Master Electrician Practice study guide with diagrams.
Wiring & Protection
Delaware Master Electrician Exam (DE-MST) — 2023 NEC (NFPA 70)
Learning Objectives
By the end of this chapter, you will be able to:
6.Apply the general requirements for services, feeders, and branch circuits as they apply to commercial and industrial installations.
7.Calculate minimum feeder and service conductor ampacity using the correct demand factors and adjustment/adjustment factors from Article 220.
8.Identify the requirements for separately derived systems (transformers, generators) and their grounding and bonding per Article 250.
9.Select and coordinate overcurrent protective devices (OCPDs) for transformers, motors, and feeders, including the application of the "next size up" rule and conductor protection.
10.Navigate the 2023 NEC efficiently to locate code requirements for supervision and plan review.
1.1 The Scope of Wiring & Protection (Chapter 1–4)
The "Wiring & Protection" domain covers the foundational infrastructure of any electrical system. For a Master, this is not just about pulling wire; it is about system architecture. This includes:
Branch Circuits (Article 210): The conductors from the final OCPD to the outlets. Masters must verify the general lighting load calculations and the maximum number of outlets per circuit for specific loads (e.g., 12 outlets for general-purpose receptacles on a 15A or 20A circuit is a common design rule, though not a strict NEC requirement for all cases).
Feeders (Article 215): The conductors between the service equipment and the branch-circuit OCPD. The minimum size is based on the computed load after applying demand factors.
Services (Article 230): The conductors and equipment connecting the utility supply to the service disconnecting means. This is where a Master's signature carries the most weight regarding safety and code compliance.
The critical shift from Journeyman to Master is understanding that you are not just installing to code minimums but designing for voltage drop, fault current, and coordination.
1.2 Services and Service Equipment (Article 230)
Service equipment is the first point of disconnect and overcurrent protection on a building. A Master must understand the specific rules governing this equipment.
Key Requirements:
Disconnecting Means (230.70): Must be at a readily accessible location nearest the point of entrance of the service conductors. For a Master, the trap is understanding that "nearest" is not always "inside." If the service enters a building and goes more than 10 feet (3.0 m) to the panel, you may need a service disconnect at the point of entry or a separate building disconnect.
Number of Disconnects (230.71): A service can have up to six disconnects to remove all power. However, in modern commercial work, a single main disconnect is standard. If you have multiple disconnects, they must be grouped.
Service Conductor Sizing (230.42): The minimum size is based on the calculated load from Article 220. However, the conductor must have an ampacity of at least the rating of the service disconnect. Master Trap: You cannot size a 400A service with 400A conductors if the calculated load is only 350A, but the conductor must be rated for the service rating (400A) unless the OCPD protects the smaller conductor.
Service Grounding (250.24): The grounded conductor (neutral) must be bonded to the grounding electrode system at the service. This is the only place where the neutral and ground are bonded together (unless it is a separately derived system).
Inspection Point: On site, verify that the service entrance conductors are protected from physical damage (230.50) and that the service panel has a main bonding jumper installed. If you see a neutral-ground bond in a sub-panel, that is a violation.
1.3 Feeder and Branch Circuit Sizing (Article 220)
This is the mathematical heart of the Master exam. You must be fluent in calculating loads for commercial and industrial occupancies.
The Calculation Process:
33.General Lighting Load (Table 220.12): For commercial spaces, this is typically 1.2 VA/ft² (for banks, offices) up to 3.5 VA/ft² (for hospitals). You must apply the demand factors from Table 220.42 for the first 3,000 VA and the next 117,000 VA.
34.Receptacle Loads (220.14): In commercial buildings, receptacles are calculated at 180 VA each. The first 10 kVA of receptacle load is typically allowed at 100% demand, but you must know when to apply the demand factors from Table 220.44 for large numbers of receptacles.
35.Fixed Appliances (220.14): Appliances are calculated at their nameplate rating. For feeders, you can apply a 75% demand factor to four or more fixed appliances (Table 220.54 for household, but for commercial, you must check the specific article).
36.Motors (Article 430): Motor loads are calculated at 125% of the motor's full-load current (FLC) for the branch circuit, and the feeder must be sized to handle the largest motor at 125% plus the sum of all other motors (430.24).
Advanced Calculation Example (Commercial):
Step 1: Compute the total connected load (lighting + receptacles + HVAC + motors).
Step 2: Apply demand factors.
Step 3: The feeder conductor must have an ampacity of at least the non-continuous load + 125% of the continuous load (215.2).
Master Trap: The "125% rule" applies to continuous loads. A continuous load is one where the maximum current is expected to continue for 3 hours or more. Most commercial lighting and some motor loads are continuous. If you forget to multiply the continuous load by 1.25, your feeder will be undersized.
1.4 Overcurrent Protection and Coordination (Articles 240 & 430)
A Master must understand the hierarchy of protection. The goal is to ensure that a fault on a branch circuit trips the branch OCPD, not the feeder OCPD.
Conductor Protection (240.4):
Conductors must be protected against overcurrent. The OCPD rating must not exceed the conductor's ampacity.
The "Next Size Up" Rule (240.4(B)): If the standard OCPD rating (e.g., 15, 20, 25, 30) does not correspond to the conductor ampacity, you can use the next higher standard rating, provided the conductor ampacity is not less than 800A and the next size up does not exceed 800A.
Tap Conductors (240.21): This is a Master-level topic. You can tap a feeder to supply a smaller panel if the tap conductors are not longer than 10 feet (3.0 m) and are enclosed in a raceway. The tap conductors must have an ampacity not less than the combined computed loads of the circuits supplied, and they cannot be longer than 10 feet.
Motor Protection (430.52 & 430.53):
Branch Circuit: The OCPD for a motor must be sized to allow the motor to start (inrush current) but protect against short circuits. The maximum rating is typically 250% of the motor's FLC for standard motors (Table 430.52). If the motor cannot start with this size, you can increase it to 400% (for standard motors) but you must then ensure the conductors are protected.
Overload Protection (430.32): This is separate from short-circuit protection. Overloads protect the motor from mechanical overload. They are typically sized at 115% to 125% of the motor's nameplate current rating.
Coordination (240.12): For emergency systems and some critical processes, the NEC requires selective coordination. This means that the OCPD closest to the fault opens without opening the upstream device. This is not just a design preference; it is a code requirement for life safety systems.
1.5 Separately Derived Systems (Article 250.30)
Transformers and generators create a new source of power. When you have a transformer secondary or a generator that is not bonded to the service, it is a Separately Derived System (SDS) .
Key Rules for SDS:
Grounding Electrode (250.30(A)): The system must be bonded to a grounding electrode. This is typically done at the first disconnecting means of the SDS. The grounding electrode conductor (GEC) must be sized per Table 250.66 based on the largest ungrounded conductor.
Bonding the Neutral (250.30(A)(1)): The grounded conductor (neutral) of the SDS must be bonded to the equipment grounding conductor (EGC) and the grounding electrode. This is the only place in the system (other than the service) where this bond is allowed.
System Bonding Jumper: The connection between the neutral and the ground must be made at the source (transformer) or at the first disconnecting means.
Master Trap: If you have a transformer feeding a panel, and you do not install a system bonding jumper at the transformer or the first panel, you will have a floating neutral. This can cause voltage spikes and is a serious safety hazard.
Generator Transfer Switches: When a generator is used as a backup, the transfer switch must be a "switched neutral" type if the generator is a separately derived system. If the generator is not an SDS (i.e., it is a portable generator with a bonded neutral), you must ensure the neutral is not bonded at the generator if it is connected to a system that is already grounded.
1.6 Transformers (Article 450)
Transformers are common in commercial buildings for stepping down 480V to 120/208V.
Protection (450.3):
Primary Protection: The OCPD on the primary side must be sized at 125% of the transformer's rated primary current. If the transformer has a primary current of 100A, the OCPD can be 125A (next size up).
Secondary Protection: If the secondary is not protected by the primary OCPD (e.g., a large transformer with a small primary), you must install secondary protection at 125% of the rated secondary current.
Conductor Sizing (450.3 & 240.21(C)):
The conductors on the secondary side must be sized to handle the transformer's output.
Tap Rule for Transformers (240.21(C)(2)): The secondary conductors can be tapped if they terminate in a single OCPD. The tap conductors must have an ampacity of at least 1/3 of the rating of the OCPD protecting the transformer primary, and they cannot be longer than 25 feet (7.5 m).
Inspection Point: Always verify the transformer's impedance (%Z). This is critical for calculating fault current. A 5% impedance transformer will have a higher fault current than a 10% impedance transformer of the same kVA rating.
As a Master, you are responsible for the final sign-off. Here are the critical items to check on site:
88.Bonding Jumpers: Verify that the main bonding jumper is installed at the service. Check that there is no neutral-ground bond in any sub-panel.
89.Grounding Electrode System: Confirm that the GEC is properly sized (Table 250.66) and connected to a code-compliant electrode (ground rod, concrete-encased electrode, etc.).
90.OCPD Sizing: Check that the breaker or fuse protecting a motor is not oversized. A 30A breaker on a 20A circuit is a violation unless it is a specific motor circuit.
91.Working Clearance (110.26): Ensure that panels and equipment have the required 36 inches of clearance in front and 30 inches of width. This is a common violation found during inspections.
92.Voltage Drop: While not strictly a code requirement for general circuits (it is a recommendation in 210.19(A) Informational Note), a Master should ensure that feeders are sized to keep voltage drop below 3% for the feeder and 5% total. This is a design responsibility.
1.9 Common Exam Traps
The 125% Rule: Do not apply the 125% factor to the conductor if the OCPD is already sized for 125% of the continuous load. You must apply it to the load first, then size the conductor to that value.
Continuous vs. Non-Continuous: Know the definition of a continuous load (3 hours or more). A 20A circuit with a 16A continuous load requires a conductor rated for 20A (16 × 1.25 = 20A), but the OCPD can be 20A.
Motor FLC vs. Nameplate: Always use the tables in Article 430 (Tables 430.247–430.250) for FLC when sizing conductors and OCPDs, not the motor nameplate. The nameplate is used for overload sizing only.
Neutral Sizing: The neutral conductor for a 3-phase, 4-wire system with nonlinear loads (e.g., fluorescent lighting, computers) must be sized to carry the maximum unbalanced load. In some cases, you may need to size the neutral to 130% of the phase conductor (220.61(C)).
The "Six Disconnect" Rule: This is a maximum, not a minimum. You can have one disconnect, but you cannot have seven.
1.10 Summary
Wiring and Protection is the foundation of the electrical code. For the Master exam, you must move beyond memorization and into application. Focus on the interaction between Articles 220 (Load Calculations), 230 (Services), 240 (OCPDs), and 250 (Grounding). Understand the flow of current from the utility to the branch circuit, and know where the neutral is bonded and where it is not. Practice the calculations until they are second nature, and use the index of the NEC to navigate to the correct tables quickly. Your ability to supervise and sign off on work depends on this knowledge.
Preparing for the Delaware Master Electrician license?
See the full licensing path, exam format, eligibility and application steps.
