What Is the 20% Rule for Solar? (Canadian Electrical Code 84-024 Explained, 2026)

In Canadian Electrical Code Rule 84-024: the sum of the main breaker rating plus 125% of the solar backfeed breaker rating cannot exceed 120% of the panel's busbar rating.

Everything else people call 'the 20% rule' is just the practical consequence of that formula. The 20% is the headroom your busbar has above the main breaker rating (120% - 100% = 20%). The solar backfeed has to fit inside that headroom.

Almost every home built in Canada since ~1985 has a 200A service. Let's work the math.

• Busbar rating: 200A
• 120% of busbar: 240A
• Main breaker: 200A
• Available for solar backfeed (240A - 200A): 40A
• 40A ÷ 1.25 (the 125% multiplier) = 32A max solar breaker

A 32A solar breaker at 240V single-phase = about 7.7 kW AC. That's the maximum solar system a typical Canadian 200A panel can accept without upgrades. Push past that and Rule 84-024 requires one of the workarounds below.

1. Downsize the main breaker (cheapest but rarely available)

If your panel has a 200A busbar but you rarely draw more than 150A of actual load, some electricians will swap the main breaker for a 150A. That frees up (240 - 150) = 90A of solar headroom, allowing a 72A breaker (~17 kW system). But: derating the main only works if the electrician can prove your peak load calculation supports it — most Canadian homes with heat pumps, EV chargers, or a hot tub can't sustain a downgrade. Sub-$500 hardware cost when it works.

2. Line-side tap (the middle option)

The solar system connects directly to the utility service lugs before the main breaker, bypassing the busbar entirely. Rule 84-024's 120% cap doesn't apply to the line-side because it isn't going through the panel's busbar. Utility approval and a licensed electrician are required, plus a separate solar-only meter socket in some provinces. Typical cost: $1,500–$3,500. Best fit for systems 8–15 kW on a 200A panel where a full service upgrade isn't warranted.

3. Smart panel replacement (Span, Lumin, Eaton)

A smart electrical panel actively manages loads in real time — shedding non-critical circuits when total draw + backfeed approaches the busbar limit. Because the panel can prove it never exceeds 120%, some inspectors will approve larger solar systems than the static math would allow. Cost: $4,000–$7,000 installed. Best fit if you're already planning a heat pump + EV + battery — the load-shedding pays back on its own.

4. Full service upgrade to 400A (the fallback that always works)

Replace the 200A service with a 400A service — new meter base, new main breaker, sometimes new service entrance conductors from the utility pole. Doubles your solar headroom to 80A of breaker (~19 kW). Cost: $6,000–$14,000 depending on how much utility work is involved. Almost always the right call if you're on a 100A service and going solar, since it's a one-time upgrade that also supports EV, heat pump, and battery for the next 30 years.

The 120% rule shows up in both Canadian Electrical Code Rule 84-024 and National Electrical Code (NEC) Section 705.12 in the US. The math is identical. Two Canadian-specific differences worth knowing:

• Provincial ESAs (Ontario ESA, Technical Safety BC, Alberta CEI) can add local amendments. Ontario, for example, requires the solar breaker to be at the opposite end of the busbar from the main — a physical placement rule NEC doesn't have.
• Canadian utilities that use combined main / meter panels (common in Alberta, Saskatchewan, and rural BC) sometimes count the meter enclosure as the effective busbar, which can either help or hurt the headroom calculation depending on the exact hardware.

'My panel is 200A, so I can just put in 20 kW'

No — busbar rating gets you 7.7 kW without workarounds regardless of unused physical breaker slots. The 120% cap is a current rating on the copper, not a space rating in the enclosure.

'I'll just get a bigger inverter'

Inverter DC input can be significantly oversized versus AC output (DC/AC ratio of 1.3–1.5 is common). But it's the AC output — inverter continuous current rating — that determines the solar breaker size, not DC input. Oversizing DC panels doesn't buy you room to grow past 84-024.

'My installer said we're fine'

Ask them to write on the quote which workaround they're using: downsize / line-side / smart panel / service upgrade. If they can't answer, or if they say 'we've done this on lots of homes so it'll pass inspection,' get a second opinion. Rule 84-024 non-compliance is one of the most common reasons a solar install gets red-tagged by a provincial ESA on inspection.

• System ≤ 7.7 kW on a 200A panel: no workaround needed. Standard install.
• System ≤ 4 kW on a 100A panel: no workaround needed but you're leaving a lot of value on the table — most homes benefit from a service upgrade regardless.
• System 8–15 kW on a 200A panel: line-side tap is usually the cheapest legal path.
• System 15+ kW on any residential service: usually a full 400A upgrade unless you have a big flat roof for micro-inverters + smart panel to shed load.
• Planning a heat pump + EV + battery in the next 5 years anyway: 400A upgrade almost always wins on total cost of ownership.