Dakota Neutral Sizing Guide for Red Seal Exam (CEC 2024)
Red Seal Exam Question Guides
The neutral conductor carries the return current in a single-phase or multi-wire circuit and helps balance current in 3-phase systems. It may or may not carry full load current — and that affects how it’s sized.
Steps to Take on a Neutral Conductor Question
1. Is the Neutral a Current-Carrying Conductor?
Ask yourself:
Is the neutral expected to carry current under normal operation?
Yes? --- It must be full-sized (same as the phase conductors)
No (e.g., balanced 3-phase loads)? --- It may be reduced
2. Use the Same Ampacity Table as for the Hot Conductors
If the neutral is current-carrying (which it often is in residential or single-phase loads), size it based on the calculated load using the appropriate ampacity table (e.g., CEC Table 2 for copper in raceways).
Example:
If your calculated neutral load is 100A, and you're using 75°C rated conductors, you might select #3 AWG copper (from Table 2).
3. Consider Unbalanced Loads and Harmonics
Residential: The neutral often carries almost as much current as the phase — so same size as phase conductors.
3-phase with nonlinear loads (like computers or VFDs): The neutral may carry more than expected due to harmonics — often it must be full size or oversized.
4. Reduced Neutral? Only When Allowed
Use Rule 4-018 to learn how to Size the Neutral Conductor
Question Examples
Example 1: 240/120V with Two 500A Loads
Determine the maximum unbalanced load
If loads are different, the maximum unbalanced load is the higher of the two loads
In this case, it is 500A
Rule 4-018 Demand Factors
⚠️ You do not reduce the neutral or apply demand factors if:
There is discharge lighting, and
Anything with harmonics (fed by a 3-phase 4-wire system)
⚠️ You cannot touch 200A of the load
Calculation:
500A – 200A = 300A
Apply 70% de-rating to anything over the 200A
300A * 0.7 = 210A
Add back what we weren’t allowed to touch earlier
210A + 200A = 410A
Size the neutral based on 410A
(Rule 4-006 applies)
Table 2 (75°C) → 600kcmil
Example 2: 240/120V with Two 500A Loads and 100A of Discharge Lighting
Calculation:
Start with 500A
⚠️ Can’t touch 200A and can’t touch discharge lighting at 100A
500A – 200A – 100A = 200A
Apply 70% de-rating
200A * 70% = 140A
Add 200A + 100A (untouchable loads) + 140A = 440A
Table 2 (75°C) → 700kcmil
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