Q I have a job in a gym that will require 10 circuits for a Stairmaster machine and tread mills, 120V 16A each.
I'm wondering if I can use a common neutral for bringing out 10 hot legs and five neutrals and a ground. In Article 225-7 (b), the ampacity of the neutral conductor shall not be less than the maximum net-computed load current between the neutral and all ungrounded conductors connected to any one phase of the circuit. Please explain this article in more detail for me.
A The 10 ungrounded conductors sharing five neutrals are essentially just five multi-wire branch circuits as defined by Article 100. Such branch circuits are permitted by Section 210-4. According to this section, the five branch circuits may also be considered as 10 individual branch circuits, and be protected by 10 separate single-pole circuit breakers. Normally, the neutrals would be sized the same as the ungrounded conductors when used in this manner. The neutral conductors don't have any specific sizing rules, but many general statements that require the neutral to be sized for the load are found in the Code. For example, Sections 210-19(a) and 310-10 require the conductors to be adequately sized without providing any specific sizing instructions. Unless significant harmonic currents are present, the neutrals and ungrounded conductors in your example could all be No. 12 copper. Section 225-7 is not applicable to these circuits. Article 225 is for outside branch circuits and feeders, and Section 225-7 is specific to outdoor lighting circuits. Since the definition of a multi-wire branch circuit allows for two or more ungrounded conductors sharing a single neutral, Section 225-7 provides a way of sizing a neutral when it's shared by more ungrounded conductors than there are phases in the system. For example, if an outdoor lighting installation requires six 20A circuits from a single phase three-wire system, six ungrounded conductors could share one neutral. If all the ungrounded conductors are equally loaded at 16A, the neutral must be sized for three times 16A or 48A minimum. Such installations are quite common in outdoor circuits feeding multiple pole lights. In interior wiring, a more common practice is to share each neutral by no more than two ungrounded conductors in a single-phase system or three ungrounded conductors in a three-phase system. In such cases, the neutral load is not likely to be larger than the load in any one phase conductor, unless, as noted above, the circuits produce significant harmonic distortion. You did not say how these conductors would be grouped in raceways or cables. In some cases, the neutrals may be counted as current carrying conductors, and their ampacities may have to be adjusted in accordance with Section 310-15(b)(2)(a) and 310-15(b)(4).