Understanding when and how to size primary protection without requiring separate secondary protection is crucial for electrical licensing exams. This specific scenario is governed by NEC Article 240.21(C)(1) and requires careful attention to voltage ratios and current calculations.
Example Primary-Only Protection Question on NEC Electrical Exams
The maximum primary overcurrent rating allowed for a 300 kVA, dry-type, 480 V - 240 V V, 3-phase, 3-wire delta - delta transformer without requiring separate secondary overcurrent protection is __________ A.
A 225 kVA, dry-type, 600 V - 480 V V, 3-phase, 3-wire delta-delta transformer can have a maximum primary overcurrent rating of _____ A where secondary protection is not required
For a 75 kVA, dry-type, 480 V-240 V, 3-phase, 3-wire transformer, the maximum primary overcurrent rating allowed without requiring separate secondary overcurrent protection is _____ A.
How to Identify Primary-Only Protection Questions on NEC Electrical Exams
Key phrases to look out for:
- Per Sections 240.4(F) and 240.21(C)(1), only the secondaries of 2-wire single-phase and 3-wire delta-delta transformers may be considered protected by their respective primary OCPD
- In general, if a transformer has a neutral, it must have secondary protection and the bottom row of Table 450.3(B) must be used
- "Without secondary protection"
- "Without requiring separate secondary overcurrent protection"
- "Primary-only protection"
- "Maximum primary overcurrent rating allowed"
When you spot these elements:
- Check if question specifically mentions absence of secondary protection
- Note both primary and secondary voltages
- Calculate voltage ratio
- Refer to NEC Article 240.21(C)(1)
Primary-Only Protection Articles: NEC 240.21(C)(1)
To correctly apply NEC Article 240.21(C)(1), focus on these main elements:
- Requirements for Omitting Secondary Protection:
- 2-wire single phase
- 3-wire delta-delta
- Primary protection must be properly sized
- Voltage ratio must be considered
- Secondary conductors must be properly sized
- Key Calculations:
- Secondary full-load current
- Secondary-to-primary voltage ratio
- Equivalent primary current
- Important Rules:
- Must round down to next standard size
- Protection must not exceed secondary conductor ampacity
- Both primary and secondary conductors must be properly sized
Steps for Primary-Only Protection Calculations
- Calculate Primary Full-Load Current:
- Three-phase: I = VA ÷ (V × √3)
- Single-phase: I = VA ÷ V
- Calculate Maximum Primary Overcurrent Protective Device Rating:
- Find primary full-load current using the appropriate formula shown above
- Multiply the FLC by the appropriate Table 450.3(B) maximum percentage for primary-only protection
- This gives maximum primary protection allowed
- 3. Select Standard Size:
- Round DOWN to next standard size
- Use Table 240.6(A) for standard sizes
- 4. Calculated Minimum Permitted Secondary Conductor Ampacity:
- Divide Primary OCPD rating by the secondary-to-primary voltage ratio
- This gives the minimum allowable secondary conductor ampacity
Walkthrough for Primary-Only Protection Question
Question #1
The maximum primary overcurrent rating allowed for a 300 kVA, dry-type, 480 V-240 V, 3-phase, 3-wire transformer where secondary overcurrent protection is not required is __________ A.
Step 1: Calculate Secondary Full-Load Current
- kVA = 300
- Primary voltage = 480V
- Using three-phase formula:
- I = 300,000 VA ÷ (480V × 1.732)
- I = 300,000 ÷ 831.36
- I = 360.9 amperes
Step 2: Calculate Maximum Permitted Primary Overcurrent Protective Device Rating Using Table 450.3(B)
- Primary FLC x 125% (primary-only over 9 amps)
- 360.9 amps x 125% = 451 amps
- Using Table 240.6(A) note 1 for 125%, maximum primary OCPD rating is 500 amps (technically the next higher rating can be used because the FLC is higher than 450)
The maximum primary overcurrent protection allowed is 500A.
Question #2
What is the minimum secondary conductor ampacity permitted for the above question using the maximum allowable OCPD?
Step 1: Solve for Minimum Secondary Conductor Ampacity per Section 240.21(B)
- Section 240.21(C)(1) states… OCPD Primary ≤ Secondary Voltage / Primary Voltage x Conductor Ampacity
- So… Conductor Ampacity ≥ OCPD Primary / (Secondary Voltage / Primary Voltage)
- CA = 500 amps / (240v / 480v)
- CA = 1000 amps
Common Mistakes to Avoid
- Using the bottom row (Primary and secondary protection) on Table 450.3(B) instead of the top (Primary only protection)
- Using the wrong ‘Primary Protection’ column
- 125% for 9 amps or more (Note 1 can be used)
- 167% for less than 9 amps (Note 1 CANNOT be used)
- 300% for less than 2 amps (Note 1 CANNOT be used)
