Services & Service Equipment

NH Master Electrician Exam Study Chapter — 2023 NEC

Learning Objectives

By the end of this chapter, you will be able to:

6.Define the boundaries of the service point, service conductors, and service equipment, and correctly apply the NEC rules that govern each.

7.Size service conductors and service disconnects for 3-phase, 4-wire commercial and industrial systems, including the neutral and grounding electrode conductor.

8.Apply the 2023 NEC rules for separately derived systems (transformers and generators), including bonding, grounding, and overcurrent protection.

9.Perform feeder sizing calculations with demand factors, continuous loads, and voltage drop considerations for multi-tenant and mixed-use buildings.

10.Coordinate overcurrent protection for services, feeders, and branch circuits to achieve selective coordination where the Code requires it.

11.Identify common inspection failures and exam traps specific to services and service equipment.

1.1 The Service Point and Service Conductors

The service point is the interface between the utility supply and the premises wiring. For a master electrician, the critical distinction is between service conductors (from the service point to the service disconnecting means) and feeder conductors (from the service disconnect to downstream distribution).

Service conductors are governed by Article 230. They must be sized per 230.42 — which requires the ampacity to be not less than the sum of the noncontinuous loads plus 125% of the continuous loads. For a 3-phase, 4-wire system, you must also account for the neutral as a current-carrying conductor when it carries the unbalanced load from line-to-neutral loads (e.g., 277 V lighting on a 480/277 V system).

Key thresholds to memorize:

Minimum service conductor size: 8 AWG copper or 6 AWG aluminum for residential, but for commercial/industrial, the calculated load governs. 230.42(B) permits a smaller neutral if the maximum unbalanced load is known, but never smaller than the grounding electrode conductor required by 250.66.

Service drop/overhead clearance: 230.24 requires 10 ft above grade, 12 ft over residential property and driveways, 18 ft over public streets. For commercial, 15 ft over parking lots is a common trap — verify against 230.24(B).

Service entrance conductors in parallel: 230.40, Exception No. 2 allows multiple service disconnects, but each set of conductors must be sized for its own load. 310.10(G) governs paralleling — all conductors must be the same length, material, cross-section, and insulation type.

Exam trap: Many candidates confuse the service point with the service disconnect. The service point is often at the utility meter or the weatherhead. The NEC does not regulate the utility side of the service point — only the premises side.

1.2 Service Disconnecting Means — Article 230.70–230.95

The service disconnecting means must:

Disconnect all ungrounded service conductors from the premises wiring (230.70).

Be located at a readily accessible point nearest the point of entrance of the service conductors (230.70(A)(1)).

Have a rating not less than the calculated load, and never less than 100 A for a service with more than two branch circuits (230.79).

Six-disconnect rule: 230.71 permits up to six service disconnects in a single enclosure or grouped in separate enclosures. For a master, the critical point is that these six disconnects must be grouped — they cannot be scattered across a building. In 2023, the NEC removed the exception for multiple occupancy buildings; all service disconnects must be grouped at one location.

Service overcurrent protection:

Each ungrounded service conductor must have an overcurrent device (230.90).

The rating of the service overcurrent device must not be less than the ampacity of the conductors, but 230.90(A), Exception 1 allows the next standard size up if the calculated load is less than the conductor ampacity.

230.95 requires ground-fault protection of equipment for solidly grounded wye services rated 1000 A or more at more than 150 V to ground. This is a mandatory requirement for commercial/industrial services — the GFP must be set to open at not more than 1200 A, and the time-current characteristic must be coordinated with the feeder and branch-circuit protection.

Master-level insight: For a 480/277 V service with a 2000 A main breaker, you must verify the GFP settings. The NEC does not require selective coordination between the service GFP and the feeder GFP unless the feeder is serving emergency systems or legally required standby systems (700.28, 701.27). However, a master should always attempt coordination to avoid nuisance tripping.

1.3 Grounding and Bonding of Services — Article 250

The grounding electrode system (250.50) must include all electrodes present: metal underground water pipe, metal frame of the building, concrete-encased electrode (Ufer), ground ring, and rod/plate electrodes.

Grounding electrode conductor (GEC) sizing — Table 250.66:

For 3/0 copper or smaller service conductors: GEC is 4 AWG copper.

For 250 kcmil to 500 kcmil copper: GEC is 2 AWG copper.

For over 500 kcmil to 1100 kcmil: GEC is 1/0 copper.

Critical change in 2023 NEC: 250.68(C) now requires that the GEC connection to the grounding electrode be accessible — but Exception 1 allows the connection to concrete-encased electrodes to be buried. Also, 250.64(E)(1) now requires the GEC to be protected from physical damage if installed in a raceway, and the raceway must be bonded at both ends.

Bonding the service:

The service neutral must be bonded to the service equipment enclosure (250.92).

The main bonding jumper must be installed at the service disconnect (250.28). It must be sized per Table 250.102(C)(1) — not less than 12.5% of the area of the largest ungrounded service conductor.

250.24(C) requires the grounded conductor (neutral) to be routed with the ungrounded conductors to the service disconnecting means. You cannot ground the neutral at the utility transformer and then run a separate ground to the service — the neutral must come to the first disconnecting means.

Exam trap: For a 400 A service with 500 kcmil copper ungrounded conductors, the main bonding jumper must be at least 1/0 copper (per Table 250.102(C)(1)). Many candidates incorrectly use Table 250.66 (which gives 2 AWG for 500 kcmil) — the bonding jumper table is different and gives a larger conductor.

1.4 Separately Derived Systems — Transformers and Generators

A separately derived system (SDS) is a premises wiring system whose power is derived from a battery, photovoltaic system, generator, transformer, or converter, and that has no direct electrical connection to the supply conductors originating from another system (Article 100).

Transformer installations — Article 450:

Primary overcurrent protection: 450.3(B) — for transformers over 600 V nominal, the primary OCPD must be set at 125% of the rated primary current (or the next standard size if 125% does not correspond to a standard rating).

Secondary protection: If the secondary is protected by a single OCPD rated at 125% of the secondary current, the primary protection can be increased to 250% (450.3(B), Exception 1).

Transformer vaults and ventilation: 450.9 requires ventilation to dissipate the transformer heat. For dry-type transformers over 112.5 kVA, 450.21(B) requires a vault or fire-resistant enclosure.

Grounding the SDS:

The system bonding jumper must be installed at the SDS source (transformer secondary or generator) — 250.30(A)(1).

The grounding electrode conductor for the SDS must be sized per 250.66 based on the area of the largest ungrounded secondary conductors.

250.30(A)(2) requires the GEC to connect to a grounding electrode — typically the nearest effectively grounded structural metal member or a concrete-encased electrode.

Master-level insight: For a 75 kVA, 480 V to 208Y/120 V transformer:

Secondary current = 75,000 VA / (208 V × 1.732) = 208 A.

The secondary conductors must be sized for 208 A × 1.25 = 260 A (continuous load).

The system bonding jumper must be sized per Table 250.102(C)(1) based on the secondary conductor area.

The GEC must be sized per Table 250.66.

Generator installations — Article 700 (Emergency), 701 (Legally Required Standby), 702 (Optional Standby):

Transfer switches: Must be listed and rated for the load. 700.5 requires the transfer switch to be automatic for emergency systems.

Grounding: A portable generator with a grounded neutral that is used as an SDS must have the neutral bonded at the generator only if the transfer switch opens the neutral. If the transfer switch does not switch the neutral, the generator neutral must be bonded to the service neutral — this is a common inspection failure.

Sizing: 700.4 requires emergency system capacity to be sufficient for all loads that are expected to operate simultaneously. The master must verify the generator can handle the locked-rotor current of the largest motor plus the steady-state load of all other connected loads.

1.5 Feeder Sizing and Demand Factors — Article 220

Feeder conductors must be sized per 215.2(A)(1) — not less than the noncontinuous load plus 125% of the continuous load. For feeders supplying multiple branch circuits, you may apply demand factors from Article 220.

Table 220.42 — Lighting demand factors for dwelling units:

First 3000 VA at 100%.

3001 to 120,000 VA at 35%.

Remainder over 120,000 VA at 25%.

Table 220.44 — Receptacle demand factors for non-dwelling occupancies:

First 10 kVA at 100%.

Remainder at 50%.

Optional method for dwelling units — 220.82:

100% of the first 10 kVA of all loads.

40% of the remainder.

100% of the heat pump or A/C (largest of the two).

100% of the dryer and range (or 40% of the nameplate if over 12 kW).

Master-level insight: For a commercial kitchen, 220.56 allows a demand factor of 80% for the total connected load of four or more kitchen equipment loads. This is a common exam question — the demand factor applies only when there are four or more pieces of equipment.

Voltage drop — 210.19(A), Informational Note No. 4 and 215.2(A), Informational Note No. 2:

The NEC recommends (but does not require) 3% for branch circuits and 3% for feeders, with a total of 5% from the service to the farthest outlet.

For a master, voltage drop is a design consideration, not a Code requirement — but many local jurisdictions in New Hampshire adopt it as a requirement. Always check the NH State amendments.

1.6 Overcurrent Protection Coordination

Selective coordination is required for:

Emergency systems (700.28): Overcurrent devices must be coordinated so that a fault on a branch circuit does not take out the feeder or service.

Legally required standby systems (701.27): Same requirement.

Critical operations power systems (708.54): More stringent requirements.

For non-emergency systems, the NEC does not require selective coordination, but 240.12 permits it. A master should always design for coordination where possible.

Coordination study basics:

The time-current curve of the upstream device must not intersect the curve of the downstream device within the fault-current range.

For fuses, this is achieved by a 2:1 ratio (upstream fuse is two sizes larger).

For circuit breakers, you must compare the instantaneous trip settings and the short-time delay settings.

Exam trap: The 2023 NEC 240.87 requires that for any circuit breaker rated 1200 A or more, the arc-energy reduction maintenance switch (or an approved equivalent) must be provided. This is a new requirement that catches many experienced electricians off guard — it applies to service disconnects and feeder breakers.

1.7 Commercial and Industrial Installations — Special Considerations

Multi-tenant buildings:

Each tenant space must have access to its own service disconnect or feeder disconnect (230.72).

The service conductors for each tenant must be sized for that tenant's calculated load.

230.82 lists the equipment that can be connected to the supply side of the service disconnect — this includes meter enclosures, instrument transformers, and surge protective devices (Type 1 or Type 2).

Motor applications — Article 430:

Feeder conductors for multiple motors: 430.24 requires the feeder to be sized at 125% of the largest motor plus 100% of all other motors plus the calculated load of other connected loads.

Motor branch circuit: 430.52 permits the branch-circuit short-circuit and ground-fault protective device to be set at up to 250% of the motor full-load current for inverse-time breakers (or 300% for fuses) — but the overload protection must be sized per 430.32 (typically 115% to 125% of the nameplate rating).

Disconnecting means: 430.102 requires a disconnect within sight of the motor and the motor controller.

Generator applications — Article 445:

445.12 requires each generator to have a disconnecting means that is lockable in the open position.

445.13 requires the generator conductors to be sized at 115% of the generator nameplate current rating.

445.18 requires the generator to be bonded and grounded per Article 250.

1.8 Code Navigation — Where to Find It

Topic	NEC Reference
Service conductors — sizing	230.42, 230.23
Service disconnects — number and location	230.70, 230.71, 230.72
Service disconnect — rating	230.79
Service overcurrent protection	230.90, 230.95
Ground-fault protection (services)	230.95
Grounding electrode system	250.50–250.60
Grounding electrode conductor — sizing	Table 250.66
Main bonding jumper — sizing	Table 250.102(C)(1)
Service neutral — bonding	250.24, 250.28, 250.92
Separately derived systems — grounding	250.30
Transformer overcurrent protection	450.3
Transformer ventilation and location	450.9, 450.21
Feeder sizing	215.2
Branch circuit sizing	210.19, 210.20
Dwelling load calculations — optional	220.82
Commercial load calculations	220.40–220.56
Motor feeder sizing	430.24
Motor branch circuit protection	430.52
Selective coordination (emergency)	700.28, 701.27
Arc-energy reduction	240.87
Generator disconnects and sizing	445.12, 445.13, 445.18

1.9 Inspection and Supervision Points

As a master electrician, you are responsible for the work you supervise. On a service installation, verify the following:

120.Service conductors are protected from physical damage — raceways must be suitable for the environment, and the service mast must be properly guyed if it extends more than 3 ft above the roof.

121.The neutral is not bonded downstream of the service disconnect — in a subpanel, the neutral must be isolated from the equipment grounding conductor (250.142(B)). This is the most common residential and commercial inspection failure.

122.The GEC is properly connected and protected — the connection to the water pipe must be within 5 ft of the point of entrance, and the pipe must be bonded on both sides of the water meter (250.53(D)).

123.The service disconnect is readily accessible — not behind a locked door, not in a bathroom, and not obstructed by storage.

124.GFP settings are verified — for services over 1000 A, the GFP must be tested and the settings recorded.

125.All terminations are torqued — loose connections are a leading cause of service failures and fires.

1.10 Common Exam Traps

128.The 125% continuous load factor applies to the conductor sizing and the overcurrent device rating — but not to the service disconnect rating if the disconnect is not the overcurrent device (e.g., a fused disconnect where the fuses are sized separately).

129.The neutral is a current-carrying conductor for ampacity adjustment purposes when it carries the unbalanced load of a 3-phase, 4-wire wye system (310.15(E)(1)). This means you must apply the 80% adjustment factor from Table 310.15(C)(1) when more than three current-carrying conductors are in a raceway.

130.The grounding electrode conductor is never required to be larger than 3/0 copper for rod, plate, or concrete-encased electrodes (250.66(A)) — but this exception does not apply to the main bonding jumper.

131.A 3-phase, 4-wire delta system with a center-tapped phase (high-leg) requires the high-leg to be identified with orange tape or a similar marking (110.15), and the high-leg cannot be used for line-to-neutral loads (230.56).

132.The service neutral must be sized for the maximum unbalanced load — but never smaller than the GEC required by Table 250.66. For a 120/240 V, 3-wire service, the neutral is a current-carrying conductor, and you cannot reduce its size below the ungrounded conductors if the load is balanced.

Summary

Services and service equipment are the foundation of every installation. The master electrician must understand not only the sizing rules but also the interaction between Article 230 (services), Article 250 (grounding), Article 220 (load calculations), and Article 450 (transformers). On the NH Master exam, expect scenario-based questions that require you to trace a system from the utility transformer through the service disconnect, to a subpanel, and out to a motor load — applying the correct demand factors, adjustment factors, and bonding rules at each step.

Final tip: Memorize the tables — 250.66, 250.102(C)(1), 310.15(B)(16), and 430.52. These four tables will answer more than half of the calculation questions on the exam. Know the exceptions, and know when the exceptions do not apply.

Services & Service Equipment