Special-Purpose Outlets- Water Pump, Water Heater--part 2: MOTOR CIRCUIT DESIGN

.

We will now take a look at the NEC requirements for all of the basics for a typical electrical motor branch circuit.

The pump circuit in this residence originates in Main Service Panelboard A, Circuit A5-7.

Current and Voltage

This information is found on the nameplate and instructions furnished with the pump.

The water pump in this residence is a 1-horse power (hp), single-phase 115/230-volt pump. Dual voltage-rated pumps can be supplied by either 115 volts or 230 volts. When connected for the higher voltage, the current draw will be half of that when connected for the lower voltage. For example, in Table 430.248, the full-load current rating of a 1-horsepower, 230-volt, single-phase motor is 8 amperes. The same motor connected to 115 volts will draw 16 amperes. Current is a big factor in determining conductor size, watts loss in the conductors, voltage drop, disconnect switch size, controller size, motor overload protection, and the size and type of branch-circuit overcurrent protection.

Some pump controllers have a simple slide switch to change the connections from 115 volts to 230 volts.

Pumps sized 1/2 horsepower are usually prewired for 115 volts. Pumps 3/4 horsepower and larger are usually prewired for 230 volts.

For the purpose of our motor circuit design, we will connect this motor for 230 volts.

Table 430.248 for 115-volt motors covers the voltage range of 110-120 volts. For 230-volt motors, the voltage range is 220-240 volts. You will also find that some motors are marked 115/208-230 volts.

The NEMA Standard MG-1 Information Guide for General Purpose Industrial AC Small and Medium Squirrel-Cage Induction Motor Standards is a condensed version of the much larger NEMA Standard MG 1-2003 Motors and Generators. These standards show that most general-purpose NEMA rated motors are designed to operate at 610% of the motor's rated voltage.

Important: For determining the conductor size, switch size, and the branch-circuit short-circuit and ground-fault protection, use the current values from the tables in the NEC. See 430.6(A)(1).

For overload protection sizing, use the motor nameplate current value. See 430.6(A)(2). For the typical submersible pump motor, the running over load protection is almost always furnished by the manufacturer in the controller.

The nameplate rating of the pump motor is 8 amperes, 230 volts.

Conductor Size (430.22):

From Table 430.248, the full-load current rating of a 1-horsepower, 230-volt, single-phase motor is 8 amperes. To calculate the conductor size, use 125% of 8 amperes.

1.25 x 8 = 10 amperes Use Type THHN/THWN conductors per the Specifications.

Checking ‘Table 310.15(B)(16)’ of the NEC, we find that a 14 AWG Type THHN/THWN conductor has an ampacity in the 608C column of 15 amperes and 20 amperes in the 758C column, more than ample for the pump motor circuit.

However, the Schedule of Special Purpose Outlets found in this text, which is part of the Specifications for the residence, indicates that 12 AWG THHN/ THWN conductors are to be used for the pump circuit. Installing larger size conductors can minimize voltage drop and might be suitable at a later date should the need arise to install a larger-capacity pump.

The two 12 AWG THHN/THWN conductors are connected to a 2-pole, 20-ampere branch-circuit breaker in Panelboard A, Circuit A(5-7).

Conduit Size:

Two 12 AWG Type THHN/THWN conductors require trade size 1/2 EMT, NEC Annex C, Table Cl. This EMT will run across the ceiling from Panelboard A to the disconnecting means located in the southeast corner of the workshop. The controller and other equipment relating to the water pump are located in this corner of the workshop. Type UF cable or PVC conduit is buried underground to the well casing location. The cable or conduit is protected at both ends from the burial depth given in Table 300.5 to the point of entry.

Motor Branch-Circuit Short-Circuit and Ground-Fault Protection (430.52, Table 430.52)

Fuses and circuit breakers are the most commonly used forms of motor branch-circuit short circuit and ground-fault protection. Should the motor windings or branch-circuit conductors short circuit or go to ground, the motor's branch-circuit short-circuit and ground-fault overcurrent device, either fuses or circuit breakers, will open.

Motor branch-circuit short-circuit and ground fault protection does not protect the motor against overload burnout! That protection is provided in the motor controller for the submersible pump.

The various choices for motor branch-circuit short-circuit and ground-fault protection are found in 430.52 and Table 430.52. In almost all instances, the values found in Table 430.52 will work just fine.

In the event that the sizing calculation results in an unavailable ampere rating fuse or circuit breaker, select the next standard ampere rating, but don’t exceed the absolute maximum sizing percentages shown in 430.52. This will be discussed in more detail after we do the calculations.

From Table 430.248, the full-load current rating of a 1-horsepower, 230-volt, single-phase motor is 8 amperes. Here are the possibilities:

• Non-Time-Delay Fuses (300% desired- maximum size: 400%) 24 amperes (use 25-ampere fuses)

• The disconnect switch would be a 30-ampere, 2-pole, 250-volt switch.

• Dual-Element, Time-Delay Fuses (175% desired-maximum size: 225%) 14 amperes (use 15-ampere fuses)

• The disconnect switch would be a 30-ampere, 2-pole, 250-volt switch.

• Inverse Time Breakers (250%) 20-ampere rating

• Maximum setting: 400% for FLA of 100 amperes or less.

• Instant Trip Breakers are never used in residential applications.

The motor branch-circuit overcurrent device must be capable of allowing the motor to start, 430.52(B). Where the values for branch-circuit protective devices (fuses or breakers) as determined by Table 430.52 don’t correspond to the standard sizes, ratings, or settings as listed in 240.6, the next higher size, rating, or setting is permitted, 430.52(C)(1), Exception No. 1.

If, after applying the values found in Table 430.52 and after selecting the next higher standard size, rating, or setting of the branch-circuit overcurrent device, the motor still won’t start, 430.52(C)(1), Exception No. 2 permits using an even larger size, rating, or setting. The maximum values (percentages) are shown in this exception.

A nuisance opening of a fuse, or tripping of a breaker, can easily be predicted by referring to time/ current curves for the intended fuse or breaker. This topic is covered in Section 28.

Motor Overload Protection (430.31 through 430.44)

Usually the manufacturer provides the necessary motor overload protection in the controller, and no additional protection is required.

Overload protection is needed to keep the motor from dangerous overheating due to overloads or failure of the motor to start. NEC 430.31 through 430.44 cover virtually every type and size of motor, starting characteristics, and duty (continuous or non continuous) in use today. The overload protection for a motor might be thermal overloads (sometimes called heaters) in the controller, electronic sensing overload devices, built-in (inherent) thermal protection, or time-delay fuses.

We find the sizing requirements for motor overload protection in 430.32(A)(1) and 430.32(B)(1). Although there are exceptions, most of the common installations require motor overload protection not to exceed 125% of the motor's full-load current draw, as indicated on the nameplate. If you need to provide this running overload protection, the running load must be obtained from the manufacturer's literature or from the nameplate on the motor. We will assume the nameplate current to be 7 amperes.

For the pump motor this would be: 8.8 amperes. Therefore, for backup motor overload protection, we could install 10-ampere, time-delay, dual element fuses in the disconnect switch. If we are interested only in motor branch-circuit short-circuit and ground-fault protection, we refer to Table 430.52 and apply the multiplier of 175 percent. 12.3 amperes.

We’re permitted to go to the next standard size, which are 15-ampere time-delay, dual-element fuses.

Instructions furnished with a particular motor or motor-operated appliance might indicate a maximum size overload protection and a maxi mum size branch-circuit protection. These values must be followed.

Disconnecting Means:

The NEC requires that all motors be provided with a means to disconnect the motor from its electrical supply, Article 430, Part IX.

NEC 430.103 requires that a disconnecting means for a motor shall:

• open all ungrounded conductors;

• be designed so all poles operate together (simultaneously); and

• be designed so that it cannot be closed automatically.

The disconnecting means for the water pump is a 30-ampere, two-pole, 250-volt switch mounted on the wall next to the pump controller. See Fgrs. 3 and 4.

Follow these two simple rules for locating the disconnect switch.

Rule #1. NEC 430.102(A): An individual disconnect must be in sight of the controller and must disconnect the controller. The NEC definition of "In Sight" means that the controller must be visible and not more than 50 ft (15 m) from the disconnect.

Rule #2. NEC 430.102(B): The disconnect must be in sight of the motor and driven machinery.

If the disconnect, as required in 430.102(A), is in sight of the controller, the motor, and driven machinery, then that disconnect meets the requirements of both 430.102(A) and 430.102(B).

How to Use the Exception to the "In-Sight" Rule

As with most good rules, there are exceptions! The exception and the Informational Notes to 430.102(B) tell us that the disconnect need not be in sight of the motor and driven machinery if the disconnect can be individually locked in the open "OFF" position. The locking provision must be of a permanent type installed on the switch or circuit breaker. Most disconnect switches have this "LOCK-OFF" feature. Listed circuit breaker "LOCK-OFF" devices that fit over the top of the circuit breaker handle also meet this requirement.

Anyone working on the pump needs to be assured that the power is off and stays off until he or she is ready to turn the power back on. This is particularly important in our installation because the motor is out of sight from the controller.

The disconnect switch for the submersible pump for this residence is located next to and in sight of the controller. It’s not in sight of the motor and can't be, because the motor is located inside the well casing below the water level. Here's where the Exception comes into play. The Exception provides that disconnecting means is not required within sight of the motor if such a location is impracticable. The disconnecting means located on the line or supply side of the controller must be capable of being locked in the open position. See FGR. 4.