Root three and the 80% rule

siting5 min read2026-09-22

Two pieces of arithmetic stand between an amperage on a drawing and kilowatts at a dispenser. Both are simple, both are routinely skipped, and skipping either one overstates your service by 20 to 40%.

Root three

Three-phase power is not three times single-phase power. The three phases peak 120 degrees apart, so the line-to-line voltage is the square root of three times the phase voltage:

Three phase:  P(kW) = 1.732 x V x I x PF / 1000
Single phase: P(kW) =         V x I x PF / 1000

For a 480 V service at 100 A with a power factor of 0.98:

1.732 x 480 x 100 x 0.98 / 1000 = 81.5 kW

Apparent power, the number the utility’s equipment is sized for, is the same sum without the power factor: 83.1 kVA. The gap between kVA and kW is reactive power, and it is why power factor appears on bills.

The 80% rule

A continuous load may only draw 80% of a breaker’s rating. Charging is the definition of a continuous load: it runs at full current for hours. So a 100 A breaker is good for 80 A continuous, and that same 480 V service delivers:

1.732 x 480 x 80 x 0.98 / 1000 = 65.2 kW

The same drawing, read two ways, differs by 16 kW. Section 3 of the calculator has a toggle for exactly this, because half the time the number someone gives you is a breaker rating and half the time it is a continuous draw, and nobody says which.

What an upgrade buys, until it does not

Here is a 250 kWh station with a 125 kW charge rate, refilling from 10 to 100% on a 480 V service, at each standard breaker size:

Breaker	Continuous	Power	Time to full
30 A	24 A	19.6 kW	11:54
60 A	48 A	39.1 kW	5:57
100 A	80 A	65.2 kW	3:34
200 A	160 A	130 kW	1:47
300 A	240 A	196 kW	1:45
400 A	320 A	261 kW	1:45
600 A	480 A	391 kW	1:45

Read the bottom of that table carefully. From 300 A upward, nothing changes. The service has outrun the battery’s 125 kW charge rate, and every further amp is spent on a breaker that cannot deliver into anything. That row is where a service upgrade stops paying and an inverter upgrade starts.

Knowing which side of that line you are on is worth more than any other number on the page, because the two upgrades have wildly different costs and lead times.

The taper comes back

One more detail: the battery does not accept constant power to 100% any more than a car does. Above about 90% state of charge the charge rate tapers, in the model down to 30% of the rate at 100%, for exactly the reasons in why fast charging slows down. The last few percent of a station refill are the slowest, which is an argument for cycling a buffer battery in a band rather than to full.

Daily throughput is a grid question

The battery does not create energy. Over a day, the grid service sets the ceiling:

24 h x 65.2 kW = 1,564 kWh from the grid
        x 0.94 charge efficiency = 1,470 kWh into the cells
        x 0.94 discharge efficiency = 1,382 kWh out of the cabinet
        x 0.94 vehicle-side losses = 1,299 kWh into packs
        / 52.5 kWh per session = about 25 sessions a day

Twenty-five sessions a day, sustained, on an 80 A continuous service. The battery decides how many of those can happen back to back and how fast each one goes. It does not add a single kilowatt-hour to the daily total.

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