Here are the latest short answers to questions posted on our Web site. In our discussion we will cover topics from Secs. 215-2, 220-2, 230-42, 250-30(a)(2) and (3), 250-52, 373-8, 384-16(a), 450-3(b), 450-9, 450-13, and 450-21(a) of the National Electrical Code (NEC).
Q. Can I make a splice within a panel?
A. Yes. Sec. 373-8 states you shall not use enclosures for overcurrent devices as junction boxes or raceways—unless you provide adequate space. Conductors inside a panelboard shall not fill the wiring space at any cross section to more than 40%, and splices and taps shall not fill the wiring space at any cross section to more than 75%.
Q. A main lug only distribution panel runs through an automatic transfer switch. The automatic transfer switch, the generator, and normal power are all in a remote location from the distribution panel. Someone told me the distribution panel doesn’t require a main circuit breaker. To disconnect the power to the distribution panel, you must open the normal and generator circuit breakers. Therefore, I believe you must install a main breaker in the distribution panel. Am I right?
A. No. Sec. 384-16(a) does not require a disconnecting means for a panelboard. But it does require a panelboard with a neutral that has more than 10% of its overcurrent devices rated 30A or less to be protected against overcurrent by a device having a rating not greater than that of the panelboard. Exception No. 1 to this rule states the overcurrent protection is not required to be located in the panelboard, if the panelboard feeder has overcurrent protection not greater than the rating of the panelboard. In your case, if the panelboard has proper overcurrent protection, the installation meets NEC requirements.
Q. I recently had an electrical inspector require the removal of a 75kVA transformer from a drop ceiling grid area, claiming the Code doesn’t allow the installation of a 75kVA transformer above a suspended ceiling. Does the Code allow a 75kVA transformer to be installed above a drop ceiling grid area, if it complies with all clearance, access, and ventilation requirements?
A. No. The general requirement of Sec. 450-13 states that transformers must be readily accessible to qualified personnel for inspection and maintenance. However, you can locate dry-type transformers, rated less than 600V and 50kVA, above a suspended ceiling—if it’s not permanently closed in, and if it meets the ventilation requirements of Sec. 450-9. You must also separate the transformer from combustible materials, as stated in Sec. 450-21(a) [450-3(b)]. Since your transformer is greater than 50kVA (75kVA), you may not install it above the suspended ceiling space.
Q. I’m confused as to how to size the neutral conductor for commercial service and feeder conductors. Am I required to size the neutral conductor at 125% for continuous loads?
A. Yes. Sec. 215-2 for feeders and Sec. 230-42 for services require the ampacity of the conductors (before the application of any adjustment or correction factors) to not be less than 125% of continuous loads, including the sum of noncontinuous loads. Since the neutral (grounded) conductor is part of the feeder or service, it must also comply with the same requirements. Therefore, you must size the neutral not less than 125% of continuous loads, and the sum of noncontinuous loads, in accordance with Sec. 220-22.
Example: If the load on the feeder/service consisted of 150A of 3-phase, 208V loads and 150A of single-phase, 120V loads, the feeder/service conductors would be, three 500kcmil conductors and one No. 3/0 conductor. Feeder conductor4500kcmil Cu, rated 380A at 75°C. 150A + 150A = 300A x 1.25 = 375A Neutral conductor = No. 3/0 Cu, rated 200A at 75°C. 150A x 1.25 = 188A.
Q. How do I ground a transformer in a building with no accessible grounded metal member of the structure, and where a grounded metal water pipe is more than 200 ft from the transformer?
A. Drive a ground rod. Sec. 250-30(a)(2) requires a grounding electrode conductor to connect the grounded (neutral) conductor of the transformer to a grounding electrode. Sec. 250-30(a)(3) requires the grounding electrode to be as near as practicable—and preferably in the same area as the transformer. Where an effectively grounded structural metal member of the structure, or effectively grounded metal water pipe within 5 ft from the point of entrance into the building is not available, you must install a ground rod in accordance with the requirements of Secs. 250-52.