Feeder Cable Sizing for Transformers Calculator – NEC

Accurate feeder cable sizing for transformers ensures safety, efficiency, and compliance with NEC standards. Proper calculations prevent overheating and voltage drop issues in electrical systems.

This article covers NEC guidelines, formulas, tables, and real-world examples for feeder cable sizing. Learn to select the right cable size for transformer feeders confidently and precisely.

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• Calculate feeder cable size for a 75 kVA, 480 V transformer with 75°C conductor insulation.
• Determine cable size for a 150 kVA transformer at 208 V, 60°C conductor rating.
• Find feeder conductor size for a 500 kVA transformer, 480 V, with 90°C rated conductors.
• Compute feeder cable size for a 300 kVA transformer, 240 V, 75°C conductor insulation.

Comprehensive Tables for Feeder Cable Sizing According to NEC

Table 1: Transformer Full Load Current (FLC) per NEC Table 450.3(B)

Table 2: NEC 310.15(B)(16) – Ampacity of Copper Conductors at 75°C Insulation

Table 3: Maximum Voltage Drop Recommendations for Feeder Conductors

Essential Formulas for Feeder Cable Sizing According to NEC

1. Transformer Full Load Current (FLC)

The full load current of a transformer is the current drawn at rated load and voltage.

• Transformer kVA: Rated power of the transformer in kilovolt-amperes.
• Voltage (V): Line-to-line voltage on the primary or secondary side.
• √3: Square root of 3 (≈1.732), used for three-phase systems.

2. Minimum Feeder Conductor Ampacity

Per NEC 450.3(B), feeder conductors must have an ampacity not less than 125% of the transformer full load current.

• 1.25: 125% multiplier to account for continuous loading and safety margin.
• FLC: Full load current of the transformer.

3. Voltage Drop Calculation

Voltage drop must be checked to ensure it does not exceed recommended limits, preserving system efficiency.

• K: Resistivity constant of conductor material (Copper ≈ 12.9, Aluminum ≈ 21.2 ohm-cmil/ft at 75°C).
• I: Load current in amperes.
• L: One-way length of the feeder in feet.
• CM: Circular mil area of the conductor.

For three-phase systems, the formula is adjusted:

4. Circular Mil Area (CM) from AWG Size

Each conductor size corresponds to a circular mil area, which is used in voltage drop calculations.

Detailed Real-World Examples of Feeder Cable Sizing for Transformers

Example 1: Sizing Feeder Cable for a 75 kVA, 480 V Transformer

A 75 kVA, 480 V, three-phase transformer requires feeder conductors with 75°C rated copper insulation. The feeder length is 150 feet. Determine the minimum conductor size considering NEC requirements and voltage drop limits.

Step 1: Calculate Full Load Current (FLC)

Step 2: Calculate Minimum Ampacity

From NEC Table 310.15(B)(16), the next standard conductor ampacity above 112.75 A at 75°C is 130 A (1 AWG copper).

Step 3: Check Voltage Drop

• Load current (I) = 90.2 A
• Length (L) = 150 ft
• Conductor size = 1 AWG copper, CM = 105,600
• Resistivity constant (K) for copper = 12.9 ohm-cmil/ft

Percentage voltage drop:

This is well below the recommended 3% maximum voltage drop, so 1 AWG copper conductors are acceptable.

Example 2: Sizing Feeder Cable for a 150 kVA, 208 V Transformer with Aluminum Conductors

A 150 kVA, 208 V, three-phase transformer requires aluminum feeder conductors rated at 75°C. The feeder length is 200 feet. Determine the minimum conductor size per NEC and verify voltage drop.

Step 1: Calculate Full Load Current (FLC)

Step 2: Calculate Minimum Ampacity

From NEC Table 310.15(B)(16), the next standard aluminum conductor ampacity above 520.25 A at 75°C is 600 A (500 kcmil aluminum conductors are rated 380 A, so we must go higher to 600 kcmil or parallel conductors).

Since 500 kcmil aluminum is insufficient, select 600 kcmil aluminum conductors (ampacity ~ 420 A) or use parallel conductors. NEC allows parallel conductors for sizes 1/0 AWG and larger.

Step 3: Voltage Drop Calculation for 600 kcmil Aluminum

• Load current (I) = 416.2 A
• Length (L) = 200 ft
• Conductor size = 600 kcmil aluminum, CM = 600,000
• Resistivity constant (K) for aluminum = 21.2 ohm-cmil/ft

Percentage voltage drop:

This is within the 3% recommended limit, so 600 kcmil aluminum conductors are acceptable.

Additional Technical Considerations for Feeder Cable Sizing

• Temperature Ratings: NEC requires using ampacity values based on the conductor insulation temperature rating and the terminal temperature rating, whichever is lower.
• Adjustment Factors: When multiple conductors are bundled or ambient temperature exceeds 30°C, ampacity must be adjusted per NEC 310.15(B)(2)(a) and 310.15(B)(3)(a).
• Conductor Material: Copper has higher conductivity and lower resistivity than aluminum, affecting ampacity and voltage drop.
• Voltage Drop Limits: While NEC does not mandate voltage drop limits, industry best practices recommend keeping voltage drop below 3% for feeders.
• Parallel Conductors: NEC 310.10(H) allows parallel conductors for sizes 1/0 AWG and larger to meet ampacity requirements.
• Grounding Conductors: Grounding conductor sizing follows NEC Table 250.122 and is separate from feeder conductor sizing.

References and Authoritative Resources

• National Electrical Code (NEC) – NFPA
• Schneider Electric Technical Publications
• Eaton Transformer Sizing Guidelines
• Copper Development Association – Ampacity Tables