Article 240, in the National Electrical Code (NEC), provides general requirements for overcurrent protection and overcurrent protective devices. Also covered in Article 240 are overcurrent protection requirements for supervised industrial installations operating at voltages of not more than 1,000 volts (V), nominal. Section 240.4(E) contains a list of sections where requirements for tap conductors are located. The most referenced section in 240.4(E) is probably 240.21. In accordance with 240.2, overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 240.21(A) through (H). While branch circuits, services, busways, motors, generators and batteries. While this section mentions branch circuits, most of the rules are for feeder taps and transformer secondary conductors.
As defined in 240.2, a tap conductor is a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4. Unless transformer secondary conductors are protected in accordance with their ampacities specified in 310.15, the secondary conductors shall be sized and installed in accordance with 240.21(C).
The fourth transformer secondary conductor rule in 240.21(C) applies to outside transformer secondary conductors. This rule is different from most of the other transformer secondary conductor rules because length is not a factor. Although there is nothing that says the conductors can be of unlimited length, the rule does not mention length. The first sentence of 240.21(C)(4) states that the secondary conductors shall be located outside of a building or structure, except at the point of load termination. This means the transformer must be outside, but the conductors shall be permitted to terminate in an overcurrent device located either outside or inside the building or structure.
Four conditions must be met when using this rule. The first condition states that the secondary conductors shall be protected from physical damage in an approved manner. The secondary conductors could be protected from physical damage by being installed in a raceway suitable for the lo cation, or they could be protected from physical damage by being installed as overhead conductors. Where branch circuits and feeders are installed outside as overhead conductors, they shall be installed in accordance with the requirements in sections 225.18 and 225.19.
Most of the feeder tap rules and transformer secondary conductor rules share the second condition. The conductors shall terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors. Like most of the other rules in this section, the conductors must terminate at a main-breaker panelboard or a safety switch (disconnect). When using this rule, terminating at main lugs of switchgear or a switchboard is not permissible. Also, like most of the other rules in 240.21, the ampacity of the transformer secondary conductors shall not be less than the rating of the overcurrent device. For example, an outside, customer-owned transformer will supply a fused disconnect located on the outside wall of a building. The fuses in the disconnect will have a rating of 125 amperes (A). The load on these transformer secondary conductors will be 112A, noncontinuous. All terminations will be rated 75°C. The length of the feeder tap conductors will be 50 feet. If all the conditions in 240.21(C)(4) will be met, what minimum size is required for the secondary conductors?
If these conductors were feeder conductors and not transformer secondary conductors, size 2 AWG conductors could be installed. In accordance with the 75°C column of Table 310.15(B)(16), a 2 AWG conductor has an ampacity of 115A, which is more than the load in this example. When the round-up rule in 240.21(B) is permitted, an overcurrent device rated 125A can be installed to protect a 2 AWG copper conductor. But, in accordance with 240.21(C), using the round-up rule is not permitted. Also, in accordance with the second condition in 240.21(C)(4), the rating of the fuse (or circuit breaker) must not be higher than the ampacity rating of the conductors. Since the fuses in the disconnect will be rated 125A, the transformer secondary conductors must be rated at least 125A. In accordance with the 75°C column of Table 310.15(B)(16), a 1 AWG conductor has an ampacity of 130A (see Figure 1).
The second condition also states that the single overcurrent device can supply any number of additional overcurrent devices on its load side. This just means that the conductors on the load side of the fused disconnect can terminate at a main-lug panelboard with breakers. The transformer secondary conductors can also terminate at a main breaker panelboard that contains any number of breakers on the load side of the main breaker. The third condition is similar to the provision in 230.91 that pertains to a service disconnecting means. The overcurrent device for the transformer secondary conductors shall be an integral part of a disconnecting means or shall be located immediately adjacent thereto. In accordance with the fourth condition, the disconnecting means shall be installed at a readily accessible location and must comply with either 240.21(C)(4)(4)(a), (b) or (c). In accordance with 240.21(C)(4)(4)(a), the disconnect can be installed on the outside of a building or structure. In accordance with 240.21(C)(4)(4)(b), the disconnect can be installed inside as long as the disconnect is nearest the point of entrance of the secondary conductors (see Figure 2).
Instead of installing the disconnect on the outside of a building or structure or instead of installing the disconnect nearest the point of entrance of the secondary conductors, the disconnecting means can be installed in accordance with 230.6, nearest the point of entrance of the conductors [240.21(C)(4)(4)(c)]. Section 230.6 provides descriptions of when conductors shall be considered being outside of a building or structure. Conductors shall be considered outside of a building or other structure under any of the five conditions in 230.6(1) through (5):
1. Where installed under not less than 2 inches of concrete beneath a building or other structure
2. Where installed within a building or other structure in a raceway that is encased in concrete or brick not less than 2 inches thick
3. Where installed in any vault that meets the construction requirements of Article 450, Part III
4. Where installed in conduit and under not less than 18 inches of earth beneath a building or other structure
5. Where installed within rigid metal conduit (Type RMC) or intermediate metal conduit (Type IMC) used to accommodate the clearance requirements in 230.24 and routed directly through an eave but not a wall of a building (see Figure 3)
With this outside transformer secondary conductor rule, there is no minimum conductor size other than having an ampere rating equal to or greater than the rating of the overcurrent device. For example, by using the outside secondary conductor rule, what are the smallest conductors that can be installed to supply an outdoor fused disconnect with fuses rated 400A?
The conductors will be secondary conductors fed from an outside, customer-owned transformer. The conductors will be protected from physical damage in an approved manner. The disconnecting means will be installed in a readily accessible location on the outside of a building. The load on the transformer secondary conductors will be 320A, noncontinuous. The conductors will be THWN-2 copper conductors, and all terminations will be rated 75°C. The length of the feeder tap conductors will be 95 feet. The rating of this transformer is sufficient for this load. Since all the conditions in 240.21(C)(4) will be met, simply select a conductor with an ampere rating equal to or greater than the rating of the fuses in the disconnect. Since the fuses will be rated 400A, the transformer secondary conductors must be rated at least 400A. In accordance with the 75°C column of Table 310.15(B)(16), a 600 kcmil conductor has an ampacity of 420A (see Figure 4).
Next month’s column continues the discussion of sizing conductors.