SizingKitElectrical · the section front door
Electrical Calculator
Five electrical relations on one free console, run one at a time: Ohm’s law, the power identity, the three-phase identity, power factor as a ratio, and apparent power to current. Each result prints the formula a second time with your own figures substituted into it, so the working can be checked or pasted into a job note — no signup, nothing to install. It settles arithmetic and takes no decisions; the 28 pages indexed below it are where the code tables, the deratings and the sizing live.
- 100% free
- No signup
- 5 relations
- 28 pages indexed
- Working shown
Five relations, one at a time
Pick the relation, pick what you want out of it, and the identity is printed back with your figures substituted into it.
V = I × R
Nominal voltages
ANSI C84.1 nominal system voltages for 60 Hz systems — 120, 208, 240, 277, 480 and 600 — and IEC 60038 for the 50 Hz world, where 230 V line-to-neutral and 400 V line-to-line are the standard pair.
The relation with your figures in it
Current — V = I × R
96 A
I = 480 V ÷ 5 Ω = 96 A
This console settles the arithmetic and stops there. The decision behind V = I × R — the twelve rearrangements, the power terms and a tolerance band on the answer — belongs to Ohm's Law Calculator, and nothing here applies a derating, a demand factor or a code table.
Every electrical page on this site
28 pages across three sections, each with the situation it answers rather than a description of it. This page routes; the pages below decide.
Electrical
- Voltage Drop CalculatorThe run is long and you want to know whether the conductor you already picked still lands inside 3 %.
- Conduit Fill CalculatorThe conductors are chosen and the question is whether they will physically go down the raceway.
- Wire Gauge ChartYou have a spool in your hand and need its area, its resistance per thousand feet or its ampacity column.
- Wire Size CalculatorYou know the load and the distance and want the smallest conductor that survives both ampacity and drop.
- Electrical Load CalculatorA dwelling service has to be justified to an inspector by the Article 220 standard method.
- Three Phase Power CalculatorWye or delta, line against winding, and any time a √3 is about to be applied or left out.
- Power Factor CalculatorThe bill has a penalty on it, or a capacitor bank has to be specified in kVAR.
- Breaker Size CalculatorThe conductor is chosen and the overcurrent device has to protect it without nuisance tripping.
- Fuse Size CalculatorA fuse rather than a breaker, and the question is which standard rating is next.
- Transformer Size CalculatorA dry-type transformer has to be picked, and the primary and secondary amps decide both sides of it.
- Short Circuit Current CalculatorAn interrupting rating has to be defended and the available fault current is the input nobody has.
Power & current
- Watts to Amps CalculatorA nameplate is in watts and a breaker is in amps, and the phase and power factor sit between them.
- Volts Amps Watts CalculatorThe load is on AC and the difference between watts, volt-amperes and reactive volt-amperes is the whole question.
- kW to Amps CalculatorA motor or a drive is rated in kilowatts and the feeder has to be sized from it.
- kVA to Amps CalculatorA transformer or a UPS is rated in kVA and you want the current on each side of it.
- Amperage CalculatorA circuit has several appliances on it and the total against the breaker is what matters.
- Watt Hour CalculatorPower has to become energy — a runtime, a consumption figure, a comparison against a battery label.
- Amp Hour CalculatorA battery bank is labeled in amp-hours and you need to know how much of that is really usable.
Electronics
- Ohm's Law CalculatorDirect current, one resistive element, and any two of V, I, R and P already in hand.
- Voltage Divider CalculatorTwo resistors have to produce a specific fraction of a rail without the load spoiling it.
- Resistor Color Code CalculatorThere is a part on the bench and the bands have to become a value, or the other way round.
- LED Resistor CalculatorAn LED needs a series resistor and the forward voltage is the figure that decides it.
- Battery Runtime CalculatorA bank is already specified and the question is how many hours it holds a given load.
- Battery Capacity CalculatorThe autonomy is specified and the bank has to be sized backwards from it.
- UPS Runtime CalculatorA UPS is in place and the runtime at a real load, rather than the badge figure, is what you need.
- Impedance CalculatorResistance is not enough because there is a coil or a capacitor in the path at a known frequency.
- Decibel CalculatorA ratio has to become decibels, or dBm, dBu and dBV have to be told apart by their references.
- RC Time Constant CalculatorA capacitor charges or discharges through a resistance and the question is how long that takes.
How to get an answer here in three moves
Pick the relation, not the answer
The five buttons are the five identities, and choosing one narrows the console to the two or three figures that identity actually needs. Nothing carries over that should not: the power factor field appears on the three-phase and power factor tabs and nowhere else, because the other three do not have one.
Say which term you want back
Ohm's law and the power identity solve for any of their three terms; the three-phase relation solves for the power or the current; the kVA relation adds a phase selector because that is what decides whether the divisor carries a √3. The fields on screen change to match, so you are never asked for the figure you are trying to find.
Check the substitution, then follow the link
Under every answer is the identity with your numbers in it, which is the part worth copying into a note or reading back over the phone. Beside it is the page that owns that relation as a subject — if what you actually needed was a conductor size, an overcurrent device or a correction capacitor, that link is the shortest way there.
What is on the console
| Relations | 5 — Ohm's law, the unity-power-factor power identity, the three-phase power identity, power factor as the ratio of real to apparent power, and apparent power to line current. |
|---|---|
| Pages indexed | 28, across the Electrical, Power & current and Electronics sections, each carrying one line on the situation it answers. |
| Working shown | Every result reprints its identity with the entered figures in place — I = 30,000 W ÷ (1.7321 × 480 V × 0.85) — and that line is what the copy button puts on the clipboard. |
| Voltage presets | 120, 208, 240, 277 and 480 V from the ANSI C84.1 nominal list, plus the 400 V of IEC 60038 for a 50 Hz three-phase system. These are nominal, not measured. |
| Phase factor | 1 for single-phase and 1.7320508 for three-phase, read from the same helper every other page on the site reads, so no two pages can drift apart on it. |
| Power factor | A decimal between 0.01 and 1.00, and it appears only on the two tabs whose identity contains it — the apparent-power relation has none, by definition. |
| Deliberately absent | No ampacity, no derating, no demand factor, no conduit fill and no code table. Each tab names the page that carries those instead. |
| After the page loads | No further network request is made, so the console goes on working in a switchroom where the phone has dropped to one bar. |
Frequently asked questions
Which page actually sizes the wire?
The wire size calculator, because sizing is a decision between three limits rather than an arithmetic result. It weighs the ampacity of NEC Table 310.16 against your voltage drop budget against the small-conductor caps of 240.4(D), and says which of the three decided the answer. This console can give you the amperes to feed it, and that is where its job ends.
Why won't this page tell me the breaker size?
Because a breaker is chosen from a list of standard ratings against the conductor it protects, not computed from the load. NEC 240.6(A) fixes the ratings that exist, 240.4 governs which of them may protect a given conductor, and continuous loads carry a 125 % factor before any of that applies. Handing back a divided number and calling it a breaker size would hide all three rules.
Is 240 V in a US house single-phase or two phases?
Single-phase, despite having two hot conductors. The service is one phase of the utility's system taken through a center-tapped transformer, so the two legs are 180° apart rather than 120° and their difference is simply twice the leg voltage. There is no √3 anywhere in it, which is why a 240 V dryer circuit uses the plain power identity on this console and not the three-phase one.
For a motor on a 208 V three-phase system, do I enter 208 or 120?
208, because the three-phase identity is written in line-to-line voltage. The 120 V is what you measure from any one line to the neutral of that same wye, and it belongs to the single-phase circuits fed from the panel, not to the motor across all three lines. Entering 120 here understates the power by a factor of 1.73.
Why is European equipment marked 230/400 V and American 120/208 V?
They are two different nominal ladders, set by IEC 60038 and ANSI C84.1 respectively, and both are line-to-neutral over line-to-line pairs of the same wye. The arithmetic joining each pair is identical — 230 × 1.732 is 400, and 120 × 1.732 is 208 — so a calculator does not need to know which country it is in. What does not travel is the code around the numbers: the tables, the device ratings and the conductor sizes behind them are national.
Can I use the kVA relation for a generator as well as a transformer?
Yes, and for a UPS too, because all three are rated in apparent power for the same reason: their limit is current, and current is what apparent power measures. Enter the kVA and the line voltage and the current comes back with no power factor involved. What the power factor then decides is how many of those volt-amperes turn into useful watts, which is a separate question the power factor tab answers.
Does the console keep working with no signal?
Yes — once the page is loaded there is no server in the loop at all. Every figure is computed by JavaScript in the tab, so a mechanical room with no reception is no different from a desk, and nothing you type is transmitted anywhere to be worked out.
About the five relations, and the line between arithmetic and a decision
All five relations here are identities: they are true by definition of the quantities in them, they need no table, and they are the same in every country. What differs between countries is only the ladder of nominal voltages the numbers tend to be drawn from — ANSI C84.1 on 60 Hz systems, IEC 60038 on 50 Hz ones — and even those are nominal rather than measured, since a utilization voltage is permitted to sit a few percent either side of its label. That is the reason the presets are buttons rather than a fixed list you must choose from: the figure that matters is the one on your meter.
Everything past that point stops being arithmetic. Whether a conductor may carry the current you just computed depends on its insulation temperature rating, the ambient, how many other current-carrying conductors share the raceway and what the terminations are listed for. Whether a service is big enough depends on demand factors that let a calculation assume not everything runs at once — the electrical load calculator applies those in the order Article 220 sets out, and none of them is derivable from a formula. The same gap shows up on the storage side: an amp-hour figure multiplied by a voltage gives watt-hours in one line, but how much of that a lead-acid bank will actually deliver depends on depth of discharge and on the discharge rate itself. A console that returned a number for those questions would be doing the thing this site exists not to do.
What the console keeps
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
Switching between the five tabs keeps the figures you have already typed, in the page’s memory only, so working through a job does not mean retyping the voltage five times. Reload the page and every field is back to its starting value.