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.

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Example Transformer Conductor Sizing Questions on NEC Electrical Exams

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What is the minimum size THWN insulated copper conductors permitted to feed a 3Ø transformer bank consisting of three 30 kVA, 480-120/240 V single-phase transformers if the transformer were protected by the maximum allowable overcurrent device?

A 225 kVA, dry-type, 480-208/120 V transformer primary must be fed with  minimum __________ THW copper conductors where the transformer terminations are rated at 167°F  if the transformer were protected by the maximum allowable overcurrent device.

An office building uses a 75 kVA, 3-phase, 600-208Y/120 V transformer terminating at 167°F (75°C). What are the minimum size RHW insulated copper conductors permitted to feed the primary of the transformer if the maximum allowable rated overcurrent protection is used.

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

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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

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Transformer Conductor Sizing Articles: NEC 450 and 310.16

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

Basic Calculations:

1. Single-phase: I = VA ÷ V
2. Three-phase: I = VA ÷ (√3 × V)
   

Configuration Considerations:

1. Individual transformers vs. transformer banks
2. Are the secondary conductors considered protected by the primary OCPD?
3. Temperature ratings and derating factors

Special Requirements:

1. Overcurrent protection
2. Transformer termination temperature ratings
3. Ambient temperature corrections

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Different Transformer Configurations and Their Calculations

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Single-Phase Transformers

1. Formula: I = VA ÷ V
2. Example: 50 kVA, 480V primary
   
   • I = 50,000 ÷ 480 = 104.17A

Three-Phase Transformers

1. Formula: I = VA ÷ (√3 × VLine)
2. Example: 75 kVA, 480V primary
   
   • I = 75,000 ÷ (1.732 × 480) = 90.32A
     

Multiple Transformer Banks

1. Calculate total kVA first
2. Example: Three 25 kVA transformers
   
   • 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 calculating
   
   • Primary: Higher voltage (600V in this case)
   • Secondary: Lower voltage (208V in this case)
2. Current: Inversely proportional to voltage
   
   • Higher voltage = Lower current
   • Lower voltage = Higher current
3. Conductor Size: Usually larger on secondary side due to higher current
   
   • Primary: Smaller conductor size typically needed
   • Secondary: Larger conductor size typically needed

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Walkthrough for NEC Electrical Exam Transformer Questions

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Three-Phase Bank Question

What is the minimum permissible size of THWN insulated copper conductors permitted to feed a 3Ø transformer bank consisting of three 30 kVA, 480-120/240 V single-phase transformers if the transformer were protected by the maximum allowable overcurrent device?

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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

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Step 2: Find maximum allowable overcurrent protective device.

• Find maximum permitted primary overcurrent protection size
• Because the transformer configuration is not a 3-wire delta-delta, secondary overcurrent protection is required
• Maximum ampacity = 108.38 x 250% = 270.95  amperes NEC Table 450.3(B)
• Next LOWER standard rating for OCPD from Table 240.6(A) = 250 ampere OCPD

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Step 3: Select Conductor Size Based off OCPD Rating

• Use Table 310.16 for THWN copper at 75°C
• Need ampacity > 250A
• 250 kcmil is rated 255A
• This is the minimum size that meets requirements

The minimum conductor size required is 250 kcmil copper.

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Single Three-Phase Transformer Question

A 225 kVA, dry-type, 480-208/120 V transformer primary must be fed with a minimum __________ THW copper conductors where the transformer terminations are rated at 167°F if the transformer were protected by the maximum allowable overcurrent device.

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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

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Step 2: Find maximum size OCPD Maximum ampacity = 270.95 × 250%

• Minimum ampacity = 667.375 amperes
• OCPD maximum standard size = 600 amps

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Step 3: Select Conductor Size

• Use Table 310.16 for THW copper
• Need ampacity > 600A NEC 240.4
• 1500 kcmil is rated 625A although 900 kcmil could be used because 600 is the next higher standard amp rating per NEC 240.4(B)
• (2) sets of parallel 350 kcmil rated at 310 A a piece could also be used
• This is the smallest size that meets requirements

The minimum conductor size required is 900 kcmil copper or parallel 350 kcmil conductors.

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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). What are the minimum size RHW insulated copper conductors permitted to feed the primary of the transformer if the maximum allowable rated overcurrent protection is used? 

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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

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Step 2: Select the maximum allowable OCPD Maximum ampacity = 208.42 × 2.5

• Minimum ampacity = 521.05 amperes
• Next lower standard size OCPD = 500 amperes per NEC Table 240.6(A)

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Step 3: Select Conductor Size 

• Use Table 310.16 for RHW copper at 75°C
• Need ampacity > 500A
• 900 kcmil is rated 520 amps although 700 kcmil could be used per 240.4(B)
• Parallel 250 kcmil conductors rated 255 amps a piece could also be used
• This is the smallest size that meets requirements

The minimum conductor size required is 700 kcmil copper or parallel 250 kcmil conductors.

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