Tapping a smaller conductor from a larger conductor is common in many electrical installations. Article 240 provides information on the minimum size of tap conductors for installations based upon the overcurrent protection device’s size and location. Article 240 does not, however, cover the sizing of an equipment grounding conductor installed with tap conductors.
Article 250, and specifically Section 250-122, would be used to determine the size of the equipment grounding conductor for a tap. To understand the concept of equipment grounding conductor sizing for a tap, one must understand what a tap conductor is and its installation requirements.
Section 240-3 requires an ungrounded circuit conductor to be protected by an overcurrent protection device sized for the conductor’s ampacity. A definition of tap conductor has been provided in the 1999 National Electrical Code (NEC), Section 240-3(e). This definition of tap conductor is only for use in Article 240; it does not apply to any tap in any other article of the Code. A tap conductor is a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the ampacity of the tap conductor.
Section 240-21 requires that the overcurrent protection device protecting the conductor be provided in each ungrounded circuit conductor at the point where the conductor receives its supply. Sections 240-21(a) through 240-21(g) permit a smaller conductor to be tapped from a larger conductor where the protection device exceeds the ampacity of the tap conductor.
If the overcurrent protective device is sized to protect both the larger conductor and the smaller conductor, as may be the case where conductors are sized to compensate for voltage drop, the smaller conductor is not a tap conductor as defined in Section 240-3(e). However, it is critical to understand that a tap conductor must never be tapped from a tap conductor. In other words, never tap a tap.
Section 240-21(a) permits tap conductors that are covered in Section 210-19 and the accompanying exceptions. Section 210-19(c), Exception No. 1 permits tap conductors with an ampacity of not less than 20 amperes to be used for electric ranges, wall-mounted ovens, and counter-mounted cooking units tapped from a 50-ampere branch circuit. These tap conductors must not be any longer than necessary to service the appliance. Remember the requirement of not tapping a tap!
If No. 6 copper conductors of a 50-ampere branch circuit are brought into a junction box and the individual electric cooktop, oven, or range is supplied with a flexible metal conduit with reduced sized conductors, these reduced-size conductors are tap conductors. The flexible metal raceway is supplied with the appliance to permit flexible connection for ease of service. This flexible metal raceway from the cooktop or oven encloses the 20-ampere or greater tap conductors that are to be connected to the 50-ampere branch circuit.
The aforementioned 50-ampere branch circuit supplying the electric cooktop, oven, or range is required by Section 250-134 or 250-138 to have an equipment grounding conductor installed with the circuit conductors. Section 250-134 provides requirements for installing an equipment grounding conductor if the appliance is permanently connected or fastened in place. If the appliance is cord and plug connected, then Section 250-138 provides similar equipment grounding requirements.
In either case, an equipment grounding conductor must be installed. It is sized based upon Section 250-122 and Table 250-122. In other words, the size of the equipment grounding conductor is based upon the rating or setting of automatic overcurrent protective device in the circuit ahead of the electrical appliance or equipment.
The equipment grounding conductor based on Table 250-122 for the 50-ampere range circuit is a No. 10 copper or No. 8 aluminum. The equipment grounding conductor for the 20-ampere tap would also normally be sized in accordance with the 50-ampere overcurrent protection device, or a No. 10 copper conductor equipment grounding conductor. However, the first sentence in Section 250-122(a) states that an equipment grounding conductor is not required to be any larger than the circuit conductors supplying the equipment. In this case, if No. 12 ungrounded circuit conductors are tapped from the 50-ampere No. 6 conductors to supply the equipment, then the tap circuit would only require a No. 12 equipment grounding conductor.
Because an equipment grounding conductor is not required to be any larger than the circuit conductors supplying the electrical equipment, this rule would also apply to any tap conductor application. Section 210-19(d), Exception No. 2 allows tap conductors smaller than No. 14 for fixture wires and flexible cords, as permitted by Section 240-4. In other words, No. 16 tap conductors supplying lighting fixtures from a 20-ampere circuit can be up to 100 feet long and No. 18 tap conductors can be up to 50 feet long. In either case, a smaller equipment grounding conductor can be installed. A No. 16 equipment grounding conductor can be used for the No. 16 tap and a No. 18 equipment grounding conductor can be installed for the No. 18 tap.
Following a few basic rules can simplify sizing of any equipment grounding conductor, even for a tap installation.
ODE is a staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at firstname.lastname@example.org.