Mike Dolt Enterprises
Mike Dolt Enterprises
User: photonboy, Location: Berkeley, CA, USA
Asked on: Friday, January 31, 2025 at 1:31 pm EST
Answered on: Thursday, March 6, 2025 at 8:05 am EST
Question: I was under the impression that the sum of all breakers rule is another way of saying the 120% rule, which says the MB and PV breaker cannot exceed 120% of bus bar rating. One of our installers is pushing back on this. There is no PCS in this system, just a SolarEdge inverter with no PCS capabilities.Scenario:200A bus200A Main Breaker1 existing 30A load breaker1 new 60A PV backfeed breakerThe installer is claiming that the "sum of all breakers rule" means that you just add up the sum of the breakers present on the bus, minus the Main Breaker, and if that is less than the bus rating then good to go.Here are the pics they uploaded to make their point and my marked up response, which is correct?:
You are correct. Your installer's interpretation of the "sum of all breakers rule" is incorrect and dangerously misleading. The correct rule to apply here is the 120% rule.Let's break down why: The 120% Rule (NEC 705.12(B)(2)(3)): This rule is specifically designed to protect the busbar of a panel from overcurrent when you have backfed power from a source like solar PV. It states that the sum of the overcurrent protection devices supplying power to a busbar (the main breaker and the PV backfeed breaker in this case) cannot exceed 120% of the busbar's rating. Your Installer's "Sum of Breakers Rule": The installer's method of simply summing all the branch circuit breakers (excluding the main) and comparing that to the busbar rating is not a recognized rule within the NEC for calculating backfeed limits. While it might prevent overloading the total current draw from the utility and PV system, it doesn't account for the potential for overcurrent on the busbar itself when both the main breaker and the PV backfeed are supplying power simultaneously.
Applying the 120% Rule to Your Scenario1. Busbar Rating
200A2.
120% of Busbar Rating
200A 1.20 = 240A3.
Sum of Main Breaker and PV Breaker
200A (Main) + 60A (PV) = 260ASince 260A > 240A, this installation violates the 120% rule.
Why the 120% Rule Matters
Imagine a scenario where your home is consuming very little power, and the sun is shining brightly. The PV system is producing its maximum output (60A backfeeding to the bus), and the utility is also supplying power to the bus through the 200A main breaker. In this case, the busbar is potentially carrying 260A, exceeding its 200A rating and potentially leading to overheating, damage, and fire.
Corrective Actions
There are a few ways to bring this installation into compliance with the 120% rule:1. Reduce the PV Breaker Size
Reduce the size of the PV backfeed breaker so that when added to the Main Breaker it is less than or equal to 240A (200A 1.2). In your case, the maximum PV breaker size is 40A: 240A - 200A = 40A2.
Derate the Main Breaker
If you can get the utility to agree to reduce the size of the main breaker, you can increase the amount of backfeed allowed from the PV system. For example, if the Main Breaker was reduced to 150A, then the max PV backfeed breaker is 90A: 240A - 150A = 90A3.
Use a Line Side Tap
A line side tap connects the PV system directly to the service conductors before the main breaker, bypassing the panel's busbar limitations. This is a more complex and expensive solution.
In Conclusion
Stand your ground on this issue. The 120% rule is a critical safety measure designed to protect electrical systems. Do not allow an installation that violates this rule. Your installer's understanding is incorrect and could lead to a dangerous situation. Contact your local electrical inspector or a qualified electrical engineer for further clarification and guidance. Flag for review