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NEC 440 Exam Examples: MCA, Max OCPD and Disconnect
Eight worked examples in the style of electrician exam questions on hermetic refrigerant motor-compressors: minimum circuit ampacity with and without BCSC, the maximum standard size of the short-circuit and ground-fault protective device, and the minimum disconnect rating — each with the rule, the arithmetic, the answer and what the Equipment Circuit tool shows for the same inputs. Exam answers follow the edition your exam uses; this page uses NEC 2023 (same in 2026). Marked nameplate values govern (440.22(C), 110.3(B)).
NEC 2023/2026 440.32 · 440.22 · 440.12
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Worked example
Exam example 1: a compressor alone, RLA 35.8 A, BCSC 39.2 A — minimum circuit ampacity, max OCPD and disconnect?
| Equipment | Hermetic motor-compressor alone (no fan, no heater) |
|---|---|
| Compressor RLA | 35.8 A |
| BCSC | 39.2 A |
| BCSC line | Using BCSC 39.2 A — it is greater than RLA (NEC 440.6(A) Exc. 1) |
|---|---|
| Minimum circuit ampacity | 49.0 A |
| Maximum overcurrent protection | 70 A — 175% of BCSC; the next higher standard size is allowed (NEC 440.22(A) Exc. 1) |
| Maximum at 225%, if 175% can't start the motor | 80 A — never above 225%, rounded down (NEC 440.22) |
| Disconnect, at least | ≥ 45.1 A — 115% of BCSC (NEC 440.12(A)(1)); ampere rating only, check the horsepower rating too |
| Breaker | 70 A |
| Conductor | 8 AWG |
Rule: BCSC is marked and greater than RLA, so it replaces RLA (NEC 2023 440.6(A) Exc. 1). MCA, 440.32: 1.25 × 39.2 = 49.0 A. Max OCPD, 440.22(A): 1.75 × 39.2 = 68.6 A — not a standard size, so the next higher one, 70 A (Exc. 1). Disconnect, 440.12(A)(1): 1.15 × 39.2 = 45.08 A, which the app shows rounded up as ≥ 45.1 A. Exam answers: MCA 49.0 A, max OCPD 70 A, disconnect at least 45.08 A. Using RLA instead would understate both: 1.25 × 35.8 = 44.75 A.
How it's calculated
The table behind the number
Every example uses the same handful of NEC 2023 Article 440 rules (unchanged in 2026). When the nameplate marks a BCSC greater than RLA, BCSC replaces RLA everywhere below (440.6(A) Exc. 1). MCA: 125% of the largest motor-compressor current plus the other motors (440.32 alone, 440.33 with other motors). Max OCPD for a compressor alone: 175%, and the next higher standard size when 175% falls between sizes (440.22(A) Exc. 1); with a fan or other loads: 175% of the largest plus the others (440.22(B)(1)), which the app rounds down to a standard size. Where 175% can't start the motor: up to 225%, never above (440.22(A) Exc. 2). Disconnect ampere rating: at least 115% of the compressor current alone (440.12(A)(1)) or of all currents added (440.12(B)(2)); the horsepower rating is a separate check. The app shows amperes to one decimal — MCA rounded to the nearest 0.1, the disconnect rounded up — so where an exam says "do not round", the unrounded figure in the arithmetic is the exam answer.
- NEC 2023/2026 440.6(A) Exc. 1Where a BCSC is marked, it is used instead of RLA to size the conductors, the disconnect and the short-circuit protection
- NEC 440.32Conductors for one motor-compressor: at least 125% of RLA or of BCSC, whichever is greater
- NEC 440.33Compressor with other motors: the largest at 125% plus the others at 100%
- NEC 440.12(A)(1)Disconnect for a motor-compressor: ampere rating at least 115% of RLA or BCSC, whichever is greater
- NEC 440.12(B)(2)Disconnect for a compressor with other loads: ampere rating at least 115% of all the currents added
- NEC 440.22(A) Exc. 1A compressor alone: up to 175% of RLA or BCSC; when that is not a standard size, the next higher standard size is allowed
- NEC 440.22(A) Exc. 2Where 175% can't start the motor, protection may be raised, but never above 225%
- NEC 440.22(B)(1)A compressor with other loads: the 175% value of the largest compressor plus the other loads
- NEC 440.22(C), 2023 110.3(B)Marked manufacturer maximums govern; listed equipment is installed per its listing or labeling instructions
Questions
Before you trust the number
Exam example 2: a hermetic compressor marked RLA 43.9 A and BCSC 46.4 A — minimum circuit ampacity?
Inputs: compressor alone, RLA 43.9 A, BCSC 46.4 A. Rule: BCSC is greater, so it replaces RLA (NEC 2023 440.6(A) Exc. 1); one compressor needs 125% of it (440.32). Arithmetic: 1.25 × 46.4 = 58.0 A. Answer: 58.0 A. What the app shows: MCA 58.0 A with the line "Using BCSC 46.4 A", max OCP 90 A (1.75 × 46.4 = 81.2 A → next higher standard size, 440.22(A) Exc. 1), disconnect ≥ 53.4 A, conductor 6 AWG.
Exam example 3: RLA 26.5 A, BCSC 28.6 A, compressor alone — minimum circuit ampacity, do not round?
Inputs: compressor alone, RLA 26.5 A, BCSC 28.6 A. Rule: BCSC replaces RLA (NEC 2023 440.6(A) Exc. 1), 125% of it (440.32). Arithmetic: 1.25 × 28.6 = 35.75 A. Answer: 35.75 A — do not round. What the app shows: MCA 35.8 A (the display rounds to 0.1; the exam answer is 35.75), max OCP 60 A (1.75 × 28.6 = 50.05 A → next higher standard size), disconnect ≥ 32.9 A.
Exam example 4: an A/C unit with a compressor RLA 34.2 A and a condenser fan FLA 2.3 A — minimum circuit ampacity?
Inputs: RLA 34.2 A, fan FLA 2.3 A, no BCSC. Rule: the largest motor at 125% plus the other motor at 100% (NEC 2023 440.33). Arithmetic: 1.25 × 34.2 + 2.3 = 42.75 + 2.3 = 45.05 A. Answer: 45.05 A. What the app shows: MCA 45.1 A (rounded to 0.1), max OCP 60 A (1.75 × 34.2 + 2.3 = 62.15 A, rounded down, 440.22(B)(1)), disconnect ≥ 42.0 A (1.15 × 36.5 = 41.975 A), conductor 8 AWG.
Exam example 5: maximum standard size short-circuit and ground-fault protective device for a compressor RLA 24.9 A with a fan FLA 2.4 A?
Inputs: RLA 24.9 A, fan FLA 2.4 A. Rule: equipment with other loads — 175% of the largest compressor plus the other loads (NEC 2023 440.22(B)(1)). Arithmetic: 1.75 × 24.9 + 2.4 = 43.575 + 2.4 = 45.975 A. Answer: 45 A, the standard size not above 45.975 A. What the app shows: max OCP 45 A, the 225% reading 50 A (2.25 × 24.9 + 2.4 = 58.425 A, rounded down), MCA 33.5 A, disconnect ≥ 31.4 A. The next-higher allowance of 440.22(A) Exc. 1 is written for a compressor alone, so the app rounds this sum down.
Exam example 6: maximum standard size OCPD for a motor-compressor RLA 37.6 A plus a fan FLA 3.1 A?
Inputs: RLA 37.6 A, fan FLA 3.1 A. Rule: NEC 2023 440.22(B)(1), 175% of the largest plus the other loads. Arithmetic: 1.75 × 37.6 + 3.1 = 65.8 + 3.1 = 68.9 A. Answer: 60 A — the next standard size, 70 A, would exceed 68.9 A. What the app shows: max OCP 60 A, the 225% reading 80 A (84.6 + 3.1 = 87.7 A, rounded down), MCA 50.1 A, disconnect ≥ 46.9 A, conductor 6 AWG.
Exam example 7: minimum ampere rating of the disconnect for a motor-compressor alone, RLA 23.8 A?
Inputs: compressor alone, RLA 23.8 A, no BCSC. Rule: at least 115% of RLA or BCSC, whichever is greater (NEC 2023 440.12(A)(1)). Arithmetic: 1.15 × 23.8 = 27.37 A. Answer: 27.37 A minimum. What the app shows: disconnect ≥ 27.4 A — rounded up to 0.1 so it never reads less than required; ampere rating only, the horsepower rating is a separate check. Also MCA 29.8 A (29.75 A), max OCP 45 A (41.65 A → next higher standard size).
Exam example 8: a compressor alone, RLA 16.0 A — maximum OCPD at 175%, and if it can't start?
Inputs: compressor alone, RLA 16.0 A. Rule: 175%, and the next higher standard size when that is not one (NEC 2023 440.22(A) Exc. 1); if the motor can't start, up to 225%, never above (Exc. 2). Arithmetic: 1.75 × 16.0 = 28 A → 30 A; 2.25 × 16.0 = 36 A → 35 A. Answer: 30 A, or up to 35 A if 30 A can't start the compressor. What the app shows: max OCP 30 A, the 225% reading 35 A, MCA 20.0 A, disconnect ≥ 18.4 A.
Which NEC edition do these answers use?
NEC 2023, checked the same in NEC 2026. Exam answers follow the edition your exam uses — check it before relying on a section number. Marked nameplate values govern over any calculation (440.22(C), 110.3(B)).
Why does the app show 35.8 A when the exam answer is 35.75 A?
The result card shows amperes to one decimal: the MCA rounded to the nearest 0.1, the disconnect rounded up so it never reads less than required. The arithmetic on this page keeps the unrounded figure for exams that say "do not round".