Lightning Protection Down Conductor Size Requirements | NFPA 780 Standards

Lightning Protection Down Conductor Size Requirements

What is the minimum size of a down conductor for lightning protection? The following conductor sizing requirements are specified under NFPA 780 (Standard for the Installation of Lightning Protection Systems):

Primary Down Conductors (Copper) Stranded copper conductors shall be at least 57,400 circular mils at 187 lbs per 1,000 ft.

Primary Down Conductors (Aluminum) Aluminum conductors shall be at least 98,600 cir. mils at 95 lbs per 1,000 ft.

• Secondary and Bonding Conductors
• The minimum size for secondary or bonding conductors shall be:
• Copper: 26,240 cir. mils
• Aluminum: 41,100 cir. mils

• Heavy-Duty Primary Conductors
• For heavy-duty applications, primary conductors shall be not less than:
• Copper: 115,000 cir. mils at 375 lbs per 1,000 ft
• Aluminum: 192,000 cir. mils at 190 lbs per 1,000 ft

• Secondary bonding conductors for heavy-duty applications:
• Copper: 26,240 cir. mils
• Aluminum: 41,100 cir. mils

What is the Individual Resistance of a Grounding Electrode?

The individual resistance of a grounding electrode depends on soil resistivity, electrode length, and electrode diameter. For a single ground rod, the resistance to earth is calculated using the Dwight formula. Typical values range from 5 ohms to over 100 ohms depending on soil conditions.

What is the Combined Resistance of Parallel Electrodes?

When multiple electrodes are connected in parallel, the combined resistance is not simply the individual resistance divided by the number of electrodes. Due to the mutual interference between electrodes, the combined resistance is higher than a simple parallel calculation would suggest. The combined resistance of N electrodes in parallel is approximately: R_combined = R_individual / N × (1 + (mutual resistance factor))

For design purposes, the NEC requires that the grounding electrode system achieve a resistance to ground of 25 ohms or less. If a single electrode does not achieve 25 ohms, a supplemental electrode must be added.

What is the Resistance of a Lightning Protection Ground System?

For lightning protection systems, NFPA 780 requires that the resistance to ground of the grounding system be as low as practicable. A resistance of 10 ohms or less is considered excellent. The resistance is measured using the fall-of-potential method or the clamp-on method for active systems.