Electrician Questions is a technical series from ElectriciansX for licensed electricians, apprentices, and inspectors working in the field. These articles walk through NEC logic step by step — not homeowner summaries.
The question: If I run more than five current-carrying conductors in a raceway, I know I must derate per NEC Table 310.15(C)(1). If that circuit also feeds a motor on continuous duty — where conductors are sized at 125% of full-load current — does an additional “80% capacity” rule stack on top of the conduit derating? Using #12 THHN, five current-carrying conductors, and a continuous motor load of 14 amps, what is the correct math?
Short Answer
Yes, both requirements apply — but they are not the same 80% applied twice to one number.
- Conduit derating (Table 310.15(C)(1)) reduces the ampacity you are allowed to use from the conductor.
- Motor continuous-load sizing (430.22) increases the ampacity the conductor must carry to 125% of motor FLA.
- The separate 80% OCPD rule (210.20(B) / 215.3) limits continuous load on a breaker — it does not replace or cancel conductor derating.
You compare: adjusted ampacity ≥ required ampacity. You do not multiply 0.80 × 0.80 on the same base ampacity unless you are incorrectly mixing unrelated rules.
The Three “80%” Ideas Electricians Mix Up
| Rule | NEC reference | What it actually does |
|---|---|---|
| Conduit fill derating | 310.15(C)(1) / Table 310.15(C)(1) | 4–6 current-carrying conductors → multiply ampacity by 80% |
| Motor branch conductors | 430.22 | Continuous-duty motor → size conductors at 125% of FLA (inverse of “only use 80%”) |
| Continuous load on OCPD | 210.20(B), 215.3 | Non-motor continuous loads → load ≤ 80% of standard OCPD rating (unless 100%-rated device) |
Motor branch circuits are primarily governed by Article 430, which has its own conductor and overcurrent rules. Do not apply 210.20(B) motor logic as if it were a second conductor derating factor.
NEC Table 310.16 — #12 THHN Copper (Selected Values)
THHN is rated 90°C dry. For ampacity selection, start with Table 310.16. Termination temperature limits in 110.14(C) may require using the 75°C column at equipment — we show both below.
| Conductor | 60°C | 75°C | 90°C |
|---|---|---|---|
| 12 AWG copper | 20 A | 25 A | 30 A |
NEC Table 310.15(C)(1) — Adjustment Factors (4–6 Conductors)
| Current-carrying conductors | Adjustment factor | % of Table 310.16 |
|---|---|---|
| 4–6 | 0.80 | 80% |
| 7–9 | 0.70 | 70% |
| 10–20 | 0.50 | 50% |
| 21–30 | 0.45 | 45% |
Five current-carrying conductors → 80% adjustment factor. Equipment grounding conductors are not current-carrying. Ungrounded conductors count. Neutral conductors count when they carry unbalanced current per 310.15(C)(1) exception notes.
Visual: How Derating and Motor 125% Interact
Worked Example — Step by Step
Given
- Conductor: #12 THHN copper
- Raceway: 5 current-carrying conductors in same conduit
- Load: motor, continuous duty, 14 A FLA
- Assume 90°C ampacity permitted for this calculation path
Step 1 — Required ampacity (motor load rule)
NEC 430.22 requires branch-circuit conductors supplying a single motor to have an ampacity not less than 125% of the motor full-load current for motors rated continuous.
Step 2 — Adjusted ampacity (derating rule)
From Table 310.16 (90°C): #12 = 30 A. From Table 310.15(C)(1) for 5 CCC: factor = 0.80.
Step 3 — Compare
Per 310.15(A) and general sizing practice: adjusted ampacity must be ≥ required ampacity.
Step 4 — 75°C termination check (often decisive in the field)
If motor starter terminals are rated 75°C (common), 110.14(C)(1)(a) may limit you to the 75°C column before derating:
20 A ≥ 17.5 A → still PASS for this example
Comparison Chart — Where #12 Would Fail
| Motor FLA (continuous) | Required (×1.25) | #12 adj. @90°C (24 A) | #12 adj. @75°C (20 A) | Result |
|---|---|---|---|---|
| 14 A | 17.5 A | 24 A | 20 A | OK |
| 16 A | 20.0 A | 24 A | 20 A | OK at limit (75°C) |
| 17 A | 21.25 A | 24 A | 20 A | FAIL @75°C → use #10 |
| 20 A | 25.0 A | 24 A | 20 A | FAIL → use #10 |
Bar Graph — Required vs Available Ampacity (14 A Motor)
(14×1.25)
(30×0.80)
(25×0.80)
Scale: 1 px ≈ 0.12 A. All three bars exceed the 17.5 A requirement for a 14 A continuous motor.
What About the Breaker “80% Rule”?
210.20(B) states that continuous loads on overcurrent devices (other than motor-specific applications covered by Article 430) shall not exceed 80% of the device rating. That governs breaker loading, not conductor ampacity derating.
For motors, overcurrent protection follows 430.52 (inverse-time breaker typically up to 250% of FLA per table, subject to exceptions). A 14 A motor does not get sized by “putting 14 A on a 20 A breaker at 80%.” The motor article is separate.
Common Mistakes in the Field
- Multiplying 0.80 × 0.80 on 30 A because “motor is continuous” — the 125% is applied to load, not as a second derate on ampacity.
- Skipping derating because “it is only a 14 A motor on #12” — five CCC still forces Table 310.15(C)(1).
- Using 30 A and calling it a 20 A circuit because of an old “#12 = 20 A” rule of thumb — 240.4(D) small-conductor rules and motor exceptions still require the ampacity calculation above.
- Ignoring 75°C termination limits at contactors and panels — especially on commercial jobs in Southwest Florida.
Inspector / Foreman Quick Checklist
- Count current-carrying conductors in the raceway segment.
- Look up base ampacity — Table 310.16 at correct temperature column.
- Apply adjustment factors — Table 310.15(C)(1), plus ambient if needed.
- Calculate required ampacity — 125% motor FLA for continuous (430.22).
- Verify Iadjusted ≥ Irequired.
- Size OCPD per Article 430 — not 210.20(B).
Bottom Line for This Example
With #12 THHN, five current-carrying conductors, and a 14 A continuous motor:
- Derated ampacity: 24 A (90°C) or 20 A (75°C)
- Required ampacity: 17.5 A
- #12 is code-compliant for ampacity in this scenario
The derating and the motor continuous factor both apply, but they work on opposite sides of the inequality — not as two stacked discounts on the same number.
References: NFPA 70 (NEC) Articles 110, 210, 310, 430 — Tables 310.16 and 310.15(C)(1). Values shown reflect commonly adopted editions (2020/2023). Always verify the edition adopted by your AHJ. This article is educational and does not replace the official NEC text or local amendments.
Have a tough code question? ElectriciansX publishes Electrician Questions for the trade. Serving licensed contractors across Lee and Collier County, Florida. Send us your next question.