Article 110—Requirements for Electrical Installations
Article 310—Conductors for General Wiring
Article 400—Flexible Cords and Cables
Article 695—Fire Pumps
Table 8 Chapter 9, NFPA 20—1999 Standard for the Installation of Stationary Pumps for Fire Protection
Running parallel conductors
Q: We have an electrical job wiring a small commercial building where the Code-calculated load is 365 amperes per phase and the voltage is 208Y/120. We ran two sets of No. 3/0 copper conductors with Type THWN insulation in 3-inch rigid metal conduit for the service. The electrical inspector told us that the wires were too small because we had to derate them. He said that there were more than three current-carrying conductors in a single raceway. We pointed out to him that part of Section 310-4 says that conductors connected in parallel are electrically joined at both ends to form a single conductor. Therefore, we have three single conductors and a neutral that carries only the unbalanced current, and derating is not required. Who is right?
A: The parenthetical phrase in Section 310-4 describes what is meant by the phrase “parallel conductors.” Here is the paragraph as it appears in Section 310-4: “Aluminum, copper-clad aluminum, or copper conductors of size No. 1/0 and larger, comprising each phase, neutral, or ungrounded circuit conductor, shall be permitted to be connected in parallel (electrically joined at both ends to form a single conductor).”
An example will show why it is necessary to derate parallel conductors in a single raceway. We will compare the I5R losses in No. 3/0 conductors with the losses in a 500kcmil conductor. Both of these conductor combinations will be assumed to be carrying 380 amperes. Since there are two No. 3/0 copper conductors per phase, each conductor carries 190 amperes. The single 500kcmil copper conductor carries 380 amperes. To keep the calculations simple, a 100-foot length will be used.
According to Table 8 “Conductor Properties,” a No. 3/0 uncoated copper conductor has a resistance of 0.0766 ohm per 1,000 feet, or 0.00766 ohm per 100 feet. Therefore, the watts loss per 100 feet for a single No. 3/0 copper conductor is (190 x 190 x 0.00766) 276.5 watts. Since there are two conductors per phase, the loss in each phase is (276.5 x 2) 553 watts. For the 500kcmil conductor the loss is (380 x 380 x 0.00253) 372.5 watts. This shows that the loss in the 500kcmil conductor is only about 67 percent of that in the No. 3/0 conductors connected in parallel. The larger loss in the parallel conductors causes overheating, which must be avoided.
Derating the No. 3/0 conductors to 80 percent of their table ampacity (200), because there are at least six current-carrying conductors in the raceway, (there are eight current-carrying conductors if the neutrals are considered to be current carrying) results in a corrected ampacity of 160. The watts loss per phase for each 100 feet is now (160 x 160 x 0.00766 x 2) 392 watts. This figure compares favorably with the 372.5-watt loss for a single 500kcmil copper conductor.
These sample calculations indicate that conductor derating is necessary where more than three current-carrying conductors are installed in a single raceway. Now that we have proved that the No. 3/0 conductors are not adequate for the calculated load of 365 amperes, let’s see what the watt loss is for the No. 4/0 copper conductors derated to 80 percent.
According to the Table 310-16, No. 4/0 copper conductors with Type THWN insulation have an ampacity of 230. The derated value is 184. The I5R loss in 100 feet is (184 x 184 x 0.00608 x 2) 412 watts. For the 500kcmil conductor carrying the calculated load of 365 amperes, the loss is (365 x 365 x 0.00258) 344 watts.
These examples show that losses are greater in parallel runs than in a single conductor, but without derating the parallel conductors, heating would be excessive.
Overcurrent protection for instantaneous water heater
Q: What is the maximum size overcurrent protective device permitted for a 16kW, 240-volt, single-phase instantaneous water heater?
A: Instantaneous water heaters are not classified as a continuous load; therefore, part (c) of Section 422-10 applies. The full load current for this water heater is 16,000 divided by 240, or 67 amperes. If the overcurrent protection rating is not marked on the appliance, the rating can be 150 percent of the full load current, or 100 amperes.
This size overcurrent device seems excessively large for a water heater with resistance type heating elements. I would use an 80-ampere overcurrent device with No. 4 copper conductors with 75 degrees Celsius insulation or No. 3 conductors with 60 degrees Celsius insulation, depending on the temperature ratings of the circuit breaker, or switch and fuse, and the terminals on the water heater. Finally, I would make sure that I have not violated the manufacturer’s installation instructions as required by Section 110-3(b).
Overcurrent protection for service equipment
Q: A 400-ampere nonfused switch feeds a group of fused disconnects that vary in ampere ratings in an electric meter room. The service conductors are 500kcmil copper and feed various sizes of conductors from a 6-foot wireway. Does this arrangement of service equipment meet the requirement in Section 230-91, which indicates that the overcurrent device must be an integral part of the service disconnecting means or be located immediately adjacent thereto?
A: This is one of the National Electrical Code (NEC) rules that requires the electrical inspector to use his or her judgment. Without seeing the electrical installation, I can’t be sure, but since all of the service overcurrent devices are in an electric meter room, I am inclined to say that the installation satisfies the requirement in Section 230-91.
Exception No. 3 to Section 230-90(a) allows up to six circuit breakers or six sets of fuses as the overload protection for the service conductors. Also, the sum of the ampere ratings of the overcurrent devices are permitted to exceed the ampacity of the service conductors, provided that the load calculated in accordance with Article 220 does not exceed the ampacity of the service conductors.
Wiring methods for fire pumps
Q: What wiring methods are suitable for fire pumps? Is rigid metal conduit required for all wiring in a fire pump room?
A: Where the fire pump circuit conductors are supplied from a separate service or a connection ahead of the service disconnecting means and the service conductors are encased in 2 inches of concrete, any wiring methods that are allowed for services and that are permitted to be encased in concrete are acceptable. Some of these wiring methods include rigid metal conduit, intermediate metal conduit, electrical metallic tubing, rigid nonmetallic conduit, Type MI cable, etc. (See Section 230-43 for a complete list of wiring methods that can be used for service-entrance conductors and the wiring method articles for wiring methods that are permitted to be encased in concrete.)
Indoor fire pumps must be separated from all other areas of the building by two-hour fire-rated construction. This separation is permitted to be reduced to one-hour where the building is protected by an automatic sprinkler system that is installed in accordance with NFPA 13. These requirements are in Section 2-7.1.1 of the 1999 edition of NFPA 20 “Standard for the Installation of Stationary Pumps for Fire Protection.”
Where a disconnecting means and overcurrent protection are provided ahead of the fire pump controller, Section 695-6(b) of the NEC applies. This Section requires encasing the wiring method in 2 inches of concrete or using a wiring method that has a one-hour fire resistance rating, or wrapping the wiring method with a circuit protective system with a minimum one-hour fire resistance rating.
The fire pump circuit conductors in the pump room do not need one-hour fire protection. In fact, the wiring methods between the fire pump controller and fire pump motor can be any of the following: rigid metal conduit, intermediate metal conduit, liquidtight flexible metal conduit, orliquidtight flexible nonmetallic conduit Type LFNC-B or Type MI cable.
Storage tank water heaters
Q: Does anything in the NEC prohibit using a cord-and-plug connection as the power supply to a 50-gallon electric water heater? This heater supplies an apartment in an apartment building.
A: I don’t know of anything in the NEC that prevents such an installation, but I don’t know of anything that allows a cord-and-plug connection, either.
Section 400-8(1) prohibits the use of flexible cords as a substitute for fixed wiring. Section 422-16 allows flexible cord connection to an appliance to facilitate frequent interchange or where the mechanical connections are specifically designed to permit ready removal for maintenance or repair, and the appliance is intended or identified for flexible cord connection.
The 1999 edition of the General Information for Electrical Equipment Directory (White Book) published by Underwriters Laboratories Inc., has this sentence under the title, “Household Storage Tank”: “They are intended for household use in ordinary locations and permanent connection to the supply source in accordance with the National Electrical Code.”
To comply with the manufacturer’s instructions provided with the product, the water heater must be permanently connected to the interior wiring system. Connection by cord and plug is a violation of Section 110-3(b). A disconnecting means must also be provided to comply with Section 422-31(b).
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at (504) 254-2132.