Full-Load Amps Calculator
Every panel needs a full-load current on its nameplate, and it is not the number most people reach for. Add up the motors off their nameplates and you are wrong twice — once because the code makes you use the table value instead, and once because the loads that never run at the same time do not all belong in the sum. Put the load list in here and get the arithmetic back with the article number beside every line.
Work through it in three steps.
The full-load amps on the nameplate is a plain sum of everything that can run at once. The minimum supply conductor ampacity puts 125% on the largest motor, and on the resistance heating too if this is a machine. The maximum overcurrent device starts from the largest branch device in the panel and rounds down. Fill in the three cards below and every line comes back with the article it came from.
The incoming supply
Pick the basis first. An industrial machine is calculated under NFPA 79 and NEC Article 670; a general control panel is calculated under UL 508A and NEC Articles 409 and 430. They differ mainly in how resistance heating is treated.
Breakers bring their lugs with them. If the supply lands on a main breaker, or a branch runs through an MCP, that device’s lug marking sets which column of Table 310.16 you may size conductors from — the results below spell it out.
Everything in the panel
Motors entered by horsepower pull their current straight out of NEC Table 430.247 / 430.248 / 430.250, which is what NEC 430.6(A)(1) requires — the motor’s own nameplate amps are only used to size its overload relay. A VFD is entered at the drive’s rated input current, not the motor it drives. Loads that are interlocked so they can never run together go in the same interlock group, and only the worst-case combination is counted.
How to treat the edge cases
Both of these change the answer, so they are shown rather than buried.
How the number was built
What goes on the nameplate
Short-circuit current rating is a separate exercise — normally the UL 508A Supplement SB method — and is not calculated here. When you have all the fields, the Legend Plate Bench will engrave the data plate →
Every load, line by line
Standard motor full-load currents NEC Tables 430.247, 430.248 and 430.250 — the same values as UL 508A Table 50.1
What each number is standing on
- NEC 430.6(A)(1)
- Conductor ampacity, switch ratings and branch-circuit short-circuit and ground-fault protection are sized on the table full-load current, not the motor’s nameplate amps. 430.6(A)(2) is the exception: overload protection is sized on the nameplate.
- NEC 670.4(A)
- Industrial machinery supply conductors: 125% of all resistance heating loads, plus 125% of the highest rated motor, plus the full-load currents of all other motors and apparatus that may be in operation at the same time.
- NEC 430.24
- General feeders and control panels: 125% of the highest rated motor, plus the full-load currents of the other motors, plus 100% of the non-continuous non-motor load and 125% of the continuous non-motor load.
- NEC 670.4(C) · 430.62(A) · 430.63
- The supply overcurrent device may not exceed the largest branch-circuit device in the panel plus everything else that can run at the same time. Taken down to a standard size from NEC 240.6(A), because it is a ceiling and not a target.
- NEC 430.52 · Table 430.52
- Maximum branch-circuit short-circuit and ground-fault device as a percentage of motor FLC — 250% inverse-time breaker, 175% dual-element fuse, 300% non-time-delay fuse, 800% instantaneous-trip. The next higher standard size is permitted under 430.52(C)(1) Exception No. 1.
- NEC 430.122(A)
- Conductors to an adjustable-speed drive are sized on 125% of the drive’s rated input current. NFPA 79 says the same thing: on the line side, the drive’s input rating governs, not the motor it happens to be turning.
- NEC 220.60 · 430.24 Ex. 3
- Loads that cannot be energized at the same time are counted once, at whichever combination produces the highest total — the “largest heater” rule.
- NFPA 79 16.4
- The machine nameplate: supplier, model or serial number, rated voltage / phases / frequency and full-load current for each supply, ampere rating of the largest motor or load, maximum rating of the short-circuit and ground-fault device, the panel SCCR, and the electrical diagram number.
- UL 508A 49.2 · NEC 409.110(2)
- A control panel’s marked full-load current is the sum of the ampere ratings of all loads able to be operated simultaneously, plus the primary ampere rating of every control transformer on the incoming supply. No 125% factor is applied to the marked value.
- NEC Table 310.16 · 110.14(C)
- Conductor sizes shown are the smallest copper conductor at the 60 °C and 75 °C columns. Which column you may use depends on the temperature rating of the terminations at both ends.
Read this before you stamp anything. This calculator is an aid to design and estimating, not a substitute for a licensed engineer, the equipment manufacturer’s instructions, or the authority having jurisdiction. It does not apply the ambient-temperature correction or the conductor-bundling adjustment of NEC 310.15, it does not size grounding or bonding conductors, it does not check voltage drop, and it does not determine short-circuit current rating — that is a separate exercise under UL 508A Supplement SB. Duty-cycle and demand-factor allowances under NEC 430.22(E) and 430.26 are not applied. Verify every value against the edition of NFPA 70, NFPA 79 and UL 508A your project is actually held to. Think-PLC, LLC accepts no liability for designs built from these results.
We do this math on every panel we ship.
UL-listed control panel fabrication out of Winston-Salem, NC — calculated, drawn, wired, tested and marked, with the nameplate right the first time.
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