By the end of this chapter, you should be able to:
4.Define ampacity and distinguish it from the actual connected load.
5.Identify the primary code articles governing conductor sizing and ampacity correction.
6.Apply the standard ampacity tables (Table 310.16) and understand their limitations.
7.Apply ambient temperature correction factors and conductor bundling adjustment factors.
8.Understand the rules for conductor sizing based on terminal temperature ratings (60°C, 75°C, 90°C).
9.Apply the rules for overcurrent protection and the "next size up" rule.
10.Calculate voltage drop for branch circuits and feeders and know when it is a code requirement versus a recommendation.
11.Recognize common exam traps related to derating, terminal ratings, and continuous loads.
1.1 Core Concepts and Definitions
Ampacity is the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. This is the single most important definition in this chapter. The conditions of use include ambient temperature, the number of current-carrying conductors in the cable or raceway, and the type of insulation.
Continuous Load is a load where the maximum current is expected to continue for 3 hours or more. This is a critical distinction because it triggers a mandatory 125% factor for branch circuits, feeders, and overcurrent protection.
Terminal Rating refers to the temperature rating of the equipment terminations (lugs, breakers, etc.) to which the conductor is connected. This is often the limiting factor in conductor sizing, not the conductor's insulation rating.
Current-Carrying Conductor is a conductor that carries current in normal operation. The neutral conductor is counted as a current-carrying conductor under specific conditions (e.g., a 4-wire 3-phase wye system with nonlinear loads).
1.2 The Standard Ampacity Table: Table 310.16
Table 310.16 is the workhorse for conductor sizing. It provides ampacities for conductors rated up to 2000 V, based on an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable.
Key Columns: The table is organized by insulation type and temperature rating:
60°C (140°F): Typically for TW, UF insulation. Used for older equipment and some small terminations.
75°C (167°F): Typically for RHW, THHW, THW, THWN, XHHW, USE. This is the most common rating for modern equipment terminations.
90°C (194°F): Typically for THHN, THWN-2, XHHW-2, RHH, RHW-2. This allows for higher ampacity only if the terminations are rated for 90°C, which is rare. The 90°C column is primarily used for derating purposes.
Copper vs. Aluminum: The table provides separate columns for copper and aluminum/copper-clad aluminum. Aluminum conductors are generally one to two AWG sizes larger than copper for the same ampacity.
The 30°C Ambient Basis: The table's base ampacity is only valid for a 30°C ambient temperature. If the ambient temperature is higher, you must apply a correction factor. If it is lower, you can apply an upward correction factor (though this is rarely done in practice).
1.3 Temperature Correction Factors
When the ambient temperature where the conductor is installed exceeds 30°C, you must multiply the ampacity from Table 310.16 by the appropriate correction factor from Table 310.15(B)(1) .
The table is organized by the conductor's insulation temperature rating (60°C, 75°C, 90°C).
The rows list ambient temperatures in 5°C increments (e.g., 31–35°C, 36–40°C, etc.).
For example, for a 90°C conductor in a 40°C ambient, the correction factor is 0.91. For a 75°C conductor in a 45°C ambient, the factor is 0.82.
Practical Field Point: This is not just a theoretical exercise. A conduit running across a hot rooftop in Texas in July can easily see ambient temperatures well above 40°C. A journeyman must account for this or the conductor will overheat and fail prematurely.
Exam Trap: The correction factor is applied to the base ampacity of the conductor, not to the load. You must first correct the conductor's ampacity and then compare it to the load.
1.4 Adjustment Factors for More Than Three Conductors
When more than three current-carrying conductors are bundled together in a raceway or cable, their heat dissipation is reduced. You must apply an adjustment factor from Table 310.15(B)(3)(a) .
4–6 conductors: 80% factor
7–9 conductors: 70% factor
10–20 conductors: 50% factor
21–30 conductors: 45% factor
31–40 conductors: 40% factor
Critical Rule: The neutral conductor is counted as a current-carrying conductor in the following situations:
46.On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads (e.g., electronic ballasts, computers, LED drivers). The neutral carries triplen harmonics.
47.On a 3-wire, 2-phase or 3-wire, 3-phase system with a grounded neutral.
Exam Trap: The neutral is not counted as a current-carrying conductor on a standard 3-wire, single-phase 120/240V circuit (line, neutral, line) where the loads are balanced, because the neutral only carries the unbalanced current.
Order of Operations: When both temperature correction and adjustment factors apply, you must apply them cumulatively. First, multiply the base ampacity by the temperature correction factor, then multiply the result by the bundling adjustment factor. You cannot simply add the percentages.
1.5 Terminal Temperature Ratings: The 60°C/75°C Rule
This is perhaps the most misunderstood concept in conductor sizing. The ampacity of the conductor is limited by the temperature rating of the equipment terminations.
The Rule (NEC 110.14(C)):
Equipment rated 100 A or less: Terminations are typically rated at 60°C. You must use the 60°C column of Table 310.16, unless the equipment is clearly marked otherwise (e.g., "75°C" on the breaker or lug).
Equipment rated over 100 A: Terminations are typically rated at 75°C. You must use the 75°C column.
The 90°C Exception: You can use the 90°C column for derating purposes only. This means you can start with a 90°C conductor's ampacity, apply the correction and adjustment factors, and then ensure the resulting ampacity is still greater than or equal to the load. However, the final conductor size must be at least as large as what the 60°C or 75°C column requires for the load.
Practical Example:
A 100 A continuous load on a breaker rated for 75°C.
The load calculation requires 100 A × 1.25 = 125 A.
You cannot use a #2 AWG THHN (90°C column = 130 A) because the 75°C column for #2 AWG is only 115 A.
You must use a #1 AWG THHN (75°C column = 130 A).
Exam Trap: Always check the terminal rating first. If the question does not specify the terminal rating, assume 60°C for equipment ≤ 100 A and 75°C for equipment > 100 A.
1.6 Overcurrent Protection and the "Next Size Up" Rule
Conductors must be protected against overcurrent in accordance with their ampacity. The general rule is that the overcurrent device rating must not exceed the conductor's ampacity.
The Exception (NEC 240.4(B)): If the conductor's ampacity does not correspond to a standard overcurrent device rating, you can use the next higher standard rating, but only if:
68.The conductor's ampacity is not less than the load.
69.The next higher standard rating does not exceed 800 A.
You can protect this conductor with a 125 A breaker, provided the load is ≤ 115 A.
Critical Limitation: This "next size up" rule does not apply to:
Motor circuits (Article 430) – they have their own rules.
Tap conductors (Article 240.21) – they have specific length and size requirements.
Luminaire and appliance cords (Article 240.5).
Exam Trap: The "next size up" rule is often tested in conjunction with derating. You must first derate the conductor, then check if the resulting ampacity allows for the next size up.
1.7 Continuous Loads and the 125% Factor
Branch circuits, feeders, and overcurrent devices must be sized to carry not less than 125% of the continuous load, plus 100% of the noncontinuous load.
This calculated value is what you compare against the conductor's ampacity (after derating) and the overcurrent device rating.
Practical Field Point: This is why a 20 A circuit cannot supply a 20 A continuous load. The load must be limited to 16 A (20 A ÷ 1.25). A journeyman must always check the nameplate of equipment (like a water heater or air conditioner) to see if it is rated as a continuous load.
1.8 Voltage Drop
Voltage drop is the reduction in voltage in an electrical circuit between the source and the load. It is caused by the resistance and reactance of the conductors.
Code Requirement vs. Recommendation:
Recommendation (Informational Note): NEC 210.19(A), Informational Note No. 4 recommends that voltage drop on branch circuits not exceed 3% and the combined drop on feeder and branch circuit not exceed 5%. This is a recommendation, not a mandatory requirement.
Mandatory Requirement: Voltage drop is a mandatory requirement for specific installations, such as:
Fire pumps (Article 695): The voltage drop must not exceed 15% during motor starting.
Sensitive electronic equipment (Article 647): Specific voltage drop requirements apply.
Calculation Formula (Single-Phase):
VD = (2 × K × I × D) / CM
VD = Voltage drop (volts)
K = Resistivity of the conductor (approximately 12.9 for copper, 21.2 for aluminum)
I = Current (amperes)
D = One-way distance (feet)
CM = Circular mil area of the conductor (from Chapter 9, Table 8)
Practical Field Point: For long runs (over 100 feet), a journeyman should always check voltage drop. The code may not mandate it, but the customer's equipment will suffer from poor performance, and the inspector may flag it as a quality issue.
Exam Trap: The formula uses the one-way distance (D), but the factor "2" accounts for the round trip. Do not double the distance again.
1.9 Conductor Sizing for Specific Equipment
While the general rules apply, specific articles have their own sizing requirements:
Air-Conditioning and Refrigeration Equipment (Article 440): The branch-circuit conductors must be sized at 125% of the rated load current (RLA) or the nameplate minimum circuit ampacity (MCA). The overcurrent device must not exceed the nameplate maximum fuse or circuit breaker size.
Motors (Article 430): Branch-circuit conductors must be sized at 125% of the motor's full-load current (FLC) as found in Tables 430.247 through 430.250, not the nameplate current.
Welders (Article 630): Conductors are sized based on the duty cycle of the welder.
Capacitors (Article 460): Conductors must be sized at least 135% of the capacitor's rated current.
1.10 Code Navigation: Where to Find It
Concept
NEC Article / Table
Ampacity Definition
Article 100
Standard Ampacity Table
Table 310.16
Ambient Temp Correction
Table 310.15(B)(1)
Bundling Adjustment
Table 310.15(B)(3)(a)
Neutral as Current-Carrying
310.15(E)
Terminal Temperature Limits
110.14(C)
Overcurrent Protection
Article 240
Standard OCPD Ratings
240.6
Next Size Up Rule
240.4(B)
Continuous Load Definition
Article 100
Branch Circuit Sizing
210.19(A)
Feeder Sizing
215.2(A)
Voltage Drop Recommendation
210.19(A) Informational Note No. 4
Conductor Properties (CM, K)
Chapter 9, Table 8
Motor Conductors
430.22
A/C and Refrigeration
440.32
1.11 Common Exam Traps and Field Pitfalls
117.The "90°C Trap": Using the 90°C column without checking the terminal rating. The 90°C rating is for derating, not for final sizing against standard terminations.
118.The "Neutral Trap": Forgetting to count the neutral as a current-carrying conductor when you have nonlinear loads on a 3-phase wye system.
119.The "Order of Operations" Trap: Applying derating factors after comparing to the load, instead of applying them to the conductor's base ampacity first.
120.The "Continuous Load" Trap: Forgetting to multiply the continuous load by 1.25 before comparing it to the conductor ampacity.
121.The "Next Size Up" Trap: Using the next size up rule for motor circuits or when the conductor's ampacity is already less than the load.
122.The "Distance" Trap: Using the total round-trip distance in the voltage drop formula instead of the one-way distance.
123.The "Ambient" Trap: Ignoring the ambient temperature on a rooftop or in an attic. Always check the location of the conductor.
124.The "Aluminum" Trap: Assuming aluminum conductors have the same ampacity as copper. Always use the correct column.
1.12 Summary of the Sizing Process
A systematic approach to conductor sizing for the exam and the field:
128.Determine the Load: Calculate the continuous and noncontinuous portions.
130.Determine Terminal Rating: Check the equipment (60°C for ≤100 A, 75°C for >100 A, unless marked otherwise).
131.Select Initial Conductor Size: Use the appropriate temperature column from Table 310.16 to find a conductor with an ampacity ≥ MCA.
132.Apply Correction and Adjustment Factors: Start with the 90°C column (if the conductor is rated for it), apply the ambient temperature correction factor and the bundling adjustment factor.
133.Verify Final Ampacity: The corrected ampacity must still be ≥ MCA.
134.Check Overcurrent Protection: Select a standard OCPD that is ≤ the final corrected ampacity, or use the "next size up" rule if applicable.
135.Check Voltage Drop: For long runs, verify that the voltage drop is within acceptable limits (3% branch, 5% total). If not, increase the conductor size.
This methodical approach will help you avoid the most common errors and ensure a safe, code-compliant installation.
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