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.

‍

Video Guide

‍

Example Transformer Conductor Sizing Questions on NEC Electrical Exams

‍

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.

‍

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:

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

‍

Different Transformer Configurations and Their Calculations

‍

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

‍

Walkthrough for NEC Electrical Exam Transformer Questions

‍

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?

‍

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: 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

‍

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.

‍

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.

‍

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

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

‍

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.

‍

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? 

‍

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: 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)

‍

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.

‍

‍

‍