Electrical
The 120% rule explained: NEC 705.12(B)(3) without the headache
Short answer
Under the 120% allowance, the rating of the overcurrent device protecting the busbar plus 125% of the inverter output current must not exceed 120% of the busbar ampacity — and the backfed PV breaker must sit at the opposite end of the busbar from the utility feed. On a 200 A busbar with a 200 A main that leaves 40 A for the PV breaker. Derating the main to 175 A raises it to 65 A.
Written against NEC 2020 and NEC 2023, where the busbar allowances sit at 705.12(B)(3). In NEC 2017 the same material is at 705.12(D)(2). Confirm which edition your jurisdiction has adopted before applying any of this.
What the rule is actually protecting
A busbar can be fed from two directions at once: the utility through the main, and the PV system through a backfed breaker. The concern is the segment of bar between those two sources carrying more current than it is rated for, with no overcurrent device seeing the total.
The 120% allowance is a permission, not a formula to be satisfied in the abstract. It says that if the two supplies are at opposite ends of the bar, the bar may be loaded to 120% of its rating, because no single segment ever sees the sum.
The arithmetic
Take the busbar ampacity, multiply by 1.2, subtract the rating of the overcurrent device protecting the busbar. What is left is the largest backfed breaker you may install.
Then work backwards to the inverter: the breaker must be at least 125% of the inverter's continuous output current, so the inverter output you can land is the available breaker divided by 1.25.
| Busbar | Main | 120% of bar | Available breaker | Max inverter output |
|---|---|---|---|---|
| 100 A | 100 A | 120 A | 20 A | 16 A |
| 200 A | 200 A | 240 A | 40 A | 32 A |
| 200 A | 175 A | 240 A | 65 A → 60 A standard | 48 A |
| 225 A | 200 A | 270 A | 70 A | 56 A |
| 400 A | 400 A | 480 A | 80 A | 64 A |
Breakers come in standard sizes, so round down to the next standard rating, never up.
The condition people forget
The backfed breaker must be at the opposite end of the busbar from the utility feed, and the installation has to be marked to say so. A set that shows the arithmetic but drops the PV breaker into the first open slot has not met the rule — and a plan checker who knows the section will catch it on the three-line.
Mark the panel. The permanent label identifying the position requirement is part of the allowance, not a nicety.
When the 120% allowance is not the answer
It is one option among several. The others are worth knowing because they are often cheaper than a service upgrade:
- The sum rule. Where the sum of all breakers supplying the busbar, excluding the main, does not exceed the busbar rating. Generous on large bars, useless on small ones.
- Derate the main. Replacing a 200 A main with a 175 A main costs a breaker and raises the available PV breaker from 40 A to 65 A. Check the actual load calculation supports the smaller main first.
- Supply-side connection. Landing ahead of the service disconnect sidesteps the busbar question entirely. It brings its own requirements and not every utility allows it on every meter configuration.
- Engineering supervision. A sealed analysis of the actual busbar. Real, but rarely the economical path on a house.
What to put on the drawing
Print the arithmetic on the three-line: busbar rating, main rating, 120% figure, the backfed breaker, and the inverter output it supports. Show the breaker at the opposite end. Call out the label. A reviewer who can read the numbers off the sheet does not raise a correction asking for them.