Received an Error Message: R749.0 "120% rule"

Error Message Received:

R749: 120% rule Your main breaker rating plus 125% of your power production source(s) continuous output current rating is larger than 120% of your busbar rating. Either choose a larger busbar rating, or make the sum of your main breaker and your power production source(s) smaller.

This article will walk you through how to identify the issue and resolve it.

Why am I receiving this error?:

This error appears when you’ve selected the “120% rule” as the compliance method under NEC 705.12(B)(2) for one or more busbars in your project...

However, the below calculation is false and the inputted design is not code compliant.

Main Breaker Rating + PV Output + ESS Output < 120% of the Busbar Rating

It’s important to note that any busbar added to your application—not just the main service panel—can trigger this error.

Which inputs have most likely triggered this error:

Found in the "Busbars" subcategory of the Electrical Section of the SolarAPP+ application:

Most likely: (Step 1)
- What is the point of interconnection compliance method in busbar #x?
Somewhat likely: (Step 2)
- What is the rating of busbar #x (Amps)?
- What is the rating of the OCPD protecting busbar #x (Amps)?

Found in the "Equipment" and "Circuit Requirements" subcategories of the Electrical Section of the SolarAPP+ application:

Least likely: (Step 2)
- What is the Inverter x1 Overcurrent Protective Device rating (Amps)?
- What is the ESS model x1 Overcurrent Protective Device rating (Amps)?

✅ Step 1: Verify that your project really is using the 120% rule.

It's not uncommon for different busbars to use separate means of compliance for safety. These include the "100% rule" under NEC 705.12(B)(1), "120% rule" under NEC 705.12(B)(2), "The Sum or Breakers Rule" under NEC 705.12(B)(2), using a "power control system (PCS)" under NEC 705.13.

Code Language 705.12(B)(2) "120% rule"

Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the busbar ampere rating. The busbar shall be sized for the loads connected in accordance with Article 220...

You can find this calculation below:

Main Breaker (Amps) + PV Breaker (Amps) + ESS Breaker (Amps) ≤ 1.2 × Busbar (Amps)

Identify each of the busbars where you’ve selected the "120% rule" as the compliance method; and

For each of these, review your plans to confirm the "120% rule" is appropriate.

🔍 Step 2: Review your input for each busbar and its breaker.

A. Identify the Affected Busbar

Go to the Busbars section in your project.
Find the service panel or subpanel where this error might be triggered. It's most likely either the main service panel a combiner panel, or a battery backed-up subpanel.

B. Double Check Input Values

You’ll need to verify:

Found in the "Busbars" subcategory of the Electrical Section of the SolarAPP+ application:
1. Busbar rating (Amps)
Found in the "Equipment" and "Circuit Requirements" subcategories of the Electrical Section of the SolarAPP+ application:
1. Main breaker rating (Amps)
2. PV and ESS OCPD (overcurrent protection device) sizes