General Electrical Knowledge

Learning Objectives

Upon completing this chapter, you will be able to:

4.Calculate service and feeder loads for commercial and industrial occupancies using the optional and standard methods.

5.Apply the rules for sizing grounded, ungrounded, and grounding electrodes for services and separately derived systems.

6.Distinguish between a separately derived system and a non-separately derived system, and apply the bonding and grounding requirements for each.

7.Perform motor circuit conductor, overcurrent protection, and disconnect sizing per Articles 430 and 440.

8.Apply the principles of selective coordination for emergency, legally required, and critical operations power systems.

9.Identify the correct NEC Article and Table for any given installation scenario during the open-book exam.

1.1 Three-Phase Systems and Voltage Drop

A master electrician must be fluent in three-phase system configurations, as the majority of commercial and industrial work involves these systems.

Common configurations:

Wye (Y): 208Y/120 V, 480Y/277 V, and 600Y/347 V. The line-to-neutral voltage equals the line-to-line voltage divided by √3 (1.732). The high-leg (wild leg) is not present in a true wye.

Delta (Δ): 240 V, 480 V. A center-tapped delta provides 120/240 V single-phase from one phase and neutral, with a high-leg (208 V to ground) on the B phase. The high-leg must be identified with orange tape or permanently marked at all termination points (NEC 110.15).

Voltage drop (informational, NEC 210.19(A) Informational Note No. 4):

Feeders: 3% maximum, branches: 3% maximum, combined: 5% maximum.

Formula for three-phase: VD = 2 × L × I × K / (CM × 1.732), where K is the conductor resistivity constant (approximately 12.9 for copper, 21.2 for aluminum at 75°C). For single-phase, omit the 1.732 factor.

Master tip: Voltage drop is not a mandatory calculation in the NEC except for specific applications (e.g., fire pumps, 695.7; sensitive electronic equipment, 647.4). However, the exam will test your ability to apply it as a design criterion.

1.2 Services and Service Equipment (Article 230)

A service is the conductors and equipment that deliver electric power from the utility to the service disconnecting means.

Key definitions and thresholds:

Service Point: The point of connection between the utility and the premises wiring.

Service Conductors: Can be overhead (230.24 clearance requirements) or underground (230.32).

Number of Services: A building can be served by only one service unless specific exceptions apply (230.2) — e.g., multiple occupancies, fire pumps, emergency systems, or capacity requirements over 2000 A.

Service Disconnecting Means (230.70-230.71): Must be at a readily accessible location, outside or inside nearest the point of entrance. For a single service, the disconnecting means must have six or fewer handles (230.71). Each disconnect must be suitable for the fault current available.

Service Overcurrent Protection (230.90): Each ungrounded service conductor must have an OCPD. The rating of the service OCPD must not exceed the ampacity of the conductor, except for motor loads where the next standard size up is permitted (240.4(G)).

Grounding Electrode Conductor (GEC) Sizing (Table 250.66): Based on the size of the largest ungrounded service conductor. For a 500 kcmil copper service, the GEC is 1/0 copper. For a 600 kcmil aluminum service, the GEC is 2/0 copper (or 4/0 aluminum).

Inspection point: Verify that the service disconnect is not located in a bathroom (230.70(D)), that working clearance is at least 36 inches in front (110.26), and that the GEC is properly bonded to the grounding electrode system.

1.3 Separately Derived Systems (SDS) — Article 250.30

A separately derived system is a premises wiring system whose power is derived from a battery, solar photovoltaic system, generator, transformer, or converter windings, and that has no direct electrical connection to the supply conductors originating from another system.

Common SDS examples: Step-down transformers (480 V to 208Y/120 V), backup generators with a transfer switch that opens the neutral, and offline UPS systems.

Grounding and bonding requirements for SDS:

37.System bonding jumper: Connect the grounded conductor (neutral) to the equipment grounding conductor (EGC) at the SDS source or at the first disconnecting means (250.30(A)(1)). This is a single point of connection.

38.Grounding electrode conductor: Run a GEC from the SDS source to a grounding electrode (250.30(A)(4)). The electrode can be the nearest effectively grounded structural metal member, a water pipe, or a ground rod (minimum 5/8" × 8').

39.Size of the bonding jumper and GEC: Per Table 250.66, based on the size of the largest ungrounded conductor of the SDS.

40.Impedance: The path to ground must have low impedance to facilitate OCPD operation.

Master trap: For a transformer feeding a panelboard, the neutral must be bonded at the transformer (or at the first disconnect) but must not be bonded at the downstream panelboard. If you bond the neutral at both locations, you create a parallel neutral path, which is a violation and a shock hazard.

Non-SDS (e.g., a generator used as a backup with a solidly connected neutral): The generator is not separately derived if the neutral is not switched. In that case, the generator frame is bonded to the service EGC, and no new system bonding jumper is installed.

1.4 Feeder and Service Load Calculations (Articles 220 and Annex D)

The master exam requires fluency in both the standard and optional calculation methods.

Standard Method (220.10-220.61):

General lighting load: Table 220.12 — 3 VA/ft² for dwelling, 1.5 VA/ft² for banks, 2 VA/ft² for offices, 1 VA/ft² for warehouses.

Receptacle loads: 180 VA per receptacle for commercial (220.14(I)).

Demand factors for dryers (220.54), ranges (220.55), and laundry (220.52(B)).

Neutral load: The maximum unbalanced load (220.61). For 3-wire DC or single-phase, the neutral carries the unbalanced current. For 4-wire 3-phase wye, the neutral carries the unbalanced current, but harmonic loads (e.g., electronic ballasts) may require a larger neutral (220.61(C)(2)).

Motor loads: 125% of the largest motor plus 100% of all others (220.50).

Optional Method for Commercial (220.86):

Permitted when the total connected load is served by a single service or feeder, and the load is not a dwelling.

The calculated load is the sum of all loads, with a demand factor applied based on the total connected load (Table 220.86). For example, the first 10 kVA at 100%, the remainder at 50% (for loads over 10 kVA).

Optional Method for Dwelling (220.82):

100% of the first 10 kVA, 40% of the remainder, plus 100% of HVAC (largest of heating or cooling), plus 100% of the fastened-in-place appliance load.

Master tip: When sizing a feeder for a commercial kitchen, remember the demand factors in Table 220.56 — you can apply a demand factor of 80% for 4 or more appliances, but the neutral must be sized for the maximum unbalanced load.

1.5 Motor Circuits (Article 430)

Motor calculations are a core competency for the master exam.

Conductor sizing (430.22):

Branch circuit conductors must have an ampacity of 125% of the motor full-load current (FLC) — not the nameplate rating. Use Table 430.248 (single-phase) or 430.250 (three-phase) for FLC values.

For a motor with a service factor of 1.15 or more, or a temperature rise of 40°C or less, the conductors must be sized at 125% of the nameplate current (430.22(A) Exception).

Overload protection (430.32):

Motors with a marked service factor of 1.15 or more: overload relay set at no more than 125% of the nameplate current.

Motors with a service factor less than 1.15: overload relay set at no more than 115% of the nameplate current.

Short-circuit and ground-fault protection (430.52):

The maximum rating of the branch-circuit protective device (fuse or breaker) is based on a percentage of the FLC, per Table 430.52. For a three-phase squirrel-cage motor, the maximum is 250% of FLC for an inverse-time breaker, 300% for a non-time-delay fuse, and 175% for a time-delay fuse.

If the calculated value does not correspond to a standard size, you may round up to the next standard size (430.52(C)(1) Exception 1).

Disconnecting means (430.102):

A disconnecting means must be located within sight of the motor and the driven machinery (430.102(B)). "Within sight" means visible and not more than 50 feet away (Article 100).

Master trap: The branch-circuit OCPD (for short circuit) is separate from the overload relay (for running protection). Do not confuse the two. The overload relay is sized off the nameplate, while the branch OCPD is sized off the table FLC.

1.6 Overcurrent Protection Coordination (Articles 240 and 700)

Selective coordination is the process of ensuring that a fault on a downstream circuit clears the downstream OCPD without opening the upstream OCPD, thus limiting the outage to the affected portion.

Mandatory coordination locations:

Emergency systems (700.28): Overcurrent devices must be selectively coordinated for all of the emergency system.

Legally required standby systems (701.27): Same requirement.

Critical operations power systems (708.54): Must be coordinated for the entire system.

Healthcare (517.26): Essential electrical systems must be coordinated.

How to achieve coordination:

Use current-limiting fuses with a ratio of 2:1 between upstream and downstream fuses.

Use circuit breakers with adjustable trip units and verify the time-current curves do not overlap.

For motor circuits, the instantaneous trip of a breaker may be bypassed or set above the motor inrush current.

Inspection point: On a job site, verify that the panelboard schedule shows the correct breaker frame sizes and trip settings. A master must ensure that the engineer's coordination study is followed, or if no study exists, that the installation meets the minimum NEC requirements.

1.7 Commercial and Industrial Installations

Panelboards (408):

The rating of the panelboard must not be less than the feeder capacity (408.30). A 200 A panelboard cannot be fed from a 400 A feeder without a main breaker rated at 200 A.

The neutral bar must be sized for the neutral load, and the equipment grounding bar must be bonded to the enclosure (408.40).

Switchboards and Switchgear (408.3):

Minimum clearance of 36 inches in front, 30 inches width of working space (110.26).

Switchboards with exposed live parts must be guarded.

Receptacle placement (210.52 for dwellings, 210.62 for commercial):

In commercial occupancies, receptacles must be installed so that no point along the floor line is more than 6 feet from a receptacle (210.62) — this is for show windows, not general areas.

Wiring methods (Articles 300-390):

EMT (Article 358) is permitted for all atmospheric conditions and occupancies.

MC cable (Article 330) is permitted in commercial and industrial, including in wet locations if listed for that purpose.

Rigid metal conduit (RMC, Article 344) is required for hazardous locations (Class I, Division 1) and for physical protection.

1.8 Generators and Power Systems (Article 445)

Generator sizing:

The generator must be sized for the load it will carry, including motor starting current (inrush). For a generator supplying a fire pump, the generator must be sized to carry the locked-rotor current of the fire pump motor (695.3(C)).

The generator must have a disconnecting means (445.18) that is lockable in the open position.

Transfer switches (Article 701):

A transfer switch must be listed for the purpose and must be mechanically held, electrically operated (for automatic transfer).

The neutral must be switched if the generator is a separately derived system (i.e., if the generator neutral is not bonded to the utility neutral).

Grounding:

If the generator is a portable unit with a grounded neutral, it must be bonded to the system grounding electrode when connected to a premises wiring system (250.30).

1.9 Code Navigation: Where to Find It

Topic	NEC Location
Service disconnects, max six	230.71
Service conductor ampacity	230.42
Grounding electrode conductor sizing	Table 250.66
SDS grounding and bonding	250.30
High-leg marking	110.15
Branch circuit voltage drop	210.19(A) IN No. 4
General lighting load (commercial)	Table 220.12
Receptacle load (commercial)	220.14(I)
Motor FLC tables	430.248, 430.250
Motor branch OCPD	Table 430.52
Motor overload protection	430.32
Selective coordination (emergency)	700.28
Selective coordination (legally required)	701.27
Panelboard rating	408.30
Working clearance	110.26
Generator disconnecting means	445.18
Fire pump generator sizing	695.3(C)
Optional commercial calc	220.86
Optional dwelling calc	220.82

1.10 Inspection and Supervision Points

As a master electrician, you are responsible for the final sign-off. On every job, verify:

118.Bonding and grounding: The system bonding jumper is installed at the SDS source only. The neutral is not bonded at downstream panels. The GEC is properly sized and connected to an acceptable electrode.

119.Service entrance: The service disconnect is within sight of the meter, the six-disconnect rule is not violated, and the service conductors are protected against physical damage.

120.Panelboard loading: The panelboard is not overloaded. The sum of the branch OCPDs does not exceed the panelboard bus rating unless the panelboard is specifically rated for that.

121.Motor circuits: The disconnect is within sight of the motor, the overload relay is set to the nameplate, and the branch OCPD is sized per Table 430.52.

122.Working clearances: Verify 36 inches in front, 30 inches wide, and 6.5 feet high for all equipment operating at 600 V or less (110.26).

123.GFCI and AFCI: All required locations are protected (210.8 for GFCI, 210.12 for AFCI in dwellings).

1.11 Common Exam Traps

Trap 1: Sizing motor conductors off the nameplate instead of the table FLC. Always use the tables for conductor sizing and OCPD, and the nameplate only for overloads.

Trap 2: Bonding the neutral at a subpanel. The neutral must be isolated from the enclosure and the EGC at all downstream panels.

Trap 3: Using 125% for a continuous load but forgetting that the OCPD must also be sized at 125% (210.20(A)).

Trap 4: Forgetting that a 3-phase 4-wire delta system has a high-leg that must be orange and cannot be used for line-to-neutral loads (110.15, 230.56).

Trap 5: Applying the optional calculation method when the load exceeds the limits of Table 220.86 (e.g., total connected load over 2000 kVA).

Trap 6: Confusing the "six disconnect rule" for services (230.71) with the "one disconnect rule" for a building with multiple occupancies (230.2).

1.12 Summary

Mastery of general electrical knowledge requires more than memorizing tables — it requires understanding the why behind the code. A master electrician must be able to look at a set of plans, identify the service configuration, calculate the loads, size the conductors and OCPDs, and verify that the grounding and bonding are correct. The NEC is a minimum standard; the master's judgment is what ensures a safe, reliable, and code-compliant installation. Use the Code Navigation table above as your roadmap during the open-book exam, and always verify your answer against the actual Article text before signing off.

General Electrical Knowledge