"Size" for surge protective devices (SPDs) does not mean physical dimensions-it refers to electrical ratings, surge current capacity, type classification, and system voltage sizing. Below is a step-by-step guide to pick the correct size.
1. First: Match System Voltage (Critical Sizing Step)
Choose an SPD with the correct MCOV (Maximum Continuous Operating Voltage) matching your power system:
• 120V single-phase (North America): MCOV ≥ 150 V
• 230V single-phase (EU, Asia): MCOV ≥ 275 V
• 400V three-phase: MCOV ≥ 440 V
Never use an SPD rated for a lower continuous voltage-it will fail prematurely.
2. Size the Surge Current Ratings (In / Imax / Iimp)
Key definitions (IEC 61643):
1. Iimp (Impulse current) – Type 1 SPD only
For direct lightning risk (main building service entrance).
• Typical rating: 12.5 kA, 25 kA (10/350 μs waveform)
Use this if your building is exposed to open areas, has a lightning rod, or rural location.
2. Imax (Maximum discharge current, 8/20 μs) – Type 1+2 / Type 2 SPDs
The largest surge the SPD can survive one time.
• Residential panel: Imax = 40 kA ~ 60 kA
• Commercial / industrial main panel: Imax = 80 kA ~ 120 kA
• High lightning zone: 120 kA or higher
3. In (Nominal discharge current, 8/20 μs)
Standard test current; minimum recommended: 20 kA for most installations.
Simple sizing rules by location:
• Type 1+2 (Service entrance, high lightning risk): Iimp 12.5 kA + Imax 100 kA+
• Type 2 (Sub-panel, standard home/office): Imax 40–60 kA
• Type 3 (Near sensitive equipment only): Small current ratings (10–20 kA), only for secondary fine protection, cannot replace panel SPDs.
3. Number of Poles (Another "size" factor)
Match poles to your electrical setup:
• 120V single hot + neutral: 2-pole SPD
• 240V split-phase home power (US): 3-pole (L1, L2, N)
• 3-phase 400V: 4-pole (3 lines + neutral)
Always include neutral and earth protection where required by code.
4. Power strip surge protectors (consumer units)
Their "size" is marked as energy rating (Joules) and clamping voltage:
• Basic small electronics: 200–400 Joules (not recommended for expensive gear)
• Computers, TVs, gaming consoles: ≥1000 Joules
• High-value home theater / servers: 2000–4000 Joules
Important: Joule rating only applies to plug-in power strips. Hardwired panel SPDs use kA ratings, not Joules.
Zone-based sizing summary table
6. Common Mistakes When Sizing
1. Buying a small Type 3 SPD for the main electrical panel (it will burn out in a large surge).
2. Choosing too low MCOV for your grid voltage.
3. Only relying on Joule rating for hardwired SPDs (Joules are not the standard industrial rating).
4. Undersizing current capacity in high-lightning areas.
Final Quick Question Checklist
1. What is your system voltage and number of phases?
2. Is this at the main incoming power or near end equipment?
3. Is the location high risk for lightning?
4. Do you need Type 1, Type 2, or Type 3 protection?