CODE CITATIONS: Article 110-Requirements for Electrical Installations; Article 210-Branch Circuits; Article 220-Branch Circuit, Feeder, and Service Calculations; Article 230-Services; Article 250-Grounding; Article 310--Conductors for General Wiring; Article 430-Motors, Motor Circuits, and Controllers

Conductor Derating

Q: We have a job to install a duplex pumping panel (two combination motor starters in a single enclosure) for two 5-horsepower motors driving sump pumps. An alternator switch is included in the pump starters that alternates starting of the motors each time the float switch reaches a predetermined level in the sump resulting from an increase in the height of the liquid in the tank. We plan to run eight conductors in a single PVC conduit from the controller enclosure to the motors. Do we have to derate the circuit conductors because there are more than three in the PVC conduit? What is the minimum size of the equipment grounding conductors? Do we need one or two equipment-grounding conductors? The voltage is 208-three phase.

A: The full-load current of a 5-horsepower, 208-volt three-phase motor is 16.7 amperes according to Table 430-150. The motor branch circuit conductors must have a minimum capacity of 125 percent of motor full-load current to comply with Section 430-22. This results in a minimum conductor ampacity of (1.25 x 16.7) 20.9. According to the Conductor Ampacity Table (Table 310-16) No. 12 copper with Type THWN insulation is suitable if derating is not required because of the number of current-carrying conductors in the raceway.

Many duplex motor controllers are internally wired so that both pumps will operate simultaneously if the liquid level continues to rise while one pump is running.

Because I am not sure about the control circuit wiring, I will assume that both pumps cannot run at the same time. The calculations and motor circuit wire sizes will also be done assuming that both pumps can operate at the same time.

If the factory installed control circuit only allows one motor to run, regardless of the height of the liquid in the sump, there are never more than three current-carrying conductors in the PVC raceway; therefore, there is no derating because of raceway fill. Derating for more than three conductors in a raceway does not apply because Table 310-15(b)(2)(a) applies only where there are more than three conductors carrying current at the same time. Under this condition the No. 12 Type THWN copper conductors are adequate.

The ampere rating of the circuit breaker in the combination starter that provides motor branch-circuit short-circuit and ground-fault protection is probably 40 amperes. According to Table 250-122, the minimum size equipment grounding conductor permitted with a 40-ampere overcurrent device is No. 10 copper, but Section 250-122(a) says that the equipment grounding conductor does not have to be larger than the circuit conductors. Therefore, a No. 12 copper equipment-grounding conductor may be used.

While on the subject of grounding, let's determine the number of equipment grounding conductors required to ground the two pump motors. The question states that two equipment-grounding conductors are provided in the single PVC conduit. Actually, Section 250-122(c) only requires one equipment- grounding conductor for the two motors. The six circuit conductors and one equipment grounding conductor (all No. 12 with Type THWN insulation) may be installed in one-half inch schedule 40 PVC conduit.

If the factory-installed control wiring allows both pump motors to operate at the same time, the wire size for each motor between the controller and the motor must be increased to No. 10 copper. This is because at some times there will be six current-carrying conductors in the conduit that serve the motors. Table 310-15(b)(2)( a) requires that the ampacities shown in Table 310-16 be derated to 80 percent where there are six current-carrying conductors in a raceway. Minimum branch circuit conductor ampacity before derating is 20.9. After applying the 80 percent derating factor, the minimum conductor ampacity
is slightly greater than 26. Therefore, No. 10 copper conductors must be used. If Type THWN insulated No. 10 copper conductors are used, the minimum conduit size is three quarters of an inch. The conduit size is obtained from Table C10 in the National Electrical Code. The equipment-grounding conductor must also be changed to No. 10 copper.

The size of the circuit conductors from the panelboard to the controllers and ampere rating of the overcurrent device in the panelboard depends on the operation of the motor starters. If the control wiring prohibits both pumps from operating at the same time, the branch circuit conductors supplying the controllers can be No.12 copper protected by a 40-ampere circuit breaker. On the other hand, if the factory installed control wiring allows both pumps to run at the same time, the supply conductors are a feeder and must be sized to comply with Section 430-24. This means that the ampacity of the feeder conductors cannot be less than (1.25 x 16.7 + 16.7) 37.6. According to Table 310-16, No. 8 copper conductors would be required. Motor short-circuit and ground-fault protection is covered in Section 430-62(a). A 50-ampere circuit breaker would be acceptable for this application. This ampere rating is based on the ampere rating of one motor short-circuit and ground-fault protective device plus the full load current of the other motor.

Temperature Limitations

Q: If twist-on connectors with a 60 degrees Celsius rating are used on conductors with 75 degrees Celsius insulation, am I limited to the 60 degrees Celsius ampacity given in Table 310-16?

A: Yes you are. Section 110-14(c) has this requirement: "The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both."

Where it is necessary to apply correction factors for elevated ambient temperature or more than three current-carrying conductors in a raceway or cable, the corrected ampacity is limited to the 60 degrees Celsius rating shown in Table 310-16 even though the corrected ampacity exceeds this value.

Circuit Breaker Handle Ties

Q: In a one-family dwelling unit may single-pole circuit breakers with or without handle ties be used to supply a 240-volt water heater, a 120/240 volt clothes dryer or range, or a multiwire small appliance branch circuit?

A: Single-pole circuit breakers with approved handle ties may be used for all loads indicated except possibly the small appliance branch circuits. Section 240-20(b)(1) and (2) are the references that permit handle ties on the circuit breakers for these loads. However, the multiwire small appliance circuit is another matter. If the small appliance multiwire branch circuit supplies split-wired duplex receptacles (two circuits connected to the same receptacle), a handle tie is required to comply with Section 210-4(b). If each ungrounded conductor and neutral are connected to duplex receptacles that are not split-wired, single-pole circuit breakers without handle ties satisfy the requirement of Section 210-4(b). Remember that Section 210-8 (a)(6) requires ground-fault circuit-interrupter protection for all receptacles that are installed to serve countertop surfaces in the kitchen.

Sizing Service-Entrance Conductors

Q: A commercial building is provided with four 200-ampere service switches that are supplied from a wireway. The service-entrance conductors feeding the wireway consist of two parallel copper conductors per phase. The phase conductors are 400 kcmil and neutral consists of two No. 4/0 copper conductors. Each switch has 200-ampere fuses. The conductors connected to the line and load terminals of each switch are No. 3/0 Type THWN copper. Is this installation code compliant? If it is, what prevents overloading the 400 kcmil service-entrance conductors?

A: The minimum ampacity of the service-entrance conductors must be calculated by using the appropriate rules in Article 220. If the result of this calculation indicates the load is 670 amperes or less, both 400 kcmil copper conductors with Type THWN insulation per phase are Code compliant. What prevents overloading these service-entrance conductors?
Sizing the conductors to handle the calculated load should prevent overloading. Adding electrical loads to the existing installation without increasing the size of the service-entrance conductors will result in overloading of these conductors.

Because service-entrance conductors are not protected from overcurrent at the point where they receive their supply, various Sections in Article 230 require that disconnecting means and overcurrent protection be provided outside of the building or as close as possible to the point where the conductors enter the building. Where this cannot be done Section 230-6 requires that the service-entrance conductors be encased in 2 inches of concrete.

The service neutral conductor ampacity is allowed to be less than the ampacity of the phase conductors by Section 220-22. However, the neutral conductor size cannot be smaller than given in Table 250-66. Section 250-24(b) explains the method to be used for determining the minimum-size grounded conductor (neutral). Since this service is for a commercial building, some thought should be given to Fine Print Note No. 2 following Section 220-22. This note advises that nonlinear loads connected to a three-phase, four-wire, wye-connected system may cause high harmonic neutral currents. Because of this possibility, it may not be proper to reduce the ampacity of the neutral to 460 amperes.
