Guide to Transformer Conductor Sizing Questions on NEC Electrical Exams

Transformer conductor sizing questions are common on electrical licensing exams because they test multiple concepts: load calculations, continuous load factors, and conductor ampacity selection. Understanding how to handle different transformer configurations (single-phase, three-phase, and multiple transformers) is crucial for passing your exam.

Example Transformer Conductor Sizing Questions on NEC Electrical Exams

1. What's the smallest permissible size of THWN copper conductor to feed a 3Ø transformer bank (three 30 kVA, 480-120/240 V single-phase transformers) from a circuit breaker?
2. A 225 kVA, dry-type, 480-208/120 V transformer's primary side must be fed with a minimum __________ THW copper conductor. Transformer terminations are rated at 167°F.
3. An office building uses a 75 kVA, 3-phase, 600-208Y/120 V transformer terminating at 167°F (75°C). The secondary side wiring requires a minimum __________ RHW copper conductor.

How to Identify a Transformer Conductor Sizing Question on NEC Electrical Exams

Key phrases to look out for:

• "Transformer primary/secondary conductors"
• "Minimum conductor size"
• "Full load current"
• "kVA rating"
• "Transformer bank"
• References to Article 450
• Temperature ratings

When you spot these elements:

1. Identify transformer configuration (single-phase, three-phase, or bank)
2. Note the kVA rating(s)
3. Check voltage ratings
4. Refer to NEC Articles 450 and 310.16

Transformer Conductor Sizing Articles: NEC 450 and 310.16

To correctly size transformer conductors, focus on these main elements:

1. Basic Calculations:some text
   • Single-phase: I = VA ÷ V
   • Three-phase: I = VA ÷ (√3 × V)
   • Apply 125% for continuous loads
2. Configuration Considerations:some text
   • Individual transformers vs. transformer banks
   • Primary vs. secondary conductors
   • Temperature ratings and derating factors
3. Special Requirements:some text
   • Overcurrent protection
   • Transformer termination temperature ratings
   • Ambient temperature corrections

Different Transformer Configurations and Their Calculations

Single-Phase Transformers

1. Formula: I = VA ÷ V
2. Example: 50 kVA, 480V primarysome text
   • I = 50,000 ÷ 480 = 104.17A
   • With 125%: 104.17 × 1.25 = 130.21A

Three-Phase Transformers

1. Formula: I = VA ÷ (√3 × V)
2. Example: 75 kVA, 480V primarysome text
   • I = 75,000 ÷ (1.732 × 480) = 90.32A
   • With 125%: 90.32 × 1.25 = 112.9A

Multiple Transformer Banks

1. Calculate total kVA first
2. Example: Three 25 kVA transformerssome text
   • Total kVA = 3 × 25 = 75 kVA
   • Then use appropriate formula based on configuration

Key Differences Between Primary and Secondary Calculations:

1. Voltage: Use the specific voltage for the side you're calculatingsome text
   • Primary: Higher voltage (600V in this case)
   • Secondary: Lower voltage (208V in this case)
2. Current: Inversely proportional to voltagesome text
   • Higher voltage = Lower current
   • Lower voltage = Higher current
3. Conductor Size: Usually larger on secondary side due to higher currentsome text
   • Primary: Smaller conductor size typically needed
   • Secondary: Larger conductor size typically needed

Walkthrough for NEC Electrical Exam Transformer Questions

Three-Phase Bank Question

Question: What's the smallest permissible size of THWN copper conductor to feed a 3Ø transformer bank (three 30 kVA, 480-120/240 V single-phase transformers) from a circuit breaker?

Step 1: Calculate Total kVA and Full Load Current

• Total kVA = 3 × 30 kVA = 90 kVA
• Primary voltage = 480V three-phase
• I = 90,000 ÷ (√3 × 480)
• I = 90,000 ÷ 831.4
• I = 108.38 amperes

Step 2: Apply 125% for Continuous Load (210.19(A))

• Minimum ampacity = 108.38 × 1.25
• Minimum ampacity = 135.47 amperes

Step 3: Select Conductor Size

• Use Table 310.16 for THWN copper at 75°C
• Need ampacity > 135.47A
• 1/0 AWG is rated 150A
• This is the smallest size that meets requirements

The minimum conductor size required is 1/0 AWG copper.

Single Three-Phase Transformer Question

Question: A 225 kVA, dry-type, 480-208/120 V transformer's primary side must be fed with a minimum __________ THW copper conductor. Transformer terminations are rated at 167°F.

Step 1: Calculate Full Load Current

• kVA = 225
• Primary voltage = 480V three-phase
• I = 225,000 ÷ (√3 × 480)
• I = 225,000 ÷ 831.4
• I = 270.95 amperes

Step 2: Apply 125% for Continuous Load (210.19(A))

• Minimum ampacity = 270.95 × 1.25
• Minimum ampacity = 338.69 amperes

Step 3: Select Conductor Size

• Use Table 310.16 for THW copper
• Need ampacity > 338.69A
• 500 kcmil is rated 380A
• This is the smallest size that meets requirements

The minimum conductor size required is 500 kcmil copper.

Secondary Conductor Sizing Question

Question: An office building uses a 75 kVA, 3-phase, 600-208Y/120 V transformer terminating at 167°F (75°C). The secondary side wiring requires a minimum __________ RHW copper conductor.

Step 1: Calculate Full Load Current for Secondary Side

• kVA = 75
• Secondary voltage = 208V three-phase
• I = 75,000 ÷ (√3 × 208)
• I = 75,000 ÷ 360.18
• I = 208.42 amperes

Step 2: Apply 125% for Continuous Load (210.19(A))

• Minimum ampacity = 208.42 × 1.25
• Minimum ampacity = 260.53 amperes

Step 3: Select Conductor Size

• Use Table 310.16 for RHW copper at 75°C
• Need ampacity > 260.53A
• 300 kcmil is rated 285A
• This is the smallest size that meets requirements

The minimum conductor size required is 300 kcmil copper.

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