Fuse Size Calculator

Calculate

Auto selects the mode from the fields you fill; the selector overrides. Motor FLC selects Motor mode; continuous/non-continuous load selects General mode.

Confirm the edition your authority having jurisdiction (AHJ) enforces. NEC 2026 is published; local adoption varies, so the newest edition is not always the one enforced.

Load expected to run 3 hours or more; the device is sized at 125% of this (NEC 210.20(A)). General Load mode only.

Load not expected to run continuously; counted at 100% (optional). General Load mode only.

Allowable ampacity of the branch-circuit conductor after any derating; enables the conductor-protection check (NEC 240.4). Optional. General Load mode only.

Use the NEC table FLC (Tables 430.247–430.250) for the motor's voltage and horsepower, NOT the nameplate FLA (NEC 430.6(A)(1)). Motor Branch Circuit mode only.

Time-delay (dual-element) fuses use 175%; non-time-delay use 300% (NEC Table 430.52). The type changes the size substantially. Motor Branch Circuit mode only.

Enables the small-conductor rule (NEC 240.4(D)). Only 14, 12, and 10 AWG have per-material caps. Advanced — General Load only.

If the motor will not start, NEC 430.52(C)(1) Exception 2 permits a higher ceiling for starting only: time-delay up to 225%, non-time-delay up to 400%. Advanced — Motor mode only.

Informational only in motor mode — the branch-circuit fuse may exceed conductor ampacity; overload is separate (NEC 430.32). Advanced — Motor mode only.

Overview

Use this calculator to size a fuse by NEC rules for a general load or a motor branch circuit. For a general load, the device is sized at 125% of the continuous load plus 100% of the non-continuous load. For a motor branch circuit, the fuse is sized from the motor's full-load current using the NEC 430.52 multiplier. In both cases the result is rounded to a standard NEC fuse rating (NEC 240.6), and for general circuits the conductor-protection rules of NEC 240.4 are checked.

How to Use This Calculator

  1. Choose the calculation mode — Auto detects it from the fields you fill: motor full-load current selects Motor Branch Circuit; continuous or non-continuous load selects General Load.

  2. Select the NEC edition your jurisdiction enforces — 2026, 2023, 2020, or 2017. Use the edition your AHJ has adopted, not necessarily the newest published one.

  3. For a general load — enter continuous load current (A) and optionally non-continuous load current. Optionally enter conductor ampacity to verify protection per NEC 240.4.

  4. For a motor branch circuit — enter motor full-load current from the NEC table (not the nameplate) and select fuse type. Use the advanced section for the Exception 2 starting option.

  5. Click Calculate — get the combined badge (validity + sizing regime), recommended standard fuse, calculated requirement before rounding, governing rule, and conductor-protection status.

This is a screening aid. Final fuse selection must be verified by a licensed engineer and the authority having jurisdiction (AHJ) against the adopted NEC edition.

Inputs & Outputs

Inputs

  • •Calculation Mode — Options: Auto — detect from inputs, General Load, Motor Branch Circuit
  • •NEC Edition — Options: NEC 2023 (default), NEC 2026, NEC 2020, NEC 2017
  • •Continuous Load Current (A)
  • •Non-Continuous Load Current (A)
  • •Conductor Ampacity (A)
  • •Motor Full-Load Current (FLC) (A)
  • •Fuse Type — Options: Dual-element / time-delay (175%), Non-time-delay (300%)
  • •Conductor Size (small-conductor rule) — Options: Not entered — skip NEC 240.4(D) check, 14 AWG, 12 AWG, 10 AWG
  • •Conductor Material — Options: Copper, Aluminum
  • •Motor Will Not Start at Calculated Size — Options: No — use standard multiplier, Yes — show Exception 2 starting range
  • •Conductor Ampacity (informational, motor mode) (A)

Outputs

  • •Recommended Fuse (A)
  • •Calculated Requirement (A)
  • •Status

Formula

Calculator Formula

This calculator implements NEC overcurrent sizing rules. The calculation path depends on the selected mode.

General Load (NEC 210.20(A)):

Adjusted Load = 1.25 × I_continuous + 1.00 × I_non-continuous
Fuse = next_standard(Adjusted Load) [NEC 240.6]

Motor Branch Circuit (NEC Table 430.52):

Calc Amps = Multiplier × FLC
Fuse = next_standard(Calc Amps) [430.52(C)(1) Exception 1]

Where Multiplier = 1.75 for dual-element time-delay, 3.00 for non-time-delay.

Standard fuse ratings (NEC 240.6(A)):

1, 3, 6, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 601, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000 A

What is Fuse Size Calculation

Fuse size calculation is the process of selecting the correct ampere rating for a fuse based on the circuit's load, conductor, and the applicable code rules. A fuse is an overcurrent protective device: it opens the circuit when current exceeds a safe value for too long, protecting the conductor and equipment from overheating and fire. Because real fuses are made in discrete standard ratings defined by NEC 240.6, sizing a fuse means matching the calculated requirement to the nearest available standard rating, not an arbitrary number.

For a general load, the key rule is the continuous-load multiplier. A load that runs for three hours or more is treated as continuous, and the overcurrent device must be rated at least 125% of that continuous current plus 100% of any non-continuous current. This extra margin accounts for the heat a sustained load generates. The result is then rounded up to the next standard fuse rating.

For a motor branch circuit, the sizing is different. Motors draw a large starting inrush current that can be several times the running current, so a fuse sized only for running current would open every time the motor started. NEC 430.52 therefore allows the motor branch-circuit fuse to be sized as a multiple of the full-load current: 175% for a dual-element time-delay fuse, or 300% for a non-time-delay fuse. This fuse is not the motor's overload protection; overload is a separate device.

The conductor relationship between general and motor circuits differs in a way that often trips people up. On a general circuit, the fuse must not be larger than the conductor can be protected at, except for two narrow allowances. On a motor circuit, the branch-circuit fuse deliberately exceeds the conductor ampacity because overload protection is separate. Applying the wrong rule to the wrong circuit type is a common source of design errors.

Key Facts

  • A fuse must be one of the standard ampere ratings defined in NEC 240.6(A); the calculator selects from that fixed list.
  • For a general load, the overcurrent device is sized at 125% of the continuous load plus 100% of the non-continuous load (NEC 210.20(A)). The 125% factor is applied once, to the continuous portion only.
  • For a motor branch circuit, the fuse is sized from the full-load current times a multiplier that depends on fuse type: 175% for a dual-element time-delay fuse, 300% for a non-time-delay fuse (NEC Table 430.52).
  • The same motor can need very different fuses depending on fuse type — at a 28 A FLC, time-delay calls for 50 A and non-time-delay for 90 A.
  • Use the NEC table full-load current for motor sizing, not the nameplate full-load amps (NEC 430.6(A)(1)).
  • On a general circuit, the fuse must not exceed conductor ampacity except where NEC 240.4(B) or 240.4(D) permits. On a motor circuit, the fuse may exceed conductor ampacity because overload protection is separate (NEC 430.32).
  • The small-conductor rule (NEC 240.4(D)) caps overcurrent protection at 15, 20, and 30 A for 14, 12, and 10 AWG copper regardless of tabulated ampacity.
  • If the motor will not start at the calculated size, Exception 2 permits a higher ceiling for starting only — up to 225% for time-delay or 400% for non-time-delay. Use the smallest rating that lets the motor start.
  • NEC editions differ; the latest published edition is not always the locally adopted edition. The calculator follows the edition you select.

Applications

  • Branch-circuit and feeder fuse sizing for general continuous and non-continuous loads.
  • Motor branch-circuit protection sizing from table FLC using NEC 430.52 multipliers.
  • Conductor-protection verification — confirming a proposed fuse protects the conductor per NEC 240.4.
  • Fuse type comparison — visualizing how time-delay versus non-time-delay fuses change the required rating for the same motor.
  • Plan review and inspection support — screening a proposed fuse rating against the load, conductor, and NEC rules.
  • Teaching and code familiarization — the output names which rule governed each step for apprentices and exam preparation.

Example Calculation

Example 1 — General Continuous Load

Given:

  • Continuous load = 80 A, no non-continuous load
  • Conductor ampacity = 100 A

Step 1: Adjusted load = 1.25 × 80 + 0 = 100.0 A

Step 2: next_standard(100.0) = 100 A (|100 − 100| ≤ 0.05 → exact)

Step 3: 100 A fuse ≤ 100 A ampacity → conductor PROTECTED

Result: NORMAL / EXACT-STANDARD (green). Recommended fuse: 100 A.

Example 2 — Motor: Time-Delay vs Non-Time-Delay

Given: Motor FLC = 28 A

Time-delay fuse: 1.75 × 28 = 49.0 A → next_standard(49.0) = 50 A

Non-time-delay fuse: 3.00 × 28 = 84.0 A → next_standard(84.0) = 90 A

Same motor, fuse type alone changes size from 50 A → 90 A. Both: NORMAL / NEXT-SIZE-UP.

Example 3 — Conductor Unprotected

Given: Continuous load = 80 A, conductor ampacity = 85 A

Adjusted load = 100 A → next_standard = 100 A fuse.

100 > 85; next_standard(85) = 90 ≠ 100 → NOT covered by 240.4(B).

Result: CONDUCTOR-UNPROTECTED / NEXT-SIZE-UP (red). Conductor must be increased so ampacity ≥ 100 A.

Example 4 — NEC 240.4(B) Next-Size-Up Allowed

Given: Continuous load = 64 A, conductor ampacity = 75 A

Adjusted = 1.25 × 64 = 80 A → next_standard(80) = 80 A.

80 > 75; next_standard(75) = 80 = fuse_calc AND 75 ≤ 800 → 240.4(B) permits.

Result: NORMAL / NEXT-SIZE-UP (green). Conductor status: PROTECTED BY 240.4(B). Verify conditions.

Standards & References

  • NEC 240.6 — Standard ampere ratings for fuses and fixed-trip circuit breakers
  • NEC 240.4 — Protection of conductors, including 240.4(B) next-standard-size allowance (≤ 800 A, conditions apply) and 240.4(D) small-conductor limits
  • NEC 210.20(A) / 215.3 — Overcurrent device at 125% continuous + 100% non-continuous
  • NEC 430.52 / Table 430.52 — Motor branch-circuit short-circuit and ground-fault protection multipliers; Exception 1 (next standard size), Exception 2 (higher ceiling for starting)
  • NEC 430.6(A)(1) — Use table FLC (Tables 430.247–430.250), not nameplate FLA
  • NEC 430.32 — Motor overload protection (separate; not sized here)
  • NFPA 70, National Electrical Code — Official standard published by the National Fire Protection Association

Units

All load and conductor values are entered and displayed in amperes (A). Conductor size uses AWG (American Wire Gauge) designations — 14, 12, and 10 AWG — for the NEC 240.4(D) small-conductor rule. There is no metric/imperial toggle for fuse ratings or load currents: the NEC 240.6(A) standard rating list is defined in amperes only. Conductor sizes above 10 AWG are entered as a free-form ampacity value in amperes, not as an AWG/kcmil size. Metric mm² equivalents are not used in NEC fuse sizing.

Limitations

  • This calculator sizes the fuse ampere rating only and is a screening aid, not a substitute for a licensed engineer or the authority having jurisdiction.
  • It does not size or derate conductors — conductor ampacity is a value you supply after applying temperature, terminal, ambient, and bundling adjustments.
  • It addresses the ampere rating only, not interrupting rating (AIC), available fault current, voltage rating, AC/DC suitability, fuse class, or current-limiting let-through behavior.
  • It does not size motor overload protection (NEC 430.32), transformer protection (NEC 450.3), tap conductors (NEC 240.21), or welder, capacitor, air-conditioning (440), or photovoltaic (690) circuits.
  • It is for fuses only, not inverse-time circuit breakers, motor circuit protectors, or supplementary protective devices.
  • It assumes equipment not listed for 100% continuous operation; listed equipment with a nameplate maximum fuse size must be protected at or below that value, which overrides generic NEC sizing.
  • The applicable rules depend on the adopted NEC edition; values must be confirmed against the edition the local jurisdiction enforces.

Common Mistakes to Avoid

  • Applying the 125% factor twice — it applies once, and only to the continuous load, not to the total of continuous plus non-continuous.
  • Using the nameplate full-load amps for motor sizing instead of the NEC table full-load current required by NEC 430.6(A)(1).
  • Ignoring the fuse type on a motor circuit — time-delay uses 175% and non-time-delay uses 300%, producing very different sizes for the same motor.
  • Upsizing a fuse to stop nuisance opening on a general circuit — this leaves the conductor unprotected. The fix is a larger conductor or corrected load, not a larger fuse.
  • Confusing the conductor rules — on a general circuit the fuse must protect the conductor; on a motor circuit the fuse may exceed conductor ampacity because overload is a separate device.
  • Overlooking the small-conductor cap — a 14, 12, or 10 AWG conductor is capped at 15, 20, or 30 A for copper regardless of any tabulated ampacity.
  • Treating the Exception 2 ceiling as the recommended size — it is a starting ceiling only; use the smallest standard rating that lets the motor start.
  • Assuming the latest published NEC applies — many jurisdictions lag the published edition by one or more cycles.

Frequently Asked Questions

What size fuse for a 20 amp continuous load?
A continuous load is sized at 125%, so 20 A × 1.25 = 25 A, which is a standard rating — a 25-ampere fuse, provided the conductor and equipment permit it. If the 20 A were non-continuous, it would be counted at 100%, requiring a 20-ampere fuse (or next size up to a standard rating).
How do I size a fuse for a motor?
Multiply the motor's NEC table full-load current by the NEC Table 430.52 factor for your fuse type — 175% for a dual-element time-delay fuse or 300% for a non-time-delay fuse — and round up to the next standard rating. Use the table full-load current, not the nameplate value, as required by NEC 430.6(A)(1).
Why does the fuse type change the motor fuse size so much?
A dual-element time-delay fuse tolerates the motor's starting inrush, so it can be sized closer to the running current at 175%. A non-time-delay fuse does not tolerate inrush as well, so it needs more headroom at 300%. The same motor therefore needs a larger non-time-delay fuse than a time-delay fuse.
Can I use the next standard fuse size above my conductor ampacity?
Sometimes. NEC 240.4(B) permits a fuse one standard size above the conductor ampacity for ampacities of 800 A or less, where the allowance's conditions are met and it is not a multioutlet receptacle branch circuit. The conductor must still satisfy NEC 240.4 overall.
Why is my motor fuse larger than the conductor ampacity?
On a motor branch circuit, the fuse provides short-circuit and ground-fault protection only and is allowed to exceed the conductor ampacity. The conductor's overload protection is a separate device under NEC 430.32. This is normal and code-compliant for motor circuits, unlike general circuits.
Which NEC edition does the calculator use?
You choose the edition your jurisdiction enforces — 2026, 2023, 2020, or 2017. The newest published edition is not always the one adopted locally; adoption can lag by years, so confirm with your authority having jurisdiction.
Does this calculator check conductor protection?
Yes, for General Load mode. If you enter the conductor ampacity, the calculator checks NEC 240.4 — whether the fuse is within the ampacity, permitted by the 240.4(B) next-standard-size allowance, or flagged as CONDUCTOR-UNPROTECTED. If you also enter conductor size and material, the 240.4(D) small-conductor cap is checked even without an ampacity value.
Does this calculator replace an engineer or check AIC and fuse class?
No. It is a screening aid that sizes the ampere rating using NEC rules. It does not address interrupting rating (AIC), available fault current, fuse voltage rating, class, or DC suitability, and it does not replace a licensed engineer or the authority having jurisdiction.

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Engineers often use these calculators in combination for complete project workflows:

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Motor Current Calculator

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