Services, Feeders & Branch Circuits

Services, Feeders & Branch Circuits — Colorado Master Electrician Exam Study Chapter

Learning Objectives

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

1.1 The Hierarchy: Service, Feeder, Branch Circuit

Understanding the three-tier structure is foundational. The service conductors run from the utility point of connection to the service disconnecting means. A feeder runs between the service equipment and the final branch-circuit overcurrent device (or, for a separately derived system, from the source to the first overcurrent device). A branch circuit runs from the final overcurrent device to the outlets or loads.

The NEC defines these in Article 100. For the master exam, you must know the exact boundaries. A common trap: conductors that pass through a panelboard without terminating are still feeders, not branch circuits. The branch circuit begins at the final overcurrent device — not at the panelboard bus.

Branch circuits are classified by the rating of the overcurrent device protecting them (210.3). A 20-ampere branch circuit is one protected at 20 amperes, regardless of conductor size. This distinction matters when applying 210.19(A) for conductor sizing.

1.2 Services: Article 230

Article 230 governs service conductors and equipment. Key sections for the master exam:

230.42 — Minimum Size of Service Conductors. Service conductors must have an ampacity sufficient to carry the computed load per Article 220, and must not be less than the rating of the service disconnecting means. The 125% continuous-load factor applies here as well. For a 200-ampere service with 180 amperes of continuous load, the conductors must be sized for 180 × 1.25 = 225 amperes, requiring 4/0 copper or 250 kcmil aluminum (per Table 310.16, 75°C column).

230.70 — Service Disconnecting Means. Each service must have a disconnecting means that is plainly identifiable, and it must be located at a readily accessible location nearest the point of entrance of the service conductors. For a master supervising work, this means verifying that the disconnect is not inside a bathroom, not behind a locked door without a key available to authorized personnel, and not located where it could be blocked by equipment.

230.71 — Maximum Number of Disconnects. The 2026 NEC retains the six-handle rule (six disconnects maximum per service), but with a critical change from earlier editions: the disconnects must be grouped. Each disconnect must be suitable for the service, and the grouping requirement eliminates the old practice of placing disconnects in different locations. For exam purposes, remember: six is the maximum, and they must be in one location.

230.79 — Rating of Service Disconnecting Means. For one-family dwellings, the minimum rating is 100 amperes. For all other occupancies, the minimum is 60 amperes unless the computed load requires more. This is a common inspection point — a dwelling with a 60-ampere service is a violation even if the calculated load is only 40 amperes.

230.95 — Ground-Fault Protection. For grounded wye services over 150 volts to ground but not exceeding 600 volts phase-to-phase, with the service disconnecting means rated 1000 amperes or more, ground-fault protection is required. This is a classic master-level question. The threshold is 1000 amperes, not 800, and it applies to the service disconnect rating, not the conductor ampacity.

1.3 Feeders: Article 215

Feeders are covered in Article 215. The minimum feeder ampacity per 215.2(A)(1) is the sum of the noncontinuous load plus 125% of the continuous load. This is the same formula as for services and branch circuits. The feeder must also have an ampacity not less than the rating of the overcurrent device protecting it, per 215.3.

Feeder neutral sizing is per 220.61. The neutral must be sized for the maximum unbalanced load. For a 3-phase, 4-wire wye system, the neutral carries the unbalanced current. You may apply demand factors from Table 220.61 for household electric ranges and dryers, but you cannot reduce the neutral below the grounding conductor size required by 250.122.

A common trap: for a 3-phase, 4-wire system supplying only line-to-line loads (e.g., 208-volt motors), the neutral carries no current and may be sized as a grounding conductor — but only if there are no line-to-neutral loads. If there is even one single-phase load connected line-to-neutral, the neutral must be sized for that load.

1.4 Branch Circuits: Article 210

Article 210 covers branch circuits. Key points for the master exam:

210.19(A)(1) — Single-phase branch circuits. Conductors must have an ampacity of not less than the noncontinuous load plus 125% of the continuous load. For a 20-ampere branch circuit supplying a 16-ampere continuous load, the conductor must be rated for 20 amperes (16 × 1.25 = 20), which requires 12 AWG copper at 60°C.

210.20 — Overcurrent protection. The branch-circuit overcurrent device must have a rating not less than the noncontinuous load plus 125% of the continuous load. This is a separate calculation from the conductor sizing — both must be checked.

210.23 — Permissible loads. A 15- or 20-ampere branch circuit can supply lighting, appliances, or a combination. A 30-ampere branch circuit can supply fixed appliances — but not cord-and-plug-connected appliances exceeding 80% of the branch-circuit rating. A 40- or 50-ampere branch circuit is limited to cooking appliances and fixed electric space-heating equipment.

210.52 — Dwelling unit receptacles. The spacing rules are a frequent exam topic. Receptacles must be installed so that no point along the wall is more than 6 feet from a receptacle (measured horizontally). This means receptacles are spaced no more than 12 feet apart. The 6-foot rule applies to wall space 2 feet or more in width. Doorways and fireplaces do not count as wall space.

1.5 Separately Derived Systems: Article 250.30

A separately derived system (SDS) is a source of power with no direct electrical connection to the supply conductors — typically a transformer or generator. The bonding and grounding requirements are in 250.30.

For a transformer, the secondary must have a system bonding jumper at the source (the transformer) or at the first disconnecting means, but not both. The grounding electrode conductor must connect the system to a grounding electrode per 250.30(A)(4). The size of the system bonding jumper is per Table 250.102(C)(1), based on the area of the largest ungrounded secondary conductor.

For a generator, the question is whether it is a separately derived system. A portable generator with a transfer switch that switches the grounded conductor is an SDS. If the grounded conductor is not switched (solidly connected), the generator is not separately derived, and the grounding rules of 250.30 do not apply — instead, the generator frame must be bonded per 250.35.

A common exam trap: the neutral of a separately derived system must be bonded to the equipment grounding conductor at the source, but if the SDS supplies a panelboard, the neutral must be isolated in that panelboard (no bonding screw).

1.6 Motor Circuits: Article 430

Motor circuits are a significant portion of the master exam. The key is understanding the relationship between branch-circuit conductors, feeder conductors, and overcurrent protection.

430.22 — Branch-circuit conductors. Conductors supplying a single motor must have an ampacity of not less than 125% of the motor's full-load current (FLC). The FLC is taken from Tables 430.247 through 430.250, not from the motor nameplate. The nameplate full-load amperes (FLA) is used for overload protection and for the motor controller, but the branch-circuit conductor sizing uses the table values.

430.52 — Branch-circuit short-circuit and ground-fault protection. The maximum rating of the branch-circuit protective device is a percentage of the FLC, depending on the motor type and the protective device. For a standard squirrel-cage motor with an inverse-time breaker, the maximum is 250% of FLC. For a non-time-delay fuse, it is 300%. If the calculated value does not correspond to a standard fuse or breaker rating, you may round up to the next standard size per 430.52(C)(1) Exception 1.

430.24 — Feeder conductors. Conductors supplying several motors must have an ampacity of not less than 125% of the largest motor FLC plus the sum of the FLCs of all other motors on the feeder. This is a critical calculation. For a feeder supplying a 20-ampere motor and a 10-ampere motor: (20 × 1.25) + 10 = 35 amperes.

430.32 — Overload protection. Overload devices must be sized at not more than 125% of the motor nameplate FLA for motors with a service factor of 1.15 or greater, or with a temperature rise of 40°C or less. For all other motors, the limit is 115%. If the calculated value does not allow the motor to start, you may use the next higher size per 430.32(C), but never exceed 140% for the first category or 130% for the second.

1.7 Generator Applications: Article 445

Article 445 covers generators. For a master, the critical sections are:

445.13 — Ampacity of conductors. Generator conductors must have an ampacity of not less than 115% of the generator's nameplate current rating. This is different from the 125% rule for continuous loads — the generator is considered a noncontinuous source, so the factor is 115%.

445.18 — Disconnecting means. Generators must have a disconnecting means that is readily accessible and that opens all ungrounded conductors. The disconnect must be capable of being locked in the open position.

445.11 — Marking. Generators must be marked with the manufacturer's name, rated voltage, frequency, power factor, and rated current. For a master signing off on an installation, verifying the nameplate data matches the overcurrent protection and conductor sizing is essential.

1.8 Load Calculations: Article 220

The load calculation methods in Article 220 are the backbone of service and feeder sizing.

220.40 — General. The calculated load must be the sum of all loads, with demand factors applied where permitted. For a dwelling, the standard method is in Part III of Article 220; the optional method is in Part IV.

220.42 — Lighting demand. For dwelling units, the general lighting load is 3 volt-amperes per square foot. The first 3000 VA is at 100%, the next 117,000 VA at 35%, and the remainder at 25%. For a 2000-square-foot dwelling: 2000 × 3 = 6000 VA. The demand is 3000 + (3000 × 0.35) = 4050 VA.

220.44 — Receptacle demand. For other than dwellings, receptacles are calculated at 180 VA each. The first 10 kVA is at 100%, and the remainder at 50%.

220.55 — Cooking equipment. Household electric ranges and cooking appliances have a demand table (Table 220.55). For up to three ranges, the demand is 8 kW per range. For 4 to 6 ranges, the demand is 7 kW each, and so on. Column C of the table applies when the nameplate ratings are between 8.75 and 12 kW. If the nameplate rating differs, adjustments per Note 1 apply.

220.61 — Neutral loads. The neutral load is the maximum unbalanced load. For a 3-phase, 4-wire system with line-to-neutral loads, the neutral is sized for the largest single-phase load. Demand factors from Table 220.61 may apply to ranges and dryers.

1.9 Overcurrent Protection Coordination

For a master, understanding coordination is critical, especially for emergency systems (Article 700) and legally required standby systems (Article 701). Selective coordination requires that the overcurrent device closest to the fault opens first, without opening the upstream device.

For emergency systems, 700.28 requires selective coordination for all overcurrent devices in the emergency system. This is a mandatory requirement — not just a recommendation. For legally required standby systems, 701.27 has the same requirement.

In practice, this means using fuses with time-current curves that are coordinated, or breakers with adjustable trip settings. A master must verify that the feeder breaker is not the first to trip on a branch-circuit fault. A common exam trap: assuming that a larger breaker automatically coordinates with a smaller one. It does not — the time-current curves must be compared.

1.10 Code Navigation: Where to Find It

For the open-book exam, speed is everything. Memorize these locations:

1.11 Inspection and Supervision Points

As a master, you are responsible for the work you supervise. On site, verify:

1.12 Common Exam Traps

Summary

Mastering services, feeders, and branch circuits requires understanding the hierarchy of the system, the specific ampacity rules for each tier, and the overcurrent protection requirements. The 125% continuous-load rule appears throughout, but motor circuits have their own factors. Separately derived systems require careful attention to bonding and grounding. Load calculations in Article 220 provide the foundation for all conductor sizing. For the open-book exam, know where to find each table and section — speed is your advantage. On the job, verify the details that keep installations code-compliant and safe.