating of all the overcurrent devices must not exceed the allowed value of a single overcurrent device.


For example, a single transformer will be used to supply power to four fused disconnects. If a single overcurrent device were to be installed on the secondary side of this transformer, the maximum rating would be 600A. This transformer will supply power to one disconnect rated 300A, one disconnect rated 125A, one disconnect rated 100A and one disconnect rated 60A. Since the total combined ratings for these disconnects do not exceed 600A (300 + 125 + 100 + 60 = 585), this installation is permitted (see Figure 4).


As previously stated, the rules for transformer secondary conductors are in 240.21(C)(1) through (6). These rules are applicable whether the transformer is supplying power to a single overcurrent device or up to six separate overcurrent devices. When sizing transformer secondary conductors, it may be necessary to install larger size conductors than would be required for other types of loads; this is especially true when supplying power to more than one overcurrent device. In the previous example, a disconnect rated 60A was going to be installed. Depending on the transformer secondary conductor rule, conductors supplying power to the 60A disconnect might need to be considerably larger than the conductor on the load side of the 60A disconnect.


The last sentence in 240.21(C) is significant when sizing transformer secondary conductors. The provisions of 240.4(B) shall not be permitted for transformer secondary conductors. The provisions in 240.4(B) are usually referred to as the round-up rules. Because of the last sentence in 240.21(C), it is not permissible to use the round-up rule. For example, if the transformer secondary conductors were supplying a 400A overcurrent device, the secondary conductors would have to be rated at least 400A. Installing a conductor with an ampacity rating of only 380A would not be permitted.


Next month’s column continues the discussion of sizing conductors.