By Mike Holt for EC&M Magazine
Q1. A 1200A main breaker is fed with 3 sets of 500 kcmil THHN conductors. Is this installation legal?
A1. No. Where the overcurrent device is rated over 800 amperes, the conductors must have an ampacity of not less than the rating of overcurrent protection device [240.4(C)]. Its true that the total ampacity of 3 sets of 500 kcmil THHN conductors at 90°C is greater than 1200A (430A x 3 = 1290A), but the conductors must be size based on the 75°C column of Table 310.16 in accordance with 110.14(C)(1)(b). At 75°C, the 500 kcmil THHN are only rated 380A x 3 = 1140A, therefore 600 kcmil conductors must be used.
Q2. I have a 1200A service and I want to supply it with 3 sets of 500 kcmil THHN conductors. I would like to set a splice box near the termination point and splice the 500 kcmil conductors to 600 kcmil THHN conductors. Is there anything wrong with this picture?
A2. This is fine if the splicing fittings are rated for 90°C and they are installed in accordance with 110.14 of the NEC.
Q3. I am moving a pool pump motor and the inspector wants me to splice the 8 AWG bonding jumper by exothermic welding or an irreversible compression type-connector listed. Is he correct?
A3. No. Connections of the 8 AWG pool bonding conductor must be made by exothermic welding or by pressure connectors or clamps that are labeled as being suitable for the purpose [680.26(C)].
Q4. My question is about the new trend for residential, under the counter, washer and dryer installations. I dont think the cord is accessible to be used as a disconnecting means when the appliance is installed? Am I correct in my interpretation of the term accessible?
A4. No. Accessible means capable of being removed or exposed without damaging the building structure or finish or not permanently closed in by the structure or finish of the building . So the installation is fine, see 422.33.
Q5. The 2002 NEC states that all branch circuits that supply 125V, 15A or 20A outlets in dwelling unit bedrooms be AFCI protected. Would this apply to smoke detectors and wall air conditioning units connected to a 125V, 15A or 20A circuit? Is AFCI protection required for switches located in the bedroom that controls a lighting outlet in another space?
A5. AFCI protection is required for all 125V, 15A and 20A outlets, and this would include the outlet for smoke detectors as well as wall air conditioners. Im not sure if the AFCI breaker will nuisance trip when it supplies large inductive reactive loads such as air conditioning equipment on dedicated circuits. Time will tell.
About the switches, if the switch controls utilization equipment in the bedroom, then it will be AFCI protected. However, if the switch operates lighting outlets for outdoor luminaire, closets or other loads not terminated in bedroom space then AFCI protection is not NEC required, because a switch is not considered an outlet*.
*According to Article 100, an outlet is defined as a point on the wiring system at which current is taken to supply equipment that utilizes electric energy for electronic, electromechanical, chemical, heating, lighting, or similar purposes . This would include a receptacle outlet, a lighting outlet, but not a switch.
Q6. Is ground-fault protection of equipment of the type specified in 230.95 required for a 2000A, 480/277V, three-phase, 4-wire generator main breaker?
A6. The general requirement is that ground-fault protection of equipment must be provided for 480/277V electrical services and feeders disconnect rated 1000 amperes or more [215.10 and 230.95]. However, such protection is not required for emergency standby power disconnects [700.26] or required standby power disconnects [701.17].
Q7. During an infrared inspection, we found a 400A breaker that had nine conductors per phase on the load side, consisting of 10 AWG, 8 AWG and 1/0 AWG conductors. The customer claimed it was legal per the 10' tap rule.
A7. This installation does not violation of the 10 ft feeder tap rule, assuming all of the requirements contained in 240.21(B)(1) are followed. But, 110.14(A) only permits one conductor per terminal unless the terminal is so identified. Naturally, there are no 400A breakers identified to be use with nine conductors.
Q8. Most dwellings in our area have plastic underground water pipes supplying the house. The internal water piping is changed to copper inside the house. My inspector requires that we bond the interior metal water pipe system at the water heater. But I fee that 250.52(A)(1) says that the bonding connection must not be made more than 5' from the point of entry. I say it is a violation to make this connection further than 5 ft from the point of entry. Am I right?
A8. No. The 5 ft from the point of entry rule contained in 250.52(A)(1) applies when the metal water pipe is used as a conductor to interconnect electrodes that are part of the grounding electrode system. This rule does not apply to the bonding requirements of 250.104, which only limits the connection to an accessible location.
Q9. What is the maximum height of an exit sign?
A9. There is no minimum or maximum height in the NEC. But Life Safety Code 101 specifies that floor proximity exit lights, if required by local codes be located with the bottom between 6 and 8 inches above the floor. In addition, NFPA 5000 section 126.96.36.199 states that "Egress markings must be located at a vertical distance of not more than 6 ft 8 in (2 m), measured from above the top edge of the egress opening..."
Q10. I am working in an old home and I want to replace the existing two prong receptacles with grounding-type (3-wire) receptacles, even though the wiring method only has only 2 wires. What do I have to do?
A10. According to 406.3(D)(3), where no
grounding (bonding) means exists in the outlet box, such as old 2-wire NM cable without
a ground, nongrounding type receptacles can be replaced with:
Note: GFCI protection functions properly on a 2-wire circuit without an equipment grounding conductor, because the equipment grounding conductor serves no purpose in the operation of the GFCI protection device.
CAUTION: Permission to replace nongrounding type receptacles with GFCI-protected grounding type receptacles does not apply to new receptacle outlets that extend from an existing ungrounded outlet box. Once you add a receptacle outlet (branch-circuit extension), the receptacle must be of the grounding type and it must have its grounding terminal grounded in accordance with 250.130(C).
Q11. I have a 20A branch circuit feeding modular office furniture receptacles that are rated 15A. The inspector tells me that I must replace the 15A receptacles with 20A receptacles. Is he right?
A11. No. Where connected to a branch circuit that supplies two or more receptacles, a 15A receptacle can be installed and protected by a 20A overcurrent protection device [Table 210.21(B)(3)].
If you have any comments, let me know Mike@MikeHolt.com
Copyright © 2002 Mike Holt Enterprises,Inc.