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Service Maintenance Company, Inc. Electrical Contractors |
| NEC Questions and Answers | 4/18/2003 |
| By Mike Holt for EC&M Magazine Q1. The luminaires in the "men's bathroom" are controlled by occupancy sensors that do not have a manual override switch and when there is no motion for 3 minutes or more, the luminaires shut off, even if the stalls are occupied! Trust me I know. Because the motion sensor is located so far from the stalls, an occupant is required to leave the stall for the sensor to "see" movement so the luminaire will turn back on. Is this permitted by the NEC? A1. Yes. This is a design issue [90.1(C)]. The NEC does not require habitable rooms in commercial occupancies to have lighting, receptacles, or switches. Therefore the bathroom occupancy sensor is not required to be equipped with a manual override switch. However, if this were a dwelling unit bathroom, then the installation would be a violation of 210.70(A)(1), which requires the bathroom wall switch to be equipped with a manual override that will allow the sensor to function as a wall switch. Note: Ultrasonic sensors are "volumetric" in that they send out sound waves to "fill" a space and then use the Doppler Effect to detect any motion that causes a shift in the returning sound waves, (that's the simplified version). This permits detection behind stall walls in bathrooms. In contrast Passive Infrared or PIR sensors are line of sight and need to "see" the area that is being detected and therefore do not work if there is a barrier (stall wall) between the sensor and what is being detected. Hope this helps, John Larsen Q2. What is the maximum ground resistance required by the NEC for the grounding electrode system? A2. There is no maximum ground resistance specified for the grounding electrode system. However, if a single ground rod is used as the required grounding electrode system, and it's resistance exceeds 25 ohms, then it is must be augmented by one additional electrode located no less than 6 ft from the original ground rod [250.56]. Example: If the first ground rod has a ground resistance of 100 ohms, you only need to add one additional ground rod, regardless of the resistance of the two ground rods. Q3. I am an electrical contractor in NJ, and the inspector wants me to drive two ground rods on a residential 200 amp service. I cannot find anything in the NEC that says I have to drive a supplemental ground rod, unless the first ground rod has a ground resistance of 25 ohms. The inspector's are not measuring the ground resistance, so what is their basis for requiring me to drive two ground rods? A3. The NEC only permits one ground rod to be used if the ground resistance does not exceed 25 ohms [250.56]. Since it's not the electrical inspectors' responsibility to verify the ground resistance, plan on driving two ground rods unless you can prove that the resistance of a single rod is not greater than 25 ohms. Q4. Can a "shared neutral" circuit be used to supply the 20A two small appliance circuits in a dwelling unit? What about the 20A dwelling unit bathroom receptacle circuit? A4. There is nothing in the NEC that prohibits the use of a "shared neutral" for this type of circuit. The NEC added Section 210.4(A) to the 1987 Code to clarify that any branch circuit required by Article 210 can be supplied by a multiwire circuit. Because of the dangers associated with a multiwire branch circuit, 300.13(B) specifies that the removal of a wiring device, such as a receptacle shall not cause an interruption of continuity for the grounded (neutral) conductor. Therefore, the grounded (neutral) conductors for multiwire branch circuits shall be spliced together, and a pigtail shall be provided for device terminations. Author's Comment: The opening of the ungrounded (hot) or grounded (neutral) conductors of a two-wire circuit during the replacement of a device does not cause a safety hazard, so pigtailing of these conductors is not required. Q5. We recently designed an upgrade to an existing Imaging Center that has 208V, three-phase service. We are adding additional equipment that requires 480V, three-phase. We designed a separate service to supply the new 480V equipment. The plans were rejected by the permit plan reviewer, stating that NEC did not allow 2 services to the same building. It is my belief that 230.2(D) permits this design for differing voltages. What's your opinion? A5. You are correct. 230.2(D) states that "additional services shall be permitted for different voltages." Just be sure to comply with 230.2(E) which requires a building supplied by more than one service, to have a permanent plaque or directory at each service disconnect location denoting all other services supplying that building and the area served by each. In addition, be sure you use the same grounding electrode for both services [250.58]. Q6. I'm an electrical designer in Greensboro, NC and recently worked on a new car dealership project. The engineer and I decided that to power the vehicle lifts we would have the contractor to provide an SO cord from a junction box above to the lift motor. The contractor says that the inspector told him that the cord must have a twist-lock connection and receptacle. I've searched the NEC and cannot find where this is required. A6. A twist-lock connector (locking type) is not required for a cord used for this purpose. However, 400.7(B) does require an attachment plug and receptacle (twist-lock not required) for any flexible cord used for the connection of utilization equipment to facilitate frequent interchange in accordance with 400.7(A)(6). However, if the Manufacturer's installation instructions call for a twist-lock plug and receptacle, then it must be provided, [110.3(B)]. Q7. Some 480V-120/208V, three-phase transformers come shipped with a bonding strap that connects the XO terminal to the case of the transformer. I was told by my boss to install a bonding jumper sized to Table 250.66 based on the secondary conductors and leave this factory bonding strap in place. Is this really required by the NEC, because this just doesn't make any sense to me? A7. No you are not required to add an additional bonding jumper if the transformer that is listed by a qualified electrical testing laboratory [90.7]. It is true that 250.30(A)(1) requires a bonding jumper sized in accordance with Table 250.66 to bond the XO terminal to the case. But if the manufacturer has installed one, then there is no need for you to add an additional bonding connection. Q8. Is an equipment grounding (bonding) conductor installed in a metal raceway required to be bonded to the metal box if all the circuit conductors are pulled thru the box without splice or termination? A8. No. The equipment grounding (bonding) conductor is not required to terminate to the metal box, unless the circuit conductors are spliced within the box, or terminated on equipment within the box [250.148]. Q9. I noticed recently that most power cords, 15A, 120V cords have small holes in the prongs of the plugs. Do you have any idea why they are there? A9. They are not there for any electrical purpose in accordance with the NEC or UL, nor are they there to provide for a lock-out device. They are just there for the manufacturing process. Q10. How do you size the conductor and protection device for a 15 kVA, 240V, three-phase space heater? A10. Conductors and protection devices for electric space heating equipment must be sized no smaller than 125% of the ampere rating of the equipment [424.3(B)]. Equipment Ampere Rating = kVA/(Volts x 1.732) = 15,000/(240V x 1.732) Equipment Ampere Rating = 36A Conductor and Protection Not Less Than = 36 x 1.25 = 45A Conductor = 6 AWG for 60C terminals or 8 AWG for 75C terminals [110.14(C)] Protection = 45 or 50A device [240.6(A)] |
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