Electrical Panel Load Calculator

What Each Field Does on the Panel

Type in your panel voltage, phase count, panel rating, and the current draw of three circuits, and the calculator hands you six numbers that map directly to decisions you make standing in front of the panel. Total connected load (45 A in the example) is the running tally you’d write on a legal pad while reading clamp-meter values off each breaker — it’s the raw sum of Circuit 1, Circuit 2, and Circuit 3 load. Estimated apparent power (10,800 VA) is that same load converted to VA at the entered Panel voltage, the number you’d quote if someone asks for the connected kVA on a load letter or a transformer-sizing memo. Phase multiplier (1x here) tells you which math the tool used to get that VA figure — flat multiplication for single-phase, a √3 factor for three-phase. Estimated connected-load current is a round-trip check: it converts the VA back to amps and should equal Total connected load, confirming the voltage and phase entries are internally consistent. Target estimated capacity (56.25 A) is the rating you’d need if you want your Total connected load to sit inside the reserve margin you set — this is the number you compare against the panel’s actual bus rating before signing off on “room for one more circuit.” Target capacity increment (11.25 A) is the headroom gap in amps between where you are now and where the reserve target sits — useful shorthand when a customer asks “how much more can I add before we’re tight.”

The One Pair That Gets Confused: Phase Multiplier vs. Current Scaling

The mix-up worth flagging is assuming that switching Phase count to three-phase will change Total connected load or Estimated connected-load current. It won’t. Because the branch loads you enter are already currents, not VA, the phase multiplier only touches the VA calculation — it scales Estimated apparent power by √3 (≈1.732), then the round-trip back to amps divides by that same √3 and the voltage, landing back on the identical current sum you started with. If you’re expecting the amp-side outputs to shrink when you flip to three-phase (because three-phase moves the same power on less current per line in a real system), that expectation doesn’t apply here — this tool sums the currents you fed it; it doesn’t re-derive them from a fixed power target.

Formula Reference

• Total connected load = Circuit 1 load + Circuit 2 load + Circuit 3 load. Field-crew shorthand: the “amp tally” or “stack total.”
• Phase multiplier = 1 for Single-phase; √3 (≈1.732) for three-phase. Often called the “phase factor” on load worksheets.
• Estimated apparent power (VA) = Panel voltage × Total connected load × Phase multiplier. This is the “connected VA” or “connected kVA” line on a load letter.
• Estimated connected-load current = Estimated apparent power ÷ (Panel voltage × Phase multiplier). This is the “sanity-check amp” — it should reproduce Total connected load exactly.
• Target estimated capacity = Total connected load ÷ (1 − Target reserve ÷ 100). Field techs often call this the “headroom target” or “80% number” when reserve is set to 20%.
• Target capacity increment = Target estimated capacity − Estimated connected-load current. This is the “spare amps” figure — how much more current you can add before hitting the reserve target.

Arithmetic vs. NEC-Governed Territory

Everything above the panel-rating comparison is plain algebra with no code citation attached: summing three currents, multiplying by voltage and a √3 factor, and dividing by (1 − reserve) are all just arithmetic operations on numbers you supplied — the tool doesn’t apply a demand factor, and it doesn’t know whether your Circuit 1/2/3 entries are continuous, noncontinuous, or motor loads.

Where NEC actually sets a boundary: the 20%-reserve convention this tool defaults toward mirrors the mandatory continuous-load rule in NEC 210.19(A)(1) and 210.20(A), which require conductors and overcurrent devices sized at no less than 125% of a continuous load — mathematically the same operation as dividing by 0.8. If any of the three circuits you’re tallying actually run three hours or more, that 20% margin isn’t a nice-to-have target from this tool; it’s a code minimum you’re already obligated to hold. Separately, the Panel rating field itself — what the busbar and enclosure are actually rated to carry — falls under NEC 408.30 (panelboard load rating) and 408.36 (overcurrent protection of panelboards, main rating not exceeding bus rating), both in the 2023 NEC. And if your Circuit 1/2/3 load entries came from anything other than direct measurement — say, a nameplate estimate or a load letter from another trade — NEC Article 220 governs how those loads should have been calculated in the first place, while NEC 220.87 is the specific path for pulling a defensible existing-load number from 12 months of utility data or a 30-day metered recording, rather than a field guess typed into this tool.

Worked Example: Adding an EV Circuit

Set 1 (as entered): Panel voltage 240 V, Single-phase (multiplier 1x), Panel rating 100 A, Circuit 1 = 20 A, Circuit 2 = 15 A, Circuit 3 = 10 A, Target reserve 20%.

Total connected load = 20 + 15 + 10 = 45 A. Estimated apparent power = 240 × 45 × 1 = 10,800 VA. Estimated connected-load current = 10,800 ÷ (240 × 1) = 45 A, matching the tally. Target estimated capacity = 45 ÷ (1 − 0.20) = 45 ÷ 0.8 = 56.25 A. Target capacity increment = 56.25 − 45 = 11.25 A. Against a 100 A panel rating, that leaves 100 − 56.25 = 43.75 A of unused rating beyond the reserve target.

Set 2 (Circuit 3 changed to a 30 A EV charger circuit): Everything else held constant — Panel voltage 240 V, single-phase, Panel rating 100 A, Target reserve 20%, but Circuit 3 load now 30 A instead of 10 A.

Total connected load = 20 + 15 + 30 = 65 A. Estimated apparent power = 240 × 65 × 1 = 15,600 VA. Estimated connected-load current = 15,600 ÷ 240 = 65 A. Target estimated capacity = 65 ÷ 0.8 = 81.25 A. Target capacity increment = 81.25 − 65 = 16.25 A. Against the same 100 A panel rating, remaining headroom drops to 100 − 81.25 = 18.75 A.

The trade-off between the two runs: swapping a 10 A circuit for a 30 A EV charger circuit doesn’t just add 20 A to your tally — it pushes Target estimated capacity up by 25 A (56.25 A → 81.25 A) and cuts your rating headroom nearly in half (43.75 A → 18.75 A), which is the difference between “plenty of room for the next tenant improvement” and “flag this panel for a load study before adding anything else.”

Field Entry Errors That Skew Results

Entering a breaker’s handle rating instead of measured load current in Circuit 1, Circuit 2, or Circuit 3 inflates every downstream number by the same gap. If the real draw on a “20 A circuit” is actually 12 A but you type 20, that’s an 8 A overstatement; repeat that pattern across all three circuit fields and you can push Total connected load 20-plus amps high, which then inflates Target estimated capacity by roughly that amount divided by 0.8 — easily a 25+ A overstatement that makes a panel look tighter than it is.

Typing the main breaker rating into Panel rating instead of the busbar rating from the nameplate creates a different failure: a panel can carry a 225 A bus behind a 200 A main breaker, and per NEC 408.30 it’s the bus rating that caps what the enclosure can carry. If you enter 200 A thinking that’s the ceiling, your headroom comparison against Target estimated capacity reads 25 A tighter than the panel can actually support.

Picking the wrong Phase count for a high-leg delta or corner-grounded three-phase panel won’t touch Total connected load or Target estimated capacity — those stay arithmetic-only — but it will throw Estimated apparent power off by a factor of about 1.732. That error stays invisible until someone downstream uses that VA figure for transformer or generator sizing, at which point it shows up as a 73% miss.

Misreading what Target reserve means is the most common conceptual slip: the field defines reserve as spare capacity above the connected load, not “percentage of the panel already used.” Someone expecting the calculator to mean “I want the panel only 20% loaded” will be surprised that a 45 A connected load with 20% reserve returns a 56.25 A target, not a 225 A one — confirm which definition you’re working from before you quote a number to a customer.

This calculator locks down the arithmetic, but the final rating decision still needs a hands-on panel inspection and sign-off against whichever NEC edition your AHJ has actually adopted — most states are still running the 2023 NEC, though a few, including Massachusetts (effective April 2026) and Texas (effective September 2026), have already moved to the 2026 edition.