Article 110 Requirement for Electrical Installations
Article 215 Feeders
Article 220 Branch-Circuits, Feeders, and Service Calculations
Article 230 Services
Article 240 Overcurrent Protection
Article 250 Grounding
Article 300 Wiring Methods
Article 310 Conductors for General Wiring
Article 408 Switchboards and Panelboards
Three-phase, four-wire delta service
Q:I have a 400-ampere service supplied by a three-phase, four-wire delta system. The three-phase load is 125 amperes, and the single-phase load is 230 amperes. Am I permitted to size the high-leg conductor based on the three-phase load or must it be the same size as the other ungrounded conductors?
A:The ampacity of service-entrance conductors must be determined from load calculations based on Article 220. The minimum size of service-entrance conductors is covered by 230.42. Conductor size for the high-leg is permitted to be smaller than the other ungrounded conductors by this rule.
The three-phase conductor with the higher voltage-to-ground (about 208 volts on a 120/240 delta system) is considered to be the “B” phase in the NEC and must occupy the center position in switchboards and panelboards. This requirement is in 408.3(E).
The “B” phase must be marked with an orange outer finish or by other effective means at each termination or junction point of the service conductors. This requirement appears in 230.56. Also, 110.15 and 215.8 require that the “B” phase be identified in a similar manner at each point where a connection is made if the grounded conductor is present.
According to Table 310.16, the minimum sizes of service-entrance conductors are No. 1 Type THWN copper for the high-leg and 400 kcmil Type THWN copper conductors for “A” and “C” phases. The grounded circuit conductor or neutral for the single-phase loads does not have to be larger than 4/0 Type THWN copper, and would be smaller if there are 240-volt single-phase loads. The sizes of the service-entrance conductors could be two 500 kcmil, one 4/0 and one No. 1 Type THWN copper wires in 3-inch, intermediate metal conduit.
Overcurrent protection must be provided for conductors connected to the load side of the service-entrance conductors. Various parts of 240.4 provide overcurrent protection for conductors under different conditions. Part (B) of 240.4 generally applies to the single-phase loads and part (G) applies to overcurrent protection for specific applications such as motor and air-conditioning loads.
If a single disconnecting means is used as the service disconnect, it will probably be necessary to use a fused switch with a fuse reducer for the center or “B” phase. A 400-ampere, three-pole with a solid neutral fused disconnect switch marked “Suitable for Use As Service Equipment” or “Suitable for Use Only as Service Equipment” with a 200-ampere fuse reducer installed in the center position for the “B” phase satisfies the Code.
Instead of a single switch, two panelboards with mains could be used. Both panels could be supplied from a wireway that contains the service-entrance conductors. The three-phase panel would have a 150-ampere main and the single-phase panel a 250-ampere main. Other options are also available for supplying all of the loads.
Parallel conductors in separate raceways
Q: Am I permitted to install separate phase conductors in nonmetallic raceways? Parallel conductors are in an auxiliary gutter above a large switchboard. Does the Code allow individual phase conductors in a single conduit nipple from the gutter to the switchboard? This will permit termination of the parallel phase conductors in the switchboard from four nonmetallic conduits that will be located above the terminals in the board. Slots will be cut in the switchboard enclosure between holes to reduce the inductive effect. The switchboard is 1,200 amperes and the feeder conductors for it are four 500 kcmil Type THWN aluminum conductors per phase.
A: Yes, parallel conductors are permitted for each set of phase conductors in a single nonmetallic raceway. This is a good way to terminate parallel conductors. An arrangement like this makes it much easier to maintain the same lengths of each conductor connected to the same terminal. An exception to 300.3(B)(1) permits isolated phase conductors to be run underground in nonmetallic raceways where parallel conductors are involved, and 300.20(B) permits a single conductor carrying alternating current to enter or leave a metal enclosure. Where the metal has magnetic properties, slots must be cut in the metal between each hole that contains conductors. You are also allowed to pass all conductors of the feeder through an insulating wall in the switchboard large enough for all conductors.
I assume the nonmetallic conduit nipples are less than 24 inches long; therefore, derating because there are four current-carrying conductors in a raceway does not apply. Exception No. 3 to 310.15(B)(2) is the reference that applies.
The question states that slots will be cut between the holes in the switchboard, but the auxiliary gutter is not mentioned. Slots are required between all PVC conduits and the holes in the auxiliary gutter if it is made of magnetic metal.
GFCIs in commercial kitchens
Q: Are GFCI circuit breakers or receptacles required on 15- and 20-ampere, 125-volt branch circuits that supply refrigerators and freezers in commercial kitchens? Since there is some concern about refrigerators and freezers tripping GFCIs, may these appliances be supplied from dedicated branch circuits with a single receptacle on each circuit and not protected by a GFCI?
A: No, all 15- and 20-ampere, 125 volt receptacles installed in kitchens of any occupancy must have ground-fault protection for personnel.
There should be no concern about tripping a GFCI that supplies a refrigerator, since listed refrigerators must pass a leakage current test to obtain a listing. This leakage current is much lower than the 4 to 6 milliampere trip current of a GFCI.
At least three proposals submitted to expand the GFCI-protection requirements to “All Occupancies” were rejected. The panel statement for rejection was: “Insufficient substantiation has been presented to extend all of the GFCI requirements to all occupancies.”
During the public comment period, the IAEI’s Michael J. Johnston provided facts on electrocutions. One involved a restaurant manager who contacted an ungrounded refrigerator with an internal ground fault that was located in the restaurant kitchen.
Code Making Panel No. 2 modified the original proposal by expanding the requirements for GFCI receptacles to include kitchens in all occupancies because the submitters’ substantiation only addressed incidents in commercial kitchens.
Underground wiring to a post light
Q: What is the minimum burial depth for Type UF cable run from a house to a post light at the end of the driveway? Is it necessary to ground the metal post?
A: The answer is found in Table 300.5 and is based on a number of variables. Let’s look at them. The maximum UF-cable burial depth is 24 inches and applies where the cable is run under streets, highways, roads, alleys, driveways and parking lots. For one- and two-family dwelling driveways and parking areas used for dwelling-related purposes, the maximum burial depth is reduced to 18 inches.
Residential branch circuits rated 120 volts or less with GFCI protection and overcurrent protection not exceeding 20-amperes qualify for a burial depth of only 12 inches, which applies where installed under dwelling driveways and outdoor parking areas associated with one- and two-family dwellings.
Part (B) of 300.5 requires grounding and bonding of underground installations. The equipment-grounding conductor in the UF cable must be connected to the metal pole supporting the luminaire (lighting fixture). This conductor must comply with 250.118 and be sized according to Table 250.122.
Grounding metal elbows
Q: I ran a 2-inch PVC conduit for service-entrance conductors underground and used a metal elbow to turn up above grade, then continued with PVC to the meter base. The electrical inspector says that I have to ground this elbow. Is this required by the NEC? Is there an exception that does not require this elbow to be grounded?
A: No, the exception does not apply to the situation you describe. Metal raceways that contain service-entrance conductors are required to be grounded by 250.80. The exception permits metal elbows installed underground in nonmetallic conduit to remain ungrounded where the minimum cover to any part of the metal elbow is 18 inches.
The elbow must be grounded to the grounding electrode system by one of the methods mentioned in 250.68 and 250.70. If the meter socket and service disconnecting means are close to the metal elbow, the grounding conductor that is sized from Table 250.66 can be connected to the (neutral) grounded circuit conductor bus in the service disconnect or meter base where permitted by the authority having jurisdiction and utility company.
Demand factors for electric clothes dryers
Q:A 30-unit apartment building is under construction. Each unit will have provisions for an electric clothes dryer. How is the demand factor calculated now that Table 220.18 is changed in the 2002 NEC?
A: The table was changed because some parts allowed a lower demand as the number of dryers increased. For example, 10 dryers rated at 5 kW had a demand of 25 kW while 11 dryers had a demand of 24.8 kW. Or 20 dryers had a demand load of 35 kW, but 19 dryers had 38 kW. The table as it appears in 220.18 is corrected so that the demand load always increases as the number of dryers increases.
Let’s use Table 220.18 to calculate the demand load for 30 electric dryers. Here is the formula: Percent = 35 – (0.5) x number of dryers – 23). Percent = 35 – (0.5) x 30 – 23 = 31.5 percent. Use 31.5 percent of the total load of 30 dryers. This results in a demand load of (0.315 x 30 x 5kW) = 47.25 kW. Using the table in the 1999 NEC edition results in a demand of 45 kW. The changes made in the table result in a demand load 2.25 kW higher than in the previous edition. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.