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120 Metres of Aluminum and a Plan to Backfill Friday

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120 Metres of Aluminum and a Plan to Backfill Friday

A hog barn sits 120 m from the meter base on a 120/240 V single-phase service. The Section 8 calculated demand for the barn panel is 100 A. The trench is open at 900 mm, the soil is sandy and dry, and the contractor has already laid in 2/0 AWG aluminum USEI direct-buried cable with a 100 A main breaker at the barn. His argument: "2/0 aluminum is good for 135 amps and you're only pulling 100." Utility no-load voltage at the meter is 246/123 V, sagging to 238/119 V when the neighbour's irrigation pivots run. The owner has two additions planned: a 40 A continuous EV charger next spring, and a 10 hp, 240 V single-phase well pump started across the line, with the pump and barn loads running together. The inspector has not been called, the soil thermal data is unknown, and the contractor wants to backfill on Friday.

1. Is the installed 2/0 aluminum acceptable? Separate the ampacity question from the voltage-drop question and show the numbers.

Read the worked answer

Ampacity: passes. Voltage drop: fails.

Ampacity check: 2/0 AWG aluminum, 75 °C column, is about 135 A in Table 2. With a 100 A calculated demand and a 100 A main, conductor ampacity exceeds the load and the overcurrent device rating. On ampacity alone the contractor is right.

Voltage drop check (single phase, round trip = 2L):

  • Resistance of 2/0 aluminum at 75 °C is roughly 0.436 ohm/km (Table 9, or convert Table 8 dc resistance and correct to operating temperature).
  • VD = 2 x I x R x L = 2 x 100 A x 0.436 ohm/km x 0.120 km = 10.5 V
  • 10.5 V on 240 V = 4.4 %

On the utility's sagging 238 V, the barn sees about 227 V at full load.

Is 3 % a legal limit? The Canadian Electrical Code does not impose a general 3 %/5 % voltage-drop rule; it appears as good practice in the appendices, plus utility standards and the owner's specification. So the answer is not "the Code fails it" - it is "this exceeds the 3 % feeder target the design and the customer's equipment need." A 10 hp single-phase motor that sees 227 V at rest will see well under 210 V on starting, which is where nuisance contactor drop-out begins.

The install is code-legal on ampacity and wrong on design.

2. Select a conductor that meets a 3 % feeder drop and justify the size against the ampacity you would otherwise have chosen.

Read the worked answer

Work backwards from 3 %. Target VD = 0.03 x 240 V = 7.2 V.

Required R = VD / (2 x I x L) = 7.2 / (2 x 100 x 0.120) = 0.30 ohm/km maximum.

Candidates at 75 °C:

  • 3/0 aluminum, about 0.348 ohm/km: VD = 2 x 100 x 0.348 x 0.120 = 8.4 V = 3.5 %. Fails.
  • 4/0 aluminum, about 0.274 ohm/km: VD = 2 x 100 x 0.274 x 0.120 = 6.6 V = 2.7 %. Passes.
  • 3/0 copper, about 0.213 ohm/km: VD = 2 x 100 x 0.213 x 0.120 = 5.1 V = 2.1 %. Passes.

Pick 4/0 aluminum USEI (or 3/0 copper if the terminations and budget favour it). Ampacity of 4/0 aluminum at 75 °C is roughly 180 A - nearly double the 100 A load. That is the point: voltage drop, not ampacity, drives the size on a 120 m run. The 100 A breaker is still correct because the load is 100 A and the conductor is now oversized, which is permitted.

Before backfilling: confirm the termination temperature rating at both ends (a 75 °C breaker lug is fine for USEI at 75 °C), confirm burial depth and mechanical protection per Table 53 and Section 12 for the actual soil and traffic condition, and confirm the sandy, dry soil assumption - if the soil is dry and poorly conducting, the direct-burial ampacity must be taken from the correct soil-temperature and thermal-resistivity columns, not the 30 °C air values.

3. The owner adds the 40 A continuous EV charger. Recalculate the feeder and state what has to change, including anything beyond the cable.

Read the worked answer

Redo the Section 8 calculation first; do not just add 40 A.

Continuous load treatment (Rule 8-104): the 40 A charger is a continuous load, so it is taken at 125 % = 50 A for conductor and overcurrent sizing. New demand = 100 A + 50 A = 150 A.

Consequences:

  1. The 100 A main breaker and barn panel are now undersized - 150 A demand needs a 150 A or larger panel and main, and the service and meter base at the house must be checked against their rating.
  2. Cable: 4/0 aluminum at 150 A gives VD = 2 x 150 x 0.274 x 0.120 = 9.9 V = 4.1 %. Fails again. Next size up, 350 kcmil aluminum at about 0.163 ohm/km: VD = 2 x 150 x 0.163 x 0.120 = 5.9 V = 2.4 %, ampacity roughly 250 A at 75 °C. Passes.
  3. The 10 hp pump is the real constraint. At about 50 A full load and roughly 300 A locked rotor, starting drop with the 350 kcmil run is 2 x 300 x 0.163 x 0.120 = 11.7 V = 4.9 % - acceptable if the utility is at 240 V, marginal if it is at 238 V with the charger drawing. Stagger the charger and the pump, or specify a soft starter.
  4. Document it. The trench should not be backfilled until the sizing is settled; re-opening 120 m of trench is the most expensive option on the table.

Cheapest correct answer today: pull the 350 kcmil now while the trench is open, and upgrade the barn panel with the charger.