Service and feeder sizing worked examples

The earlier lessons each took one calculation at a time. Real exam questions, and real permit applications, string them together: a demand calculation gives a load, the load gives a current, the current gives a conductor size after corrections, the conductor is checked for voltage drop, and the overcurrent device and equipment ratings follow. This lesson works four complete examples from start to finish, each based on a Parkland Valley job, so that you can see the whole chain and the decisions at each link.

The examples use the Code's methods as summarised in lessons 3 to 10. Where a step needs a table value, the value is illustrative and labelled; on the exam you will look up the real one. The point of this lesson is the sequence and the habits, not the particular numbers.

The chain

Example 1: Aspen Ridge with its basement suite

In lesson 4, Aspen Ridge gained a basement suite. The combined load before heating and cooling was 32 825 W. The house has 12 kW of baseboards upstairs and the suite adds 6 kW, so the total heating is 18 kW; the 5 kW air conditioner is not interlocked. The EV charger stays in the main dwelling load. Now finish the calculation and size the service.

Because 224.3 A exceeds the existing 200 A service, the conversion needs a service upgrade, which is common when suites are added. Priya proposes two options to the owners. The first is a 400 A service with a main panel and a suite subpanel, which handles any future additions. The second is to install an energy management system for the EV charger, which the 2024 Code's Section 8 and Section 86 rules allow to reduce the load counted for EV supply equipment, and keep the load within a 225 A or 250 A service. The owners choose the 400 A service. For the exam, the point is that a calculated load slightly above a standard size is not rounded down: the service must meet or exceed the calculated load.

Example 2: Riverbend's parkade EV chargers

Two years after Riverbend Condos opened, the condo board asked Parkland Valley to add 24 Level 2 EV chargers in the parkade, one per stall, each 7.2 kW at 208 V. At full value that is 172.8 kW, more than the building's whole gas-heated service could take. The calculated load from lesson 4 was 243 361 W on an 800 A, 208 V service. Here, Rule 8-106 provides two tools: the recorded maximum demand over the last 12 months for the existing load, and an electric vehicle energy management system to limit the chargers' combined load.

The EV feeder itself must be sized for the load the management system allows, with the continuous-load rule from lesson 6 in mind, because EV charging runs for hours. If the system limits the parkade to 60 kVA at 208 V, the current is 60 000 / 360.3 = 166.5 A, and if the feeder breaker is not marked for 100% continuous operation, it must be rated at least 166.5 / 0.80 = 208 A, so a 225 A feeder breaker and conductors to match.

Example 3: the clinic feeder from the mall's 600 V distribution

The clinic transformer from lesson 9 is fed from the mall's 600 V distribution panel, 40 m away through the ceiling space. The transformer is 75 kVA with primary-only protection. Size the primary feeder and check its voltage drop.

Then the secondary: conductors of at least 260.3 A from the transformer to the clinic panel, a short run, and the panel's main breaker. The panel must be rated for the available fault current of about 6 kA from lesson 9, and its breakers must have an interrupting rating to match.

Example 4: Hartley Farms main service

Hartley Farms' original service was 200 A at 120/240 V single-phase, which had been fine for the house and shop. The new grain drying system brought three-phase motors, a dryer burner blower and a lot of aeration. The utility extended three-phase to the farm, and Parkland Valley installed a new 347/600 V three-phase service for the farm buildings, keeping the house on its own single-phase service. Farm loads are calculated as an occupancy with specific loads: the motor loads by the Section 28 feeder method, plus heating and lighting.

That example contains a simplification worth noticing: a single-phase load on a three-phase system does not spread evenly. A careful calculation puts the 41.7 A on the two lines it actually uses, which makes those lines heavier than the third. Exams usually tell you which approach to take; where they do not, the worst line governs.

The neutral

Every example above sized the line conductors. The neutral of a service or feeder can sometimes be smaller, because it carries only the unbalanced current, but the Code limits how much smaller and lists loads that do not qualify for a reduction. Electric ranges and dryers on 120/240 V circuits put some current on the neutral but much less than on the lines, while non-linear loads such as electronic equipment, LED drivers and EV chargers with switching power supplies can put more on the neutral than on any line, and for those no reduction is permitted and an increase may be needed. The rule in Section 4 on neutral conductor sizing sets out how to calculate the maximum unbalanced load and when the neutral may be reduced.

For the exam, the safe habits are these. Size the neutral equal to the line conductors unless the question asks you to apply the reduction. If it does, calculate the maximum unbalanced load as the rule directs, check whether any of the loads are excluded from a reduction, and never go below the minimum the rule sets in relation to the bonding requirements. At Riverbend, with electric baseboards and ranges on 208 V, the neutral reduction would have been allowed for much of the load; with the EV chargers added, the board's engineer kept the parkade feeder neutral full size.

Continuous loads, dwellings and the 80% rule

One question comes up in every example: is the load continuous, and does the 80% limit of Rule 8-104 apply? For single dwellings and apartments calculated by the Section 8 dwelling methods, the Code says the calculated load is not treated as continuous for that rule, because the demand factors already reflect diversity. For other occupancies, decide by the nature of the load: lighting in a store, EV charging, process loads and heating are usually continuous. Equipment marked for 100% continuous operation may carry its full rating; otherwise, size at 125% of the continuous load.

import math

def three_phase_amps(kva, volts):
    return kva * 1000 / (math.sqrt(3) * volts)

def capacity_kva(amps, volts, phases=3):
    return (math.sqrt(3) if phases == 3 else 1) * volts * amps / 1000

# Example 2: Riverbend EV chargers
cap = capacity_kva(800, 208)
print(f'Riverbend capacity {cap:.0f} kVA; managed {150 + 60} kVA; unmanaged {150 + 24 * 7.2:.1f} kVA')
ev = three_phase_amps(60, 208)
print(f'EV feeder {ev:.1f} A, breaker at 125%: {ev / 0.8:.0f} A')

# Example 3: clinic primary feeder voltage drop
vd = math.sqrt(3) * 40 * 72.2 * 1.0 / 1000
print(f'Clinic feeder drop {vd:.1f} V = {vd / 600:.2%}')

Riverbend capacity 288 kVA; managed 210 kVA; unmanaged 322.8 kVA
EV feeder 166.5 A, breaker at 125%: 208 A
Clinic feeder drop 5.0 V = 0.83%

A checklist for complete questions

After the four examples, the study group writes a checklist for complete service and feeder questions, and Priya tapes it into the red tab of her code book. Identify the occupancy and the method. Calculate the load, writing every line. Decide whether the load is continuous. Convert to current at the right voltage. Choose the conductor from the right table and column with corrections, and limit it to the termination column. Check voltage drop. Choose the overcurrent device, protecting the conductor. Check equipment ratings, including interrupting rating. Then do Denise's three checks: units, size and direction.

Tyler, watching the group work through these, observes that every step is something he has seen on the job, just never written down in one place. Denise tells him that is exactly what a master electrician is: the person who writes it down, gets the permit and signs for it.