Calculator Inputs
Example Data Table
Example values illustrate a typical building down-conductor sizing scenario.
| Material | LPL | Peak Current (kA) | Pulse (s) | Down Conductors | Safety | Required (mm²) | Suggested Standard (mm²) |
|---|---|---|---|---|---|---|---|
| Copper | III | 100 | 0.05 | 2 | 1.25 | ≈ 61.0 | 70 |
| Aluminum | II | 150 | 0.05 | 4 | 1.30 | ≈ 71.8 | 95 |
| Galvanized Steel | I | 200 | 0.10 | 4 | 1.40 | ≈ 151.5 | 150 |
Example results are approximate and depend on your chosen options.
Formula Used
This calculator uses a simplified adiabatic conductor sizing approach for short-duration current pulses:
S = ( Ishare × SF × √t ) ÷ k
- S = required conductor cross-sectional area (mm²).
- Ishare = peak current shared per down conductor (A).
- SF = safety factor (≥ 1.0).
- t = pulse duration (s).
- k = material constant (simplified, depends on thermal limits).
Equivalent round diameter is estimated by d = √( 4S / π ). Standard sizes are chosen as the first available size ≥ S.
How to Use This Calculator
- Select the conductor material used for the air terminals and down conductors.
- Choose the protection level, or enable a custom peak current if you have project data.
- Enter an appropriate pulse duration and the number of down conductors sharing the strike.
- Set a safety factor and optionally enforce a common minimum practice size.
- Press Calculate to see results above the form.
- Use Download CSV or Download PDF to document the outcome.
This tool provides an estimate for early design and comparison. Final lightning protection design should follow applicable standards and manufacturer guidance.
Professional Article
Project Context and Design Inputs
Lightning protection sizing begins with the expected strike current, impulse duration, and how many down conductors share the event. For early design, teams map risk to protection levels and confirm with site exposure, roof geometry, and bonding details. Conservative peak currents and realistic pulse durations help prevent overheating during high-energy events. Shorter impulses can still be severe because peak current drives thermal rise and magnetic forces.
How Current Sharing Affects Conductor Area
When multiple down conductors are installed and well-bonded, the peak current can split across paths. This calculator applies a simple equal-sharing assumption: peak current divided by the number of down conductors. Adding conductors can reduce required area, but only if routing is symmetrical and connections are low impedance.
Material Selection and Practical Minimums
Copper, aluminum, and galvanized steel have different thermal and mechanical behavior. Copper often supports smaller areas for the same pulse, while aluminum may require larger cross-sections and careful corrosion control at joints. Many projects enforce minimum conductor sizes to handle mechanical loads and installation tolerances, even when the thermal calculation is smaller.
Safety Factor and Standard Size Selection
Safety factors (often 1.2–1.5) add margin for uncertain distribution, connection resistance, and aging. After the required area is computed, the next larger standard size is recommended to align with commonly stocked conductors and accessories. The calculator also converts area to an equivalent round diameter for quick checks against clamps and routing constraints.
Documentation and Construction Coordination
Exporting results to CSV or PDF supports design reviews, QA/QC records, and handover documentation. Coordinate sizing with air terminal layout, roof membrane details, bonding to structural steel, and the grounding electrode system. Field teams should verify continuity, secure fastening, and protected transitions to reduce impedance and maintain a reliable current path.
FAQs
What does this calculator size?
It estimates the required conductor cross-sectional area for down conductors based on peak current, pulse duration, material constant, current sharing, and a safety factor, then recommends the next larger standard size and equivalent round diameter.
Should I always enforce the minimum practice size?
Use it when you need mechanical robustness, corrosion allowance, and standard compliance, especially on exposed rooftops. For purely comparative studies you can disable it, but final designs typically follow a minimum conductor size for durability.
How do I choose pulse duration?
If you have project or utility data, use that value. Otherwise select a conservative impulse duration in the common range of 0.01–0.2 seconds. Longer duration increases required area because heating scales with the square root of time.
Does current really split equally across down conductors?
Not always. Equal sharing is a simplified assumption that works best with symmetrical routing, short bonds, and low-impedance connections. If one path is longer or poorly bonded, it can carry more current; add margin or review the layout.
Why does aluminum often require larger sizes?
Aluminum has different thermal properties and joint considerations, so the sizing constant is lower in this simplified model. It may also need larger cross-sections for mechanical strength and to manage corrosion risks at connectors and bimetallic interfaces.
Can I size strip conductors with this tool?
Yes. Choose the strip option and enter strip width. The tool converts the required area into an estimated thickness and also reports thickness for the recommended standard area, supporting quick feasibility checks against available stock and fixing hardware.
Is this a replacement for detailed lightning protection design?
No. It is an early-stage sizing aid. Final design should follow applicable lightning protection standards, verified risk assessment, bonding and grounding requirements, and manufacturer installation details, plus inspection and testing during construction.
FAQs are provided for planning and documentation support.