Motor Full-Load Current Calculator (NEC Table 430 Reference)
No nameplate yet, but the overload relay still has to be sized. A 25 HP pump motor is on the submittal, its horsepower, voltage and phase are known, the unit itself hasn't shipped, and the panel schedule needs a full-load current figure before the job can move forward.
That figure is the motor's full-load current, or FLA: the steady amperage it draws once running at rated horsepower and voltage. Once the physical motor is on site, its nameplate settles the question in one glance. Until then, the calculator below produces a working estimate from horsepower, voltage, phase and typical efficiency and power-factor assumptions, and, further down this page, explains why that estimate still isn't the number a code-compliant breaker or conductor gets sized against.
Estimated full-load current
41.41A
This is an estimate from nameplate HP, not the NEC Table 430.250 / 430.248 published figure. Code work uses the table value even when it differs from the calculated current.
Where this estimate comes from
A motor's horsepower rating describes mechanical output at the shaft, not electrical input at the terminals, and a motor is never a perfectly efficient converter between the two. The first step of the estimate turns HP into an input kW figure by dividing the mechanical output by an assumed efficiency, since some of the electrical power in is lost to heat before it ever reaches the shaft.
From there, the estimate is an ordinary kW-to-amps conversion, exactly like sizing any other AC load: divide by voltage and power factor for single-phase, or by √3 × voltage × power factor for three-phase, since a three-phase line-to-line voltage measurement already reflects a vector sum across the three phases rather than a plain arithmetic one.
The calculator above defaults to a 90% efficiency and 0.87 power factor, typical assumed values for a mid-size induction motor, and both are editable so the estimate can be tightened once better data is available.
Both assumptions move the answer more than they might seem to. Nudging efficiency from 90% down to 85% raises the estimated input kW, and therefore the estimated amps, by roughly six percent, since a less efficient motor needs more electrical power in to deliver the same mechanical output. Power factor works the same way from the other direction: a lower assumed PF pushes the estimated current up, because more apparent current is needed to deliver the same real power. Neither knob is wrong to adjust, but it's worth remembering that every point moved is a guess standing in for a number a real nameplate would simply state.
Worked example: a 25 HP pump motor
25 HP, three-phase, 460 V, assumed 90% efficiency and 0.87 power factor.
- 1
Convert HP to input kW
(25 × 0.7457) / 0.90 = 20.71 kW
- 2
Apply the three-phase current formula
A = (20.71 × 1000) / (1.732 × 460 × 0.87)
- 3
Solve
A ≈ 29.88 A (estimated)
29.88 A is this page's from-scratch estimate, not the NEC Table 430.250 published figure for a 25 HP, 460 V, three-phase motor. Code work uses the table value; see the next section for why.
Reading the nameplate once the motor is on site
Every motor nameplate carries a short list of figures that settle everything this page can only estimate: rated horsepower, voltage, phase, a measured FLA, and usually a service factor and a nominal efficiency. That FLA line is the manufacturer's own test result for that exact unit, and it is the figure NEC 430.6 points to for sizing the overload relay that protects the motor's windings, not this page's estimate, and not necessarily the code table figure either.
It's worth checking that nameplate FLA against the estimate above as a sanity check, not a replacement. If a nameplate figure and the estimate for the same HP, voltage and phase are wildly far apart, not the few percent a different efficiency or power-factor assumption would explain, but two or three times off: that's usually a sign the wrong HP, voltage or phase was entered into the calculator, not that the motor is unusual. A close match confirms the estimate was a reasonable stand-in during planning; it doesn't retroactively make the estimate the number to build a panel schedule from.
Three numbers, all called "FLA"
The phrase "full-load current" gets used for three different figures on a real job, and mixing them up is an easy way to size something wrong. It's worth keeping them straight before touching a panel schedule.
Nameplate FLA
Stamped on the specific motor after the manufacturer tests it. The most accurate figure available, but it doesn't exist until a physical unit has been selected and built.
Calculated / estimated FLA
What this page's calculator produces: a from-scratch approximation built from HP, voltage and assumed efficiency and power factor, useful for early planning before a motor model is chosen.
NEC table FLA
Looked up from NEC Table 430.250 (three-phase) or 430.248 (single-phase) by HP, voltage and phase alone, not calculated at all, and not tied to any specific motor's actual efficiency.
The reason the third figure is the one code work actually uses is worth stating plainly, because it can look like a contradiction the first time it's noticed: a real motor's nameplate FLA and the code table's FLA for the same HP, voltage and phase are frequently close but rarely identical, and the table value governs conductor ampacity and short-circuit protection regardless of which one is smaller.
That's not an inconsistency; it's the point. NEC Table 430.250 and 430.248 were set to cover the range of motors on the market for a given HP, voltage and phase, so the same standardized figure applies whether the motor that eventually gets installed is a slightly more efficient model or a slightly less efficient one, and whether it gets swapped for a different unit five years into the equipment's life. A wire and breaker sized off the table figure keep working across that range without being re-verified every time the specific motor changes; a wire sized off one motor's own nameplate wouldn't offer the same guarantee for its replacement. The table value's built-in margin is what makes it usable as a fixed, code-mandated reference rather than a number that has to be recalculated per unit.
Estimated FLA by horsepower
Estimated FLA, assuming 90% efficiency and 0.87 power factor, the same defaults used by the calculator above. These are general-reference estimates, not NEC Table 430.250/430.248 published values; look up the actual code figure for any real conductor, breaker or overload sizing.
| Motor HP | Single-phase, 230 V (est.) | Three-phase, 460 V (est.) |
|---|---|---|
| 1 HP | 4.14 A | 1.20 A |
| 2 HP | 8.28 A | 2.39 A |
| 3 HP | 12.42 A | 3.59 A |
| 5 HP | 20.70 A | 5.98 A |
| 7.5 HP | 31.06 A | 8.96 A |
| 10 HP | 41.41 A | 11.95 A |
| 15 HP | 62.11 A | 17.93 A |
| 20 HP | 82.81 A | 23.91 A |
| 25 HP | 103.52 A | 29.88 A |
| 30 HP | 124.22 A | 35.86 A |
Reference estimates only, not measured or code-table values. Confirm any real installation against the motor's nameplate and the applicable NEC table.
Motor and load sizing references
Questions
Motor full-load current FAQ
Questions that come up once an estimated FLA has to be reconciled with a nameplate or a code table.
Why doesn't the NEC table FLA change if my motor is actually 92% efficient instead of a typical 90%?
Because NEC Table 430.250 and 430.248 aren't calculated from any specific motor's efficiency at all: the published value is fixed by horsepower, voltage and phase alone, full stop. That's a deliberate design choice, not an oversight: the table figure already carries its own conservative margin, built from the spread of motors that were on the market when the table was set. An unusually efficient motor doesn't earn a smaller wire, and a slightly less efficient one isn't left undersized: every motor of that HP, voltage and phase gets the same code-mandated number, which is what makes conductor and breaker sizing predictable across a whole installation.
What's the difference between full-load current and locked-rotor current?
Full-load current (FLA) is the steady amperage a motor draws once it is up to speed and carrying its rated load; locked-rotor current (LRA) is the brief inrush at the instant it starts, and it is typically several times higher. FLA is what sizes continuous ampacity and overload protection, since that current flows for as long as the motor runs. LRA matters for a different job entirely: picking a starter, a contactor and an instantaneous-trip or motor-circuit-protector setting that can ride through the starting surge without tripping on a normal start.
What does a motor's service factor mean, and does it change the FLA I should use?
Service factor (SF) is how far above its nameplate horsepower a motor can be run continuously without exceeding its insulation temperature limits. An SF of 1.15 means a sustained 15% overload is tolerated by design. It doesn't change the base FLA figure itself, which stays tied to rated (not overloaded) horsepower. Where it does matter is overload-relay sizing, where code allows a different percentage above FLA depending on the nameplate service factor, a detail worth confirming against NEC 430.32 and a licensed electrician rather than assuming a fixed number.
Why is overload protection sized from FLA while the branch-circuit breaker is sized differently?
Because the two devices are protecting against different failure modes on the same circuit. Overload protection guards the motor's windings against a sustained, moderate overcurrent that would slowly overheat the insulation, so it is set close to FLA. Branch-circuit protection, the breaker or fuse, exists to clear a short circuit or ground fault fast, and is deliberately allowed to sit well above FLA so normal starting inrush doesn't nuisance-trip it. Two devices, two different multipliers of the same FLA number, both on the same conductor.
Does a motor running below full load draw proportionally less current?
Not proportionally, and not down at low load. A motor idling at 20% of its rated output can still draw a surprisingly large share of its FLA. A portion of a motor's current is magnetizing current, which sustains the magnetic field in the stator and has to flow whether the shaft is doing work or not. That fixed component means current tracks load fairly closely near full load but flattens out at partial load, so a lightly loaded motor draws noticeably more current than a simple straight-line guess would suggest.
What actually separates nameplate FLA, calculated FLA and NEC table FLA?
Three different numbers, all called "FLA," each answering a different question. Nameplate FLA is stamped on a specific motor after the manufacturer tests it, the most accurate figure that exists for that exact unit. Calculated FLA, like the estimate this page produces, is a from-scratch approximation useful before a motor model has even been picked, built from horsepower, voltage and assumed efficiency and power factor. NEC table FLA is neither measured nor calculated: it's looked up from the published code table by horsepower, voltage and phase, and it's the figure code work for conductor ampacity and short-circuit protection is actually built on.
Can I use this page's estimated FLA to size a real breaker or conductor?
Not for the final number: treat it as a planning figure for early-stage layout, before a motor model or its nameplate is available. For an actual installation, code-compliant sizing comes from looking up the horsepower, voltage and phase in NEC Table 430.250 (three-phase) or 430.248 (single-phase), then applying the relevant NEC 430 sizing rules. A licensed electrician confirms the final breaker, conductor and overload selection against the applicable code.