SPD Wire Length vs. Let-Through Voltage
Issue:
How far can a surge protective device be installed from the panel?
Product Line:
Surge Protective Devices
Environment:
Surge Protective Devices Installation
Resolution:
When installing an SPD (Surge Protection Device), it is important to understand the impact of wire length on the SPD's performance. Longer wires
reduce SPD performance because of inductive effects on conductors, so SPDs should be installed using the shortest possible
wire lengths. Because overvoltage transients occur at high frequencies, the associated inductances are high. Inductance
limits the propagation of short-duration, high frequency, overvoltage transients. When propagation is limited, high amounts
of energy remain in localized areas and can impact nearby equipment. Rule-of-thumb estimates for inductance-related
voltage drops range from 100 to 170 volts per foot, which adds to SPD let-through voltage and decreases protection for
equipment. For these reasons, Article 242.24 of 2020 National Electric Code requires SPDs to be installed with the shortest possible lead lengths, as follows:
"242.12 Routing of Connections. The conductors used to connect the SPD to the line or bus and to ground shall not be any longer than necessary and shall avoid unnecessary bends."
References
1. National Electric Code 15th Edition, 2020
2. ASCO Power Technologies, WP-50019_Locating SPDs
https://www.se.com/us/en/faqs/FA301358/
https://www.se.com/us/en/faqs/FA125734/
Correct Installation Incorrect Installation
How far can a surge protective device be installed from the panel?
Product Line:
Surge Protective Devices
Environment:
Surge Protective Devices Installation
Resolution:
When installing an SPD (Surge Protection Device), it is important to understand the impact of wire length on the SPD's performance. Longer wires
reduce SPD performance because of inductive effects on conductors, so SPDs should be installed using the shortest possible
wire lengths. Because overvoltage transients occur at high frequencies, the associated inductances are high. Inductance
limits the propagation of short-duration, high frequency, overvoltage transients. When propagation is limited, high amounts
of energy remain in localized areas and can impact nearby equipment. Rule-of-thumb estimates for inductance-related
voltage drops range from 100 to 170 volts per foot, which adds to SPD let-through voltage and decreases protection for
equipment. For these reasons, Article 242.24 of 2020 National Electric Code requires SPDs to be installed with the shortest possible lead lengths, as follows:
"242.12 Routing of Connections. The conductors used to connect the SPD to the line or bus and to ground shall not be any longer than necessary and shall avoid unnecessary bends."
References
1. National Electric Code 15th Edition, 2020
2. ASCO Power Technologies, WP-50019_Locating SPDs
https://www.se.com/us/en/faqs/FA301358/
https://www.se.com/us/en/faqs/FA125734/
Correct Installation Incorrect Installation
Released for: Schneider Electric USA
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