Calculating Backfeed Breaker Multiple Inverters

When installing multiple solar inverters in a system, proper backfeed protection is essential to prevent power from flowing back into the utility grid during outages. This guide explains how to calculate the appropriate breaker size for backfeed protection.

What is Backfeed in Solar Systems?

Backfeed occurs when power from your solar system flows back into the utility grid during a power outage. This can happen if your solar system is not properly isolated from the grid. Backfeed protection is crucial for safety and compliance with electrical codes.

Why Backfeed Protection is Important

Prevents electrical hazards to utility workers
Protects your solar system from damage
Ensures compliance with electrical codes
Prevents power quality issues

Types of Backfeed Protection

The two main types of backfeed protection are:

Isolation transformer: Physically separates your solar system from the grid
Backfeed breaker: A circuit breaker that opens when backfeed is detected

Formula and Assumptions

The backfeed breaker size is typically calculated based on the total capacity of your solar inverters. The formula used is:

Backfeed Breaker Size (amps) = (Total Inverter Capacity × 1.25) + 20

Where:

Total Inverter Capacity = Sum of all inverter ratings in amps
1.25 is a safety factor
20 amps is added for system losses and protection margin

Note: This calculation assumes a standard 120V system. For 240V systems, the formula would be different.

Worked Example

Let's calculate the backfeed breaker size for a system with three inverters:

Inverter 1: 20A
Inverter 2: 25A
Inverter 3: 30A

Total Inverter Capacity = 20 + 25 + 30 = 75A

Backfeed Breaker Size = (75 × 1.25) + 20 = 93.75 + 20 = 113.75A

You would need a 125A backfeed breaker for this system.

FAQ

What size backfeed breaker do I need for my solar system?

Use the calculator above or apply the formula: (Total Inverter Capacity × 1.25) + 20. Round up to the nearest standard breaker size.

Can I use a smaller breaker than the calculation suggests?

No, you should never use a breaker smaller than the calculated size. This could create a fire hazard and violate electrical codes.

What happens if I don't have backfeed protection?

Without proper backfeed protection, power from your solar system could flow back into the utility grid during outages, creating safety hazards and potential damage to equipment.

Is backfeed protection required by code?

Yes, backfeed protection is typically required by electrical codes in most jurisdictions to ensure safety and compliance.