Sizing the Neutral and Earth Bars in a Switchboard
Neutral and earth bar sizing: Determining neutral and earth bar cross-sections, the influence of harmonics and continuity requirements.
ContentsSizing the neutral and earth bars is the most frequently glossed over subject in switchboard design. Phase bars are calculated carefully, while the neutral is chosen on the assumption that half is enough and the PE is picked as a standard profile. In modern installations both assumptions are dangerous: electronic loads push neutral current above expectations, and the protective conductor carries fault current of the same order as the phases.
The system type drives the decision
Sizing starts with the type of earthing system. In a TN-S system neutral and protective conductor run separately throughout the board and two separate bars are required. In TN-C-S a combined PEN is used up to the main distribution point and the separation point must be defined inside the board; beyond it neutral and PE are never joined again. In a TT system the protective conductor goes to the installation own earth electrode, and coordination with residual current devices inside the board becomes mandatory.
The neutral bar: harmonics change the answer
With balanced three phase linear loads the neutral current is negligible. But switched mode power supplies, LED drivers, computer loads and single phase electronics generate third harmonic, and instead of cancelling across the three phases that harmonic adds up in the neutral. In installations with high third harmonic content the neutral current can exceed the phase current, and the neutral bar then has to match the phase bars, or in some cases be larger.
| Load profile | Typical neutral current | Recommended neutral size |
|---|---|---|
| Balanced motor and heating loads | Below 10% of phase current | Half the phase cross-section is enough |
| Mixed loads with moderate electronics | 30 to 60% of phase current | At least half the phase size, safely the full size |
| Predominantly single phase electronics | Close to or above phase current | Equal to or larger than the phase size |
| Data centres, lighting drivers | Above phase current | 1.3 to 1.7 times the phase size |
The earth bar: continuity is everything
- PE cross-section follows the phase size: equal up to 16 mm², at least 16 mm² between 16 and 35 mm², and half the phase size above that
- The PE bar must run uninterrupted along the full length of the board; internal separation must not break continuity
- Every cubicle door, frame member and mounting plate is bonded to the PE bar with its own conductor
- On painted surfaces the coating is removed or serrated washers are used at bonding points
- Continuity is verified with a low resistance ohmmeter and the results recorded in the technical file
Key takeaways: neutral and earth bar sizing
- The system type drives the decision — Sizing starts with the type of earthing system.
- The neutral bar: harmonics change the answer — With balanced three phase linear loads the neutral current is negligible.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Can neutral and PE be joined inside the board?
Only in a TN-C-S system and only at the defined separation point. Joining them again anywhere downstream lets fault current circulate through the neutral and destroys the function of residual current protection.
Should the PE bar be checked for short circuit withstand as well?
Yes. The protective conductor must survive the fault thermally, which is checked against the fault current and the operating time of the protective device. This is handled with the overall rating of the board, covered in our article on short circuit withstand.
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- All technical articles
At Devpan we size neutral and PE bars together with the load profile, harmonic measurements and system type, and we record continuity test results in the technical file. Share your load schedule and we can establish what the neutral cross-section really needs to be.
