RACEWAY SIZING AS PER NEC BASIC INFORMATION

In keeping with the emphasis of the metric system of measurements in the NEC, a Metric Designator has been introduced to provide an equivalency to the inch system of measurement for circular raceways
used for many years as shown in table below.

You may have noticed the NEC uses the term “trade size” rather than simply “size” or “size in
inches” to indicate the size of circular raceways. There is a simple explanation for this designation.
Trade size 2 conduit is not two inches!

You should know that the circular raceway with the largest internal size in trade sizes 1⁄2 through 2 (metric designator 16 through 53) size is IMC and from trade size 21⁄2 through 4 (metric designator 63 through 103) is EMT. Thus, these raceways allow a larger number of conductors in some cases.

It will be useful to review some of the terms used in the NEC concerning raceways:

• A ferrous conduit is made of iron or steel; a nonferrous conduit is made of a metal other than iron or steel.

• Common nonferrous raceways include aluminum, brass and stainless steel.

• Metal or metallic would include both ferrous and nonferrous.

• Nonmetallic raceways would include PVC, ENT, and fiberglass.

• Couplings are used to connect sections of a raceway.

• Locknuts, metal bushings, or connectors are used to connect raceways to boxes or fittings.

• Integral couplings are formed into some raceways and cannot be removed.

• Associated fittings such as couplings and connectors are separate items.

• A running thread is a longer-than-standard length thread cut on one conduit and is sometimes referred to as “continuous-thread” or “all-thread.” Connection to another conduit is achieved by screwing a coupling on the running thread, butting the conduits together, and then backing the coupling onto the second conduit.

As shown in Figure 6-1, running threads are not permitted for connecting conduits together with a coupling as a tight connection is not assured, NEC 344.42(B). Running thread can be used to connect two enclosures by cutting the conduit to length, installing the outer locknuts on the running thread, and sliding the conduit into one enclosure, then into the other.

Caution: If running thread is created by cutting threads in the field, no galvanizing will remain on the conduit,
and protection against rusting should be provided. An option to installing running thread between enclosures is to connect two conduit nipples together with a split coupling.

WIRING DIAGRAMS FOR A TYPICAL STANDBY GENERATOR

For simplicity, the diagrams presented are one-line diagrams. Actual wiring consists of two ungrounded conductors and one grounded “neutral” conductor. Equipment grounding of all of the components is accomplished through the metal raceways that interconnect the components.

The neutral bus in a panelboard that serves selected loads must not be connected to the metal panelboard enclosure. Connecting the neutral conductor, the grounding electrode conductor, and the equipment grounding conductors together is permitted only in the main service panelboard.

If you were to bond the neutral conductor and the metal enclosures of the equipment (panelboard, transfer switch, and generator) together beyond the main service panelboard, you would create a parallel path. A parallel path means that some of the normal return current and fault current will flow on the grounded neutral conductor and some will flow on the metal raceways and other enclosures. This is not a good situation!

Manufacturers of generators in most cases do not connect the generator neutral conductor lead to the metal frame of the generator. Instead, they connect it to an isolated terminal. Then it is up to you to determine how to connect the generator in compliance with the NEC and/or local electrical codes.

Most inspectors will permit the internal neutral bond in a portable generator to remain in place. In fact, when you purchase a portable generator, you should insist the neutral-to-case bond is in place.

Without the bond, an overcurrent device cannot function on a ground fault because a return path does not
exist. For permanently installed generators, inspectors will generally not permit a bond between the
generator neutral and the metal frame of the generator because of the explicit requirements in the NEC.

A sign must be placed at the service-entrance main panelboard indicating where the standby
generator is located and what type of standby power it is.

The total time for complete transfer to standby power is approximately 45–60 seconds. To further give the homeowner assurance that the standby generator will operate when called upon, some systems provide automatic “exercising” of the system periodically, such as once every 7 or 14 days for a run time of 7 to 15 minutes.


WARNING: When an automatic type of standby power system is in place and set in the automatic
mode, the engine may crank and start at any time without warning. This would occur when the utility
power supply is lost.

To prevent possible injury, be alert, aware, and very careful when working on the standby generator equipment or on the transfer switch. Always turn the generator disconnect to the “Off” position, then lock out and tag out the switch, warning others not to turn the switch back “On.” In the main panelboard, locate the circuit breaker that s upplies the transfer equipment, turn it “Off,” then lock out and tag out the circuit breaker feeding the transfer switch.

UNGROUNDED SYSTEM BASIC INFORMATION

In this type of system, a phase-to-earth fault only produces a weak current through the

phase-to-earth capacity of the fault-free phases.

It can be shown that Id = 3 CwV

V being the simple voltage,

C the phase-to-earth capacity of a phase,

wthe frequency of the system (w = 2* pi *f).

The Id current can remain for a long time, in principle, without causing any damage since

it does not exceed a few amperes (approximately 2 A per km for a 6 kV singlepole

cable, with a 150 mm2 cross-section, PRC insulated, with a capacity of 0.63 mF/km).

Action does not need to be taken to clear this 1st fault, making this solution advantageous in

terms of maintaining service continuity.

However, this brings about the following consquences:

c if not cleared, the insulation fault must be signalled by a permanent insulation monitor, c subsequent fault tracking requires device made all the more complex by the fact that it is automatic, for quick identification of the faulty feeder, and also maintenance personnel qualified to operate it, c if the 1st fault is not cleared, a second fault occurring on another phase will cause a real two-phase short circuit through the earth, which will be cleared by the phase protections.

Advantage

The basic advantage is service continuity since the very weak fault current prevents automatic tripping.

Drawbacks

The failure to eliminate overvoltage through the earth can be a major handicap if overvoltage is high. Also, when one phase is earthed, the others are at delta voltage (U = V* sqrt 3) in relation to the earth increasing the probability of a 2nd fault.

Insulation costs are therefore higher since the delta voltage may remain between the phase and earth for

a long period as there is no automatic tripping. A maintenance department with the equipment

to quickly track the 1st insulation fault is also required.

Applications

This solution is often used for industrial systems (< or = 15 kV) requiring service continuity.

ELECTRICITY METERING FOR THREE-WIRE NETWORK SERVICE SYSTEM DIAGRAMS

Two-Stator Meter
Three-wire network service is obtained from two of the phase wires and the neutral of a three-phase, four-wire wye system, as shown in Figure 7-2. It is, in reality, two two-wire, single-phase circuits with a common return circuit and it has voltages that have a phase difference of 120 electrical degrees between them.

The voltage is commonly 120 volts line-to-neutral/208 volts line-to-line.

The normal method of metering a network service is with a two-element (two-stator) meter connected as shown in Figure 7-2. With this connection, which follows Blondel’s Theorem, each stator sees the voltage of one phase of the load. The phasors representing the load phase currents, IAN and IBN, are shown in the diagram lagging their respective phase voltages.

Figure 7-2 Two-Stator Meter on Three-Wire Network Service.

The meter current conductors carry the line currents, IAN and IBN, and inspection of the circuit shows that these currents are identical to the load phase currents. Hence, the meter correctly measures the total load power. Any loads connected line-to-line, between A and B in Figure 7-2, will also be metered properly. With this type of meter there are no metering errors with imbalanced load voltages or varying
load currents and power factors.


As such, the watt metering formula for any instance in time is:

Watts  (VAN X IA) + (VBN X IB)

Accumulating the watts over time allows the metering of watthours.


Single-element (single-stator) meters for measuring network loads have been developed and may be used with reasonable accuracy under particular load conditions.  The conventional three-wire, single-element (single-stator), single-phase meter cannot be used for network metering.

It will, of course, measure the 208-volt load correctly; but the two 120-volt loads are metered at 104-volts rather than at 120 volts and at a phase angle which is 30 degrees different from the actual. Therefore, for 120-volt balanced loads, meter registration will be close to 75 percent of the true value; but with imbalanced loads, the resulting meter error varies, rendering such metering useless.

Single-Stator Meters for Three-Wire Network Service
Single-stator meters have been developed for use on three-wire network services. These meters do not conform to Blondel’s Theorem and are subject to metering errors under certain conditions noted in the following paragraphs.

The schematic connections for the two types of meters now in use are shown in Figure 7-3. Each meter has one voltage coil and two current coils. In one case, Figure 7-3a, the meter is designed to use line-to-line voltage (208 V) on the voltage coil and the other, Figure 7-3b, uses one line-to-neutral voltage (120 V) on its voltage coil.

The currents in the meter current coils are shifted in phase to provide correct metering. Obviously, any imbalance in line-to-neutral voltages will cause metering errors and, where imbalanced voltages exist, a two-stator meter should be used for accurate results.

In electromechanical meters, the current phase shifting is accomplished by impedance networks of resistors and inductors along with the current coils to split the total line currents and shift the phase position of the meter-current-coil current the desired amount. The number of turns and the impedance of the current coil may also be varied in design to obtain a usable meter.

The current-impedance networks are shown in Figure 7-3. Electronic meters can accomplish the phase shifting using a variety of techniques. Phase sequence of voltages applied to these meters is extremely important since such meters can usually only be designed to provide the correct phase shift of meter-coil current for only one phase sequence.

If they are installed on the wrong phase sequence their energy registration is useless. All meters of this type have a built-in phase-sequence indicator.

Figure 7-3 Single-Stator, Three-Wire Network Meters.