Branch circuit calculations

Tyler, Brightwater's second-year apprentice, asked Dana a question on the Copper Kettle job that every master candidate should be able to answer instantly. The dining room's cove lighting is 16 amperes of LED drivers on a 20-ampere breaker. Why does the inspector's checklist flag it as fine, while the walk-in cooler's evaporator heaters, also about 16 amperes, needed a bigger breaker? Dana's answer covered the idea at the heart of this lesson. Both loads draw the same current, but they are not the same kind of load, and the code sizes branch circuits by what the load is and how long it runs, not just by how many amperes it draws.

Before you can calculate a feeder or a service, you need to calculate branch circuits. Article 210 sets the rules for branch circuit ratings and permissible loads, and the first parts of Article 220 tell you how much load to assign to each kind of outlet. This lesson works through continuous loads, the loads Article 220 assigns to receptacles and other outlets, the number of branch circuits a dwelling needs, and the special branch circuit rules for water heaters, space heating and electric vehicle chargers.

Continuous and noncontinuous loads

Article 100 defines a continuous load as one where the maximum current is expected to continue for three hours or more. Store lighting, office lighting, parking lot lighting, electric heat and electric vehicle charging are classic examples. A receptacle that powers a toaster for five minutes is not. The distinction matters because conductors, terminals and overcurrent devices are tested and rated assuming they will not carry their full rating for hours at a time.

Sections 210.19(A) and 210.20(A) give the same basic arithmetic for branch circuit conductors and overcurrent devices. The minimum rating is the noncontinuous load plus 125% of the continuous load. Put another way, a continuous load may use only 80% of a standard breaker's rating. There are exceptions for assemblies and devices listed for operation at 100% of their rating, which are uncommon on branch circuits and almost never the subject of exam questions unless the question says so.

The standard ampere ratings for fuses and inverse-time breakers are listed in 240.6(A). For branch circuits, the common sizes are 15, 20, 25, 30, 35, 40, 45, 50 and 60 amperes, and the list continues in larger steps above that. When a calculation gives a value between sizes, check whether the rule you are applying lets you round up to the next standard size; Lesson 9 covers 240.4(B), which governs that choice for conductors.

Loads Article 220 assigns to outlets

When an outlet's actual load is not known, Section 220.14 tells you what load to assume. Outlets for specific appliances are calculated at the ampere rating of the appliance or load served. Receptacle outlets in nondwelling occupancies are calculated at 180 VA for each single or multiple receptacle on one yoke, the strap that mounts the device. A duplex receptacle is one yoke and counts as 180 VA, not 360. A device with four or more receptacles on it counts at 90 VA per receptacle. Other outlets not covered elsewhere also count at 180 VA each. Show windows, signs, multioutlet assemblies and heavy-duty lampholders have their own entries in 220.14 that come up in Lesson 7.

In dwellings, the general-use receptacles in living areas are not counted outlet by outlet. They are already included in the general lighting load computed from floor area, which Lesson 4 covers. That is why a house with 60 receptacles does not get 10,800 VA added for them.

Many designers put fewer than the maximum on each circuit, and some local codes set lower limits, but the exam answer is the arithmetic. Note that the receptacle load is not a continuous load by default. If a question tells you that the receptacles serve equipment that will run for three hours or more, apply the 125% factor as well.

Lighting at its actual current

Section 220.18 has a rule that catches people who learned the trade with incandescent lamps. For inductive and LED lighting, the load on a branch circuit is based on the total ampere rating of the luminaires, not on the total wattage of the lamps. A driver or ballast draws more current than the lamp wattage alone suggests, because of its own losses and power factor. On an exam, if a fixture's input current is given, use it.

Permissible loads on a branch circuit

Section 210.23 limits what you may connect to a branch circuit of a given rating. On a 15 A or 20 A circuit, a single cord-and-plug connected piece of utilization equipment may not exceed 80% of the circuit rating. Equipment fastened in place, other than luminaires, may not exceed 50% of the circuit rating where the circuit also supplies lighting or cord-and-plug equipment that is not fastened in place. The idea is to leave room for the other loads on the circuit. Larger circuits, 25 A and up, have their own limits on what they may serve, and individual branch circuits may serve any load within their rating.

How many circuits a dwelling needs

Section 210.11(A) requires enough branch circuits to supply the calculated load, and 210.11(B) requires the load to be evenly proportioned among them as far as practical. For a dwelling's general lighting and general-use receptacles, the load is the floor area times the unit load from Table 220.12, which for dwelling units is a value in volt-amperes per square foot. The number of circuits is that load divided by the capacity of one circuit.

In addition, 210.11(C) requires specific circuits in every dwelling: at least two 20 A small-appliance branch circuits for the kitchen, pantry, dining and similar areas, at least one 20 A laundry circuit, and at least one 20 A circuit for the bathroom receptacles. Recent editions add a garage circuit requirement as well. These are in addition to the general lighting circuits.

The floor area is measured using the outside dimensions of the dwelling, and under 220.5(C) in the 2023 edition it does not include open porches or unfinished spaces that are not adaptable for future use. Garages are also excluded from the dwelling floor area for this calculation. Exam questions sometimes give the dimensions of a house plus a garage and a porch, and the test is whether you leave those out.

Branch circuits with their own sizing rules

Several kinds of equipment have rules in their own articles that tell you to treat them as continuous loads. Section 422.13 treats a fixed storage water heater of 120 gallons or less as a continuous load, so its branch circuit must be rated at least 125% of the nameplate. Section 424.4(B) does the same for fixed electric space heating. Article 625 treats electric vehicle charging as continuous and requires the branch circuit to be sized at 125% of the equipment's maximum load. Exam writers like these because the 125% factor is easy to forget when the question does not use the word continuous.

import math
STANDARD = [15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100]

def min_rating(noncontinuous, continuous):
    return noncontinuous + 1.25 * continuous

def next_standard(amps):
    return next(s for s in STANDARD if s >= amps)

for name, nc, c in [("Cove lighting", 0, 16), ("Mixed circuit", 3, 12),
                    ("Water heater", 0, 4500 / 240), ("EV charger", 0, 48)]:
    need = min_rating(nc, c)
    print(f"{name}: needs {need:.2f} A, use {next_standard(need)} A")
print("Receptacles on 20 A:", math.floor(20 * 120 / 180))

Cove lighting: needs 20.00 A, use 20 A
Mixed circuit: needs 18.00 A, use 20 A
Water heater: needs 23.44 A, use 25 A
EV charger: needs 60.00 A, use 60 A
Receptacles on 20 A: 13

Dana's answer to Tyler, then, was this. The cove lights are continuous, 16 A times 1.25 is exactly 20 A, and a 20 A breaker is permitted. The walk-in evaporator heaters were rated slightly higher, 17 A, and electric heating elements in that equipment are treated as continuous, so the 125% factor pushed them past 20 A to a 25 A circuit. The amperes looked the same on paper, but the arithmetic did not.