Service Conductor Sizing: The NEC 230 Chain the Exam Tests

Why service questions eat your clock

A service sizing question is never one lookup. It chains three articles: the load lives in Article 220, the conductor rule lives in Article 230, and the wire itself lives in Article 310. Guys who hesitate are guys who treat each question as a fresh puzzle, flipping between articles hoping to bump into the answer. Guys who move fast run the same four steps on every service question, because every service question is the same question wearing a different job site.

Name the problem before you touch the numbers. If the question says service, service-entrance conductors, or service disconnect, you are in the chain below. Run it in order.

The four-step chain

1. Compute the load. Article 220 gives you the calculated load in amperes.
2. Set the minimum conductor ampacity. NEC 230.42(A): the service-entrance conductors must carry that calculated load.
3. Check the floor. NEC 230.79 sets minimum service disconnect ratings, and a one-family dwelling never goes below 100A per 230.79(C).
4. Pick the conductor. Table 310.16 for the general case, or the 83% dwelling shortcut at 310.12 when it applies.

Four steps. Load, ampacity, floor, wire. The rest of this page walks each step, then runs a full 200A worked example.

Step 1: Compute the load (Article 220)

Every service sizing problem starts with a number from Article 220: general lighting at 3 VA per square foot, small appliance and laundry circuits, appliance demand factors, larger of heat or AC. On many exam questions this work is already done for you and the question hands you a calculated load. When it is not done for you, it is a dwelling load calculation first and a service question second. We cover that full calculation in the Article 220 guide linked below, so this page assumes you have the load number in hand.

Step 2: Minimum ampacity (NEC 230.42)

NEC 230.42(A) is the anchor: service-entrance conductors must have an ampacity at least equal to the load calculated under Article 220. Continuous loads count at 125 percent, the same cushion the code applies everywhere a load runs three hours or more.

NEC 230.42(B) then ties the conductors to the disconnect: the service-entrance conductor ampacity cannot fall below the rating of the service disconnecting means required by 230.79(A) through (D). That linkage is the whole reason step 3 exists.

Step 3: The 230.79 floor

The code does not let a service shrink to whatever the load calculation says. NEC 230.79 sets minimum ratings for the service disconnecting means:

• 230.79(A): a single-circuit installation gets a 15A minimum. Think a sign or a small outbuilding with one circuit.
• 230.79(B): a two-circuit installation gets a 30A minimum.
• 230.79(C): a one-family dwelling gets a 100A minimum, 3-wire. This is the one the exam tests.
• 230.79(D): everything else gets a 60A minimum.

The trap: a question gives you a small house, the Article 220 math lands at 72A, and answer choice B offers a tidy 80A service. Wrong. One-family dwelling means 100A minimum, full stop, per 230.79(C). The calculated load sets the size only when it exceeds the floor.

Step 4: The 83% rule (NEC 310.12)

Here is the dwelling shortcut in plain language. For a 120/240V single-phase service rated 100A through 400A, where the service conductors supply the entire load of a one-family dwelling, or the entire load of an individual dwelling unit in a two-family or multifamily building, NEC 310.12 permits the conductors to have an ampacity of at least 83 percent of the service rating.

Why 83 percent? A dwelling never runs everything at once. The range, the dryer, the AC, and the water heater do not all peak together, so the code allows a conductor smaller than the rating would otherwise demand. Same logic as the demand factors in Article 220, applied one more time at the conductor.

When the shortcut is off the table

• Not a dwelling. Commercial and industrial services size from the calculated load and Table 310.16, no 83% discount.
• The conductors do not carry the entire load. A feeder to a detached garage sub-panel carries part of the load, so it sizes the normal way.
• Service rating outside 100A through 400A. Below 100A or above 400A, the rule does not apply.
• Adjustment or correction factors apply. Table 310.12 assumes none. Rooftop heat, more than three current-carrying conductors, or high ambient pushes you back to the long-form ampacity work in 310.15.

Worked example: 200A dwelling service

A one-family dwelling in Round Rock gets a 200A, 120/240V single-phase service. The service-entrance conductors carry the entire load. What is the minimum copper conductor size at 75C terminations?

1. Name the problem. Dwelling, single-phase 120/240V, service conductors carrying the entire load, 200A rating. That is a 310.12 question.
2. Apply the 83% rule. 200A x 0.83 = 166A. The conductors need an ampacity of at least 166A.
3. Go to Table 310.16, 75C copper column. 1/0 copper is rated 150A. Too small. 2/0 copper is rated 175A. 175A covers 166A.
4. Answer: 2/0 AWG copper.

Table 310.12 hands you the same answer without the math: 2/0 copper or 4/0 aluminum for a 200A dwelling service. Use the table when the question is clean. Know the multiplication anyway, because the exam can hand you a 175A or 350A rating the table handles but your memory of common answers does not.

Two details that separate a pass from a near miss. First, the 75C column: residential service equipment terminations are rated 75C, and 110.14(C) holds you to the lowest termination rating, so the 90C column is bait. Second, the multiplier hits the service rating, not the calculated load. If the load calculation came out to 152A and the service is 200A, the conductor check is 200 x 0.83, not 152 x 0.83.

Where candidates lose the points

• Applying 83% to a commercial service. The rule is dwelling-specific. Read 310.12 before you discount anything.
• Applying 83% to a feeder that carries part of the load. Entire load or no shortcut.
• Multiplying the calculated load by 0.83 instead of the service rating. The rule keys off the rating.
• Flipping to 310.15(B)(7) in a 2023 codebook. The rule moved to 310.12 in 2020.
• Reading the 90C column of Table 310.16. Terminations are 75C. Use the 75C column.
• Forgetting the 100A floor at 230.79(C) when the calculated load is small.
• Skipping the 125% continuous load cushion in the 230.42(A) ampacity check.

The memory hook

Write this on your scratch paper before the first question:

220 gives the load. 230.42 sizes the wire to it. 230.79 sets the floor. 310 picks the conductor.

And for the shortcut: 83% of the rating, not the load. Then the 75 degree column. Ten words. They settle the two decisions that stall most guys on service questions.