Why Correct SPD Sizing Matters for Your Equipment Safety
In today’s increasingly electrified environment, a reliable and stable power supply is essential. From whole house residential installations to advanced industrial electrical systems, modern equipment relies on continuous and safe electrical operation. However, transient overvoltages, commonly known as electrical surges, remain one of the most serious threats to system integrity and equipment safety.
Even a brief or relatively small voltage rise can create a damaging surge event. Such sudden surges may cause equipment failure, data loss, system downtime, and in severe cases, pose safety risks to personnel and infrastructure, resulting in costly disruptions.
Proper SPD sizing is key to ensuring effective protection. Understanding how to size surge protection devices ensures that your system receives reliable defense against unpredictable overvoltages.
What Is a Surge Protection Device (SPD)?
A Surge Protection Device (SPD) is the primary safeguard against transient overvoltages. Installed within distribution boards, SPDs are designed to limit voltage spikes and divert surge currents away from sensitive electrical systems and equipment.
Modern SPDs typically use MOV or GDT technology to maintain a low voltage protection level (Up), ensuring that connected devices remain within safe operating limits. Selecting the correct SPD type and rating is essential for protecting your system from damage caused by electrical surges.
Why Surge Protection Devices Are Needed and How They Work
Surge Protection Devices (SPDs) reduce surge voltages to levels that the electrical system and connected equipment can safely handle. They protect critical devices such as PLCs, power supplies, transformers, and sensitive I/O modules.
During a surge event, an SPD detects the abnormal voltage and diverts excess energy away from connected equipment. Once the surge ends, the SPD resets, ready for future events.
Correct SPD sizing is essential. An undersized SPD may fail and expose your system to damage, while an oversized SPD may be inefficient and fail to provide optimal protection. A properly sized SPD absorbs surge energy and safeguards critical infrastructure.
Types of Surge Protection Devices (SPD)
SPD Classification by Installation Location
Based on installation location, protection level, and compliance with IEC/EN 61643-11 standards, SPDs are generally classified into the following three types:
- Type 1 SPD (Class I) – Installed at the main service entrance, designed to handle high-energy surges from direct lightning strikes or utility switching operations and evaluated using lightning impulse current (Iimp, 10/350 µs).
- Type 2 SPD (Class II) – Installed in sub-distribution panels or downstream of Type 1 SPDs. Limits residual overvoltages and protects against internal operational surges (8/20 µs current wave).
- Type 3 SPD (Class III) – Installed upstream of sensitive end devices. Provides precision transient voltage suppression for low-level surges, acting as the final line of defense.
In practice, a multi-layer protection strategy is recommended: Type 1 SPDs as primary barrier, with Type 2 and Type 3 providing secondary protection.
Decoding Key Parameters: Essential Factors in SPD Sizing
To perform effective SPD selection and SPD sizing, engineers should understand key parameters:
- Lightning Impulse Current (Iimp, 10/350 µs) – Core metric for Type 1 SPDs.
- Nominal & Maximum Discharge Current (In / Imax, 8/20 µs) – Defines SPD handling capacity.
- Maximum Continuous Operating Voltage (Uc) – Must exceed system nominal voltage to prevent nuisance trips.
- Voltage Protection Level (Up) – Maximum voltage at SPD terminals during operation.
- Short-Circuit Current Rating (Isccr) & Response Time – Ensures SPD safety under fault conditions.
How to Size Surge Protection Devices
Besides the SPD’s own technical parameters mentioned above, what other factors can help us choose the appropriate surge protection device?
Step 1 – Assess Your System and Equipment Load
Selecting the correct SPD rating starts with understanding your main breaker capacity, which reflects the overall power demand of your electrical system. Assessing the rated power and operational current of all connected equipment is essential to ensure coverage of peak load and startup currents.
In addition, consider whether the equipment operates continuously or intermittently, as well as any potential future system expansion. The main breaker rating should exceed the total system load to provide an adequate safety margin. Once the main breaker capacity is defined, the required SPD capacity can be determined as the foundation of proper surge protection design.
Step 2 – Identify the Power System Configuration
Before selecting an SPD, identify the earthing and power system configuration, such as TT, TN-S, or TN-C-S (PME) systems. Different system configurations affect the required protection mode, wiring method, and SPD arrangement.
Correct identification of the system type ensures proper coordination between the SPD and the grounding system, which is essential for effective surge current diversion and system safety.
Step 3 – Determine Single-Phase or Three-Phase Application
SPD selection must match the system’s phase configuration. Single-phase applications typically require SPDs designed for L-N or L-PE protection, while three-phase systems require coordinated protection across all phases and, where applicable, the neutral conductor.
Selecting the correct phase configuration ensures balanced protection and prevents uneven stress on the electrical system during surge events.
Step 4 – Select the Appropriate SPD Type and Rating
Based on the installation location and protection requirements, select the appropriate SPD type (Type 1, Type 2, or Type 3). Key parameters such as Lightning Impulse Current (Iimp), Nominal Discharge Current (In), Maximum Discharge Current (Imax), Maximum Continuous Operating Voltage (Uc), and Voltage Protection Level (Up) must be matched to the system conditions.
Proper selection of SPD type and rating ensures that surge energy is effectively limited while maintaining voltage levels within equipment tolerance.
Step 5 – Selecting the Correct SPD Backup Protection (MCB or Fuse)
Once the SPD rating has been determined based on the main breaker and system load, proper coordination with a dedicated MCB or fuse becomes critical. According to IEC/EN 61643-11, surge protection devices must be installed with suitable backup protection to ensure short-circuit safety and controlled disconnection.
The selection of an SPD backup breaker or fuse depends on the SPD’s discharge capability and the system’s prospective short-circuit current. In systems with larger main breaker ratings, appropriately rated backup protection ensures that the SPD can withstand surge events while remaining safely protected under fault conditions.
All selected SPDs and their associated backup protection devices must comply with IEC/EN 61643-11 to ensure safe coordination, reliable operation, and long-term system protection.
SPD Selection Table
| Main Switch (ACB / MCCB / MCB) | Dedicated MCCB / MCB or Fuse | Surge Protective Device (SPD) | ||||||
| MCCB / MCB | Fuse | TN-S Network | TN-C Network | TT Network | ||||
| Single Phase | 3 Phase | Single Phase | 3 Phase | Single Phase | 3 Phase | |||
| ACB ≥ 630A MCCB: 630A ~ 315A | 200A | 315A ~ 250A | Type 1 SPD FLP25-275/1(S), FLP25-275/2(S), FLP25-275/1(S)+1 | Type 1 SPD FLP25-275/4(S), FLP25-275/3(S)+1 | Type 1 SPD FLP25-275/1(S) | Type 1 SPD FLP25-275/3(S) | Type 1 SPD FLP25-275/1(S), FLP25-275/1(S)+1 | Type 1 SPD FLP25-275/3(S)+1 |
| MCCB: 400A ~ 200A | 125A ~ 100A | 125A | Type 1+2 SPD FLP12,5-275/1(S), FLP12,5-275/2(S), FLP12,5-275/1(S)+1 | Type 1+2 SPD FLP12,5-275/4(S), FLP12,5-275/3(S)+1 | Type 1+2 SPD FLP12,5-275/1(S) | Type 1+2 SPD FLP12,5-275/3(S) | Type 1+2 SPD FLP12,5-275/1(S), FLP12,5-275/1(S)+1 | Type 1+2 SPD FLP12,5-275/3(S)+1 |
| MCCB: 200A | 80A ~ 50A | 80A | Type 1+2 SPD FLP7-275/1(S), FLP7-275/2(S), FLP7-275/1(S)+1 | Type 1+2 SPD FLP7-275/4(S), FLP7-275/3(S)+1 | Type 1+2 SPD FLP7-275/1(S) | Type 1+2 SPD FLP7-275/3(S) | Type 1+2 SPD FLP7-275/1(S), FLP7-275/1(S)+1 | Type 1+2 SPD FLP7-275/3(S)+1 |
| MCCB: 100A ~ 63A | 40A ~ 32A | 40A ~ 32A | Type 2 SPD SLP40-275/1(S), SLP40-275/2(S), SLP40-275/1(S)+1 | Type 2 SPD SLP40-275/4(S), SLP40-275/3(S)+1 | Type 2 SPD SLP40-275/1(S) | Type 2 SPD SLP40-275/3(S) | Type 2 SPD SLP40-275/1(S), SLP40-275/1(S)+1 | Type 2 SPD SLP40-275/3(S)+1 |
| MCB: 32A | 20A | 20A | Type 2+3 SPD SLP20-275/1(S), SLP20-275/2(S), SLP20-275/1(S)+1 | Type 2+3 SPD SLP20-275/4(S), SLP20-275/3(S)+1 | Type 2+3 SPD SLP20-275/1(S) | Type 2+3 SPD SLP20-275/3(S) | Type 2+3 SPD SLP20-275/1(S), SLP20-275/1(S)+1 | Type 2+3 SPD SLP20-275/3(S)+1 |
| / | 16A ~ 10A | 16A ~ 10A | Type 3 SPD TLP-275/2（S) | Type 3 SPD TLP-275/2(S) | Type 3 SPD TLP-275/2(S) | |||
What Size Surge Protector (SPD) Do I Need
Selecting the correct surge protection device is largely determined by the main circuit breaker rating, which provides a clear reference for SPD capacity and coordination.
Based on the table above, the appropriate SPD can be identified quickly according to the breaker size, simplifying the surge protector selection process. The selection examples below show how different breaker ratings correspond to specific SPD types and upstream disconnectors, providing a practical reference for how to size a surge protection device in real-world installations.
1. At the incoming distribution box, if the main air circuit breaker is greater than 630 A or MCCB is between 315 A and 630 A, the upstream dedicated disconnector of SPD should use a 200 A circuit breaker or a 250~315 A fuse. SPD can be selected Type 1 SPD FLP25 series.
Main ACB ≥630A or MCCB 315-630A → 200A breaker or 250-315A fuse → Type 1 SPD (FLP25 series)
2. At the outgoing distribution box, if the main circuit breaker is between 200 A and 400 A, the upstream dedicated disconnector of SPD should use a 100~125 A circuit breaker or a 125 A fuse. SPD can be selected Type 1+2 SPD FLP12,5 series. At the main distribution box, if the main circuit breaker is rated at 200 A, the upstream dedicated disconnector of SPD should use a 50~80 A circuit breaker or an 80 A fuse. SPD can be selected Type 1+2 SPD FLP7 series.
Outgoing distribution
MCCB 200-400A → 100-125A breaker or 125A fuse → Type 1+2 SPD (FLP12.5 series)
3. At the main distribution box, if the main circuit breaker is rated at 200 A, the upstream dedicated disconnector of SPD should use a 50~80 A circuit breaker or an 80 A fuse. SPD can be selected Type 1+2 SPD FLP7 series.
Main distribution board
MCCB 200-400A → 100-125A breaker or 125A fuse → Type 1+2 SPD (FLP12.5 series)
4. At the sub-distribution box, if the main circuit breaker is rated at 63~100 A, the upstream dedicated disconnector of SPD should use a 32~40 A circuit breaker or fuse. SPD can be selected Type 2 SPD SLP40 series.
MCCB 63-100A → 32-40A breaker or fuse → Type 2 SPD (SLP40 series)
5. At the sub-distribution box, if the main circuit breaker is 32 A, the upstream dedicated disconnector of SPD should use a 20 A circuit breaker or fuse. SPD can be selected Type 2+3 SPD SLP20 series.
MCB 32A → 20A breaker or fuse → Type 2+3 SPD (SLP20 series)
6. At the sub-distribution box, for surge protection of industrial control equipment, the upstream dedicated disconnector of SPD should use a 10~16 A circuit breaker or fuse. SPD can be selected Type 3 SPD series.
MCB 10-16A → Industrial control equipment protection → Type 3 SPD
Advanced Cascaded Protection
Using a single SPD may not be sufficient to protect sensitive electronics from transient overvoltages. For enhanced system reliability, multiple SPDs must work together across the main distribution board, sub-distribution panels, and near critical devices.
Cascaded SPD Design Principles
Cascaded SPD systems combine SPDs of different types and protection levels. A typical setup uses:
- Type 1 SPD – at the service entrance to handle large surges from lightning or utility switching.
- Type 2 SPD – at sub-distribution panels to mitigate residual surges and internal switching transients.
- Type 3 SPD – close to sensitive equipment to suppress any remaining low-level surges.
Combination SPD Products
Combination SPDs integrate two protection levels within a single device, such as Type 1+2 SPD or Type 2+3 SPD. These units simplify installation and ensure coordinated protection across different points in the electrical system. By combining protection levels, one device can handle both high-energy surges at the service entrance and low-energy surges near sensitive electronics.
AC System
- Type 1+2 AC SPD – FLP7-275/4S
Key Info:
- Three Phase, high discharge capacity
- Use at main distribution boards
- TN-S system, protects L-PE and N-PE
- IEC 61643-11 compliance
Installation:
2. Type 2+3 AC SPD – SLP20-275/2S
Key Info:
- Single Phase, double protection
- Use at sub-distribution boards
- TN-S system, protects L-PE and N-PE
- IEC 61643-11 compliance
Installation:
DC System
Type 1+2 DC SPD – FLP25-DC75/1S+1
Key Info:
- 48V DC system, DIN rail mount
- Use at string boxes or inverters
- High discharge capacity (MOV + GDT)
- IEC/EN 61643-41 compliance
Wiring diagram:
Special Considerations for Solar & DC Systems
When sizing SPDs for solar photovoltaic (PV) systems), DC strings, or battery and energy storage systems, there are additional considerations. DC systems often require SPDs rated for continuous DC voltage, and surge currents may behave differently compared to AC networks. Selecting the correct SPD for these applications ensures reliable protection for inverters, controllers, and batteries, without compromising system performance.
In practice, a combination of Type 1, Type 2, or Type 3 SPDs may be required, depending on whether the system is at the service entrance, in sub-distribution panels, or near sensitive equipment. Understanding how to size DC SPDs correctly is critical to safeguard expensive solar and storage components.
Conclusion – Ensuring Reliable Surge Protection
Selecting the correctSPD is not just a matter of installation—it is a key part of system reliability management. A correctly sized SPD protects against unpredictable transient overvoltages that could damage electrical systems or sensitive electronics.
A comprehensive SPD selection strategy involves evaluating risks, choosing the appropriate type, and considering key parameters such as voltage protection level (Up), maximum continuous operating voltage (Uc/MCOV), nominal discharge current (In), maximum discharge current (Imax), and short-circuit current rating (SCCR).
Oversized SPDs do not guarantee better protection, while undersized SPDs may fail under high surge conditions. Ensuring that the selected SPD matches your system requirements is essential for effective protection.
LSP Surge Protection Solutions – Technical Overview
LSP provides comprehensive surge protection solutions for residential, commercial, and industrial applications, covering both AC and DC systems. All LSP SPDs are designed and tested in strict compliance with IEC/EN 61643-11 standards. The product portfolio includes Type 1, Type 2, and Type 3 SPDs, as well as combination solutions such as Type 1+2 and Type 2+3 SPDs, enabling engineers to implement cascaded protection strategies that provide coordinated defense at main distribution boards, sub-distribution panels, and near sensitive equipment.
For DC SPD selection in photovoltaic strings and energy storage systems, LSP’s solutions ensure safe and reliable long-term operation of PV systems and batteries. These solutions help maintain system integrity, protect sensitive electronic devices, and support both standard and customized SPD configurations.
FAQ: Common SPD Sizing and Selection Questions
How to select SPD rating?
SPD rating is determined by: system voltage, maximum continuous operating voltage (Uc), expected surge currents (In/Imax), and voltage protection level (Up). Choose ratings to ensure the SPD can safely absorb surges without exceeding equipment insulation limits. Backup protection (MCB/fuse) must match SPD capacity.
How to properly size surge protective devices?
Proper sizing ofSPDs involves:
- Understanding the main breaker rating and system load.
- Identifying system configuration (TT, TN-S, TN-C-S) and phase count.
- Selecting SPD type and rating based on installation location and expected surges.
- Matching key parameters: Up, Uc/MCOV, In, Imax, Joule rating, SCCR.
- Coordinating with backup protection for safety.
3-phase surge protector selection – what to consider?
For three-phase systems, SPDs must provide coordinated protection across all phases and, if necessary, the neutral conductor. Key considerations include: system voltage, maximum surge current, installation point (main or sub-distribution), and compliance with IEC/EN 61643-11. Correct selection prevents unbalanced stress during surges.
What size whole house surge protector (SPD) do I need?
The whole-house surge protector (SPD) should match the main service entrance breaker. Typical guidance:
- For 200A service → Type 1+2 SPD (FLP7 series) with 50-80A upstream breaker.
- For 315-630A service → Type 1 SPD (FLP25 series) with 200A breaker or 250-315A fuse.
Surge arrester (SPD) selection calculation – how to perform it?
To perform the surge arrester (SPD) selection calculation, follow these steps:
- Determine the system voltage and type (single-phase or three-phase).
- Identify the maximum continuous operating voltage (Uc) and insulation level of protected equipment.
- Calculate the expected surge current based on historical lightning data or standards.
- Select the SPD type (Type 1, Type 2, Type 3, or combination) and rating to ensure residual voltage (Up) is within safe limits.
- Ensure backup protection (MCB/fuse) is coordinated with SPD capacity.
Does every SPD require an MCB?
Yes, according to IEC/EN 61643-11, all SPDs must be installed with suitable backup protection such as an MCB or fuse. The backup device ensures short-circuit safety and controlled disconnection in the event of SPD failure. The rating of the MCB or fuse should match the SPD’s discharge capacity and the system’s prospective short-circuit current.
How do I choose SPD for solar applications?
For solar PV systems, DC SPDs must be rated for the system’s continuous DC voltage, typically 48V for small installations. Consider whether the SPD is installed at the inverter, string box, or near the battery. Type 1+2 or Type 2+3 SPDs are commonly used in cascaded arrangements to protect against both high-energy surges from external sources and low-energy surges near sensitive electronics. Ensure the SPD complies with IEC/EN 61643-41 for DC applications.
What size surge protector (SPD) do I need for my house?
The size of the surge protector (SPD) depends primarily on your main circuit breaker rating and the total load of your household electrical system. For a typical home with a 200A main breaker, a Type 1+2 SPD rated for that service level is recommended, with an upstream disconnector such as a 50-80A breaker or an 80A fuse. This ensures the SPD can safely divert surges from lightning strikes or utility switching without being damaged.
