6.2 Small Conductor Rules & Protection Limits (NEC 240.4(D))

6.2 Small Conductor Rules & Protection Limits (NEC 240.4(D))

Exam Focus: Small conductor limits (240.4(D)), interaction with Table 310.16, derating calculations, and Table 240.4(G) exceptions (HVAC Article 440, motors Article 430, welders Article 630).

One of the most frequently tested concepts on the Michigan Journeyman Electrician exam is the application of NEC 240.4(D) Small Conductor Rules. While Table 310.16 lists raw allowable ampacities for conductors under various temperature columns (60°C, 75°C, and 90°C), NEC 240.4(D) imposes absolute maximum limits on overcurrent protective devices for small wire sizes under general wiring applications.

---

NEC 240.4(D) Small Conductor Overcurrent Protection Limits

Unless specifically permitted in NEC 240.4(E) or 240.4(G), overcurrent protection for small copper and aluminum conductors shall not exceed the values specified below after any correction or adjustment factors have been applied:

                  SMALL CONDUCTOR OVERCURRENT PROTECTION RULES
+-------------------------------------------------------------------------+
| 14 AWG Copper  --> Max 15A Breaker / Fuse (NEC 240.4(D)(3))             |
| 12 AWG Copper  --> Max 20A Breaker / Fuse (NEC 240.4(D)(5))             |
| 10 AWG Copper  --> Max 30A Breaker / Fuse (NEC 240.4(D)(7))             |
| 12 AWG Aluminum--> Max 15A Breaker / Fuse (NEC 240.4(D)(4))             |
| 10 AWG Aluminum--> Max 25A Breaker / Fuse (NEC 240.4(D)(6))             |
+-------------------------------------------------------------------------+

---

Table 310.16 vs. NEC 240.4(D): Understanding the Dual Role

Exam candidates are frequently confused when looking up ampacities in NEC Table 310.16:

• 14 AWG THHN Copper: Table 310.16 shows 25A in the 90°C column, 20A in the 75°C column, and 15A in the 60°C column.
• 12 AWG THHN Copper: Table 310.16 shows 30A in the 90°C column, 25A in the 75°C column, and 20A in the 60°C column.
• 10 AWG THHN Copper: Table 310.16 shows 40A in the 90°C column, 35A in the 75°C column, and 30A in the 60°C column.

How to Use 90°C Ampacity for Derating:

You are permitted to use the higher 90°C ampacity (e.g., 25A for 14 AWG THHN, 30A for 12 AWG THHN, 40A for 10 AWG THHN) as the starting point when calculating ambient temperature correction factors (Table 310.15(B)(1)) or adjustment factors for more than 3 current-carrying conductors in a raceway (Table 310.15(C)(1)).

However, after completing all derating math, the final circuit breaker or fuse selection for general wiring CANNOT EXCEED the small conductor limit established in 240.4(D)!

---

Worked Derating Calculation Example

Question: Four 12 AWG THHN copper current-carrying branch circuit conductors are installed in a single EMT conduit passing through an industrial room with an ambient temperature of 104°F (40°C). What is the maximum rating of the overcurrent protective device allowed to protect these general lighting branch circuit conductors?

• Step 1: Find starting ampacity from Table 310.16:
  • 12 AWG THHN has 90°C rating = 30 Amperes.
• Step 2: Determine Ambient Temperature Correction Factor (Table 310.15(B)(1)):
  • 104°F (40°C) factor for 90°C insulation = 0.91.
• Step 3: Determine Adjustment Factor for Number of Conductors (Table 310.15(C)(1)):
  • 4 current-carrying conductors = 80% (0.80).
• Step 4: Calculate Adjusted Ampacity: $\text{Adjusted Ampacity} = 30\text{ A} \times 0.91 \times 0.80 = 21.84\text{ Amperes}$
• Step 5: Compare with NEC 240.4(D) Limit:
  • Under 240.4(D)(5), 12 AWG copper general branch circuit protection cannot exceed 20 Amperes.
• Conclusion: Even though the calculated derated ampacity is 21.84A, the maximum permitted circuit breaker rating is 20 Amperes.

---

Table 240.4(G) Specific Conductor Applications (Exceptions)

NEC 240.4(G) lists specialized electrical equipment circuits that are EXEMPT from standard small conductor limits in 240.4(D). In these specific applications, conductors may be protected by overcurrent devices rated significantly higher than 15A, 20A, or 30A.

                      KEY TABLE 240.4(G) EXCEPTIONS MATRIX
+-------------------------------------------------------------------------+
| Article 440: Air Conditioning & Refrigeration Equipment                 |
|   - Conductors protected by Maximum Overcurrent Protection (MOP) rating |
| Article 430: Motors & Motor Circuits                                    |
|   - Conductors sized @ 125% FLC; Breaker sized up to 250% per 430.52    |
| Article 450: Transformer Secondary Conductors                           |
|   - Secondary protection governed by 240.21(C) and Article 450          |
| Article 630: Electric Welders                                           |
|   - Ampacity reduced by duty cycle; Breaker sized per 630.12/630.22     |
| Article 725/760: Remote-Control, Signaling & Fire Alarm Circuits        |
|   - Overcurrent protection governed by specific power limits            |
+-------------------------------------------------------------------------+

1. Air Conditioning & Refrigeration (Article 440)

An A/C condensing unit has a nameplate stating:

• Minimum Circuit Ampacity (MCA): 18.5 Amperes
• Maximum Overcurrent Protective Device (MOP): 30 Amperes

Calculation:

• Per NEC 440.32, branch circuit conductors are sized based on MCA (18.5A). A 12 AWG Cu conductor (rated 20A at 60°C/75°C) is installed.
• Per NEC 440.22 and Table 240.4(G), the overcurrent device is sized based on the nameplate MOP (30 Amperes).
• Result: 12 AWG Cu wire protected by a 30A circuit breaker is a 100% compliant installation under Article 440!

2. Motor Branch Circuits (Article 430)

A 5 HP 230V single-phase motor has a Full-Load Current (FLC) of 28A (Table 430.248).

• Conductor Sizing (430.22): $28\text{ A} \times 1.25 = 35\text{ A}$ $\rightarrow$ 8 AWG Cu required.
• Short-Circuit / Ground-Fault OCPD (Table 430.52): Inverse time breaker allowed up to 250% of FLC: $28\text{ A} \times 2.50 = 70\text{ Amperes}$
• Result: 8 AWG Cu wire protected by a 70A circuit breaker is compliant under Article 430 because motor overload protection is handled separately by thermal overload relays.

---

Common Exam Traps on Small Conductor Sizing

1. General Receptacle Circuit Trap: If a question asks for the maximum circuit breaker size for a 12 AWG THHN copper wire supplying general wall convenience outlets, the answer is ALWAYS 20A (due to 240.4(D)). Do NOT select 30A just because Table 310.16 shows 30A at 90°C!
2. A/C Unit Nameplate Trap: If a question explicitly states an A/C compressor unit nameplate MCA is 17A and MOP is 30A, wire size is 12 AWG Cu. The max breaker is 30A (exempt under 240.4(G) / Article 440).
3. Aluminum 10 AWG Trap: 10 AWG Aluminum wire for general circuits is limited to 25 Amperes (NEC 240.4(D)(6)), NOT 30A!