Article 110 Requirements for Electrical Installations
Article 225 Outside Branch Circuits and Feeders
Article 240 Overcurrent Protection
Article 250 Grounding
Article 450 Transformers and Transformer Vaults
Article 680 Swimming Pools, Fountains, and Similar Installations
Article 700 Emergency Systems
Article 701 Legally Required Standby Systems
Working space around electrical equipment
Q: Where the working space is 6 feet in front of a 208Y/120 volt, 1600 ampere switchboard that is more than 6 feet wide, a single entrance is permitted by 110.26(C)(2)(b). Does the door have to swing outward and is panic hardware required?
A: The minimum working space for a 208 volt switchboard is listed as 3 feet in Table 110.26(A)(1). And part (C)(2) requires two entrance doors at least 2 feet wide and 6.5 feet high at each end of the working space. These doors must open in the direction of egress and be equipped with pressure plates, panic bars, or other devices that allow the doors to open under pressure. However, only one door is required where the working space is twice the space required by the Table previously mentioned. This single door must be at least 2 feet wide and 6.5 feet high; must open in the direction of exit from the room, and be equipped with a pressure plate or panic hardware. It is also necessary to locate the door so that the nearest edge of the opening is not less than 3 feet from the switchboard.
Grounding separately derived systems
Q: What size grounding electrode conductor is required for a 500 kVA transformer with a 480 volt, 3 phase primary and a 240 volt delta secondary? The secondary conductors are 3-sets of 400 kcmil copper conductors with Type THWN insulation, and the grounding electrode for the building is structural steel. The plans and specifications require a grounded conductor (corner ground) on the secondary.
A: Although the secondary voltage and delta connections are not required to be grounded by 250.20, the secondary is permitted to be grounded, provided that grounding complies with applicable provisions of Article 250. The Fine Print Note in 250.20 mentions corner grounding delta systems and references 250.26 (4) for the conductor to be grounded. It says that on a multiphase system that is grounded, one phase conductor shall be grounded.
The grounding electrode conductor and bonding jumper must be connected at the same point on the separately derived system. This is required by Item 2(a) of 250.30(A). This part also requires the use of table 250.66 to obtain the size of the grounding electrode conductor. Note A to this Table advises that the Table applies to the derived conductors of a separately derived AC system. Therefore, 3-400 kcmil copper conductors require a 3/0 AWG copper or 250 kcmil aluminum grounding electrode conductor. This grounding electrode must be run to the nearest effectively grounded structural metal member of the building. Also, a bonding jumper (the same size as the grounding electrode conductor) must be provided to bond the metal water pipe in the vicinity of the separately derived system to the metal frame of the building if the metal water pipe is not bonded to effectively grounded structural steel of the building in the area served by the separately derived system. This requirement is in 250.104(A)(4)
Supplying emergency lighting
Q: Article 700 requires unit equipment for emergency lighting to be supplied from a normal lighting branch circuit. Is this branch circuit an emergency circuit? Does the circuit breaker that supplies this branch circuit have to be provided with a lock-on device?
A: According to 700.12(E) unit equipment consists of a battery, a battery charger, provisions for one or more lamps mounted on the unit or have provisions for remote lamps, and a relay to energize the lamps when there is a power outage. The branch circuit that supplies the unit equipment must be the same branch circuit that serves the normal lighting in the area, and be connected to the circuit ahead of any wall switches or any other switches that control the normal lighting in the area covered by the emergency lighting.
A lock-on device is not required for the circuit breaker that energizes the circuit, but this circuit breaker must be clearly identified to indicate that it supplies emergency lighting. However, a lock-on device is required if a separate circuit feeds the unit equipment under the exception to 700.12(E). This exception permits a separate branch circuit for the unit equipment where at least three normal lighting circuits are provided in an uninterrupted area, all branch circuits originate at the same panelboard, and a lock-on feature is provided for the branch circuit overcurrent device that supplies the unit equipment.
The branch circuit wiring that supplies the unit equipment along with normal lighting for the area must be installed to comply with the appropriate wiring method in Chapter 3. If there are remote lamps that are energized from the unit equipment battery, the wiring for these lamps must comply with 700.9(A)(B) and (C).
Disconnect for an emergency generator
Q: Several changes in the 2002 NEC were made concerning the disconnecting means for an outdoor emergency generator that are not very clear. What are the rules for disconnecting means for an outdoor generator that supplies emergency power to a building?
A: I admit that the revision to 700.12(B) is not perfectly clear. The intent of the revision which appears as Item (6) in 700.12(B) is to not require a disconnecting means on or in the building served at the point where the generator feeder enters the building or structure if the outdoor generator is provided with a readily accessible disconnecting means and is within sight of the building or structure. To remove the ambiguity, the words “enter or” should be added near the end of the sentence. The revision would read like this: “Where an outdoor housed generator set is equipped with a readily accessible disconnecting means located within sight of the building or structure supplied, an additional disconnecting means shall not be required where ungrounded conductors enter or pass through the building or structure.”
A similar change appears in 701.11 (B) (5) for Legally Required Standby Systems.
Both of these Articles require compliance with other parts of the Code unless modified by these Articles. This means that 225.31 and 225.32 apply to the location of disconnecting means for an outdoor feeder that enters a building. And 225.32 requires that a disconnecting means be installed either inside or outside of the building or structure served. The substantiation for this change indicated that a disconnecting means and overcurrent protection were provided on the generator which was about 10 feet from the building, but another disconnect was required on the building by the inspector. This revision eliminates the requirement for an additional disconnect at the building or structure where the disconnect provided with the generator is readily accessible and within sight of the building served.
Connecting swimming pool pump motors
Q: May a 240-volt single phase pump motor for a permanently installed swimming pool be cord and plug connected? The pool is in the back yard of a one-family residence. The nameplate rating on the motor is 8 amperes, 240 volts, single phase. Is GFCI protection required for this motor circuit?
A: Cord and plug connections are permitted for pool pump motors provided that the cord is not more than 3 feet long, the equipment grounding conductor in the cord complies with 250.122, and the outer end of the cord terminates in a grounding-type attachment plug.
The receptacle for the pump is permitted to be located not less than 5 feet from the inside walls of the pool if it is a single receptacle of the locking type, is a grounding type, and has Ground-Fault Circuit-Interrupter protection, otherwise it must be located not less than 10 feet from the inside walls of the pool.
Item (5) in 680.22 requires GFCI protection for 15- and 20-amperes, 120 and 240-volt receptacles supplying pool pump motors.
Transformer secondary overcurrent protection
Q: What size overcurrent protection is required for the secondary of a 75 kVA, 480 volt delta primary 208Y/120 volt secondary transformer?
A: Overcurrent protection for the secondary windings may not be required. Overcurrent protection for transformers rated 600 volts or less is covered by 450.3(B) and Table 450.3(B).
The primary full-load current for this transformer is about 90 amperes (75000 divided by 480 x 1.73). If the primary overcurrent protection does not exceed 125 amperes, which is 125 percent of full-load current (1.25 x 90) plus permission by Note 1 to the Table to increase the overcurrent protection to the next larger size as shown in 240.6, overcurrent protection for the secondary windings is not required. However, overcurrent protection for the secondary conductors is required by 240.4(F).
Maximum overcurrent protection for this transformer is 225 amperes. Any primary overcurrent device that exceeds 125 amperes but does not exceed 225 amperes requires secondary winding protection that does not exceed 300 amperes. This figure is obtained by multiplying the secondary full-load current by 1.25 percent and increasing the overcurrent protection to the next larger standard size fuse or circuit breaker shown in 240.6. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.