Interactive selector
Calculate Fuse Size
Convert the load to amperes, apply a declared sizing factor, and identify the first matching nominal rating.
Important
This is a preliminary nominal ampere rating, not a complete fuse selection. Verify conductor ampacity, equipment instructions, fuse class, AC/DC voltage rating, interrupting rating, time-current behavior, ambient conditions, inrush, and applicable code.
Nominal ratings
Standard Fuse Size Chart
Use the table to find the first nominal rating at or above the calculated minimum, then complete the application checks.
Search by amp rating or filter by rating band. The recommended row is highlighted after calculation.
No listed fuse ratings match the current search or filter.
| Current values | Classification and basis | Required verification | Copy | ||||
|---|---|---|---|---|---|---|---|
| Source scope | |||||||
| 1 A | 1 | 0.8 | Low amp | Additional fuse-only rating | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 3 A | 3 | 2.4 | Low amp | Additional fuse-only rating | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 6 A | 6 | 4.8 | Low amp | Additional fuse-only rating | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 10 A | 10 | 8 | Low amp | Additional fuse-only rating | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 15 A | 15 | 12 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 20 A | 20 | 16 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 25 A | 25 | 20 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 30 A | 30 | 24 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 35 A | 35 | 28 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 40 A | 40 | 32 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 45 A | 45 | 36 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 50 A | 50 | 40 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 60 A | 60 | 48 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 70 A | 70 | 56 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 80 A | 80 | 64 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 90 A | 90 | 72 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 100 A | 100 | 80 | Branch / equipment | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 110 A | 110 | 88 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 125 A | 125 | 100 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 150 A | 150 | 120 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 175 A | 175 | 140 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 200 A | 200 | 160 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 225 A | 225 | 180 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 250 A | 250 | 200 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 300 A | 300 | 240 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 350 A | 350 | 280 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 400 A | 400 | 320 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 450 A | 450 | 360 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 500 A | 500 | 400 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 600 A | 600 | 480 | Industrial | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 601 A | 601 | 480.8 | High current | Additional fuse-only rating | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 700 A | 700 | 560 | High current | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 800 A | 800 | 640 | High current | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 1000 A | 1000 | 800 | High current | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 1200 A | 1200 | 960 | High current | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 1600 A | 1600 | 1280 | High current | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 2000 A | 2000 | 1600 | High current | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 2500 A | 2500 | 2000 | High current | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 3000 A | 3000 | 2400 | High current | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 4000 A | 4000 | 3200 | High current | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 5000 A | 5000 | 4000 | High current | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
| 6000 A | 6000 | 4800 | High current | Standard fuse / inverse-time breaker sequence | NEC 240.6 sequence summary | Verify application, conductor, equipment, voltage, class, and interrupting rating. | |
The “Load at 125% sizing” column equals nominal rating ÷ 1.25. It is a reverse-calculation aid only—not a universal permissible load, continuous-load rule, or substitute for the applicable code and product instructions.
Table tools
Key takeaways
What the Result Means
- Load current is converted from the entered electrical or mechanical value.
- Calculated minimum equals load current multiplied by the selected preliminary sizing factor.
- First standard rating is the smallest listed nominal rating not below that minimum.
- Final selection still depends on the protected conductor or equipment and the exact fuse family.
Why the Calculated Fuse Size May Still Be Wrong
A higher nominal rating can violate conductor or equipment limits even when it is the next standard value. Motor, transformer, HVAC, photovoltaic, semiconductor, and automotive circuits can use application-specific rules, nameplate instructions, and time-current requirements that override a general multiplier.
Fuse ampere rating is only one selection property. The fuse must also be appropriate for circuit voltage, AC or DC interruption, available fault current, response speed, ambient conditions, pulse or inrush energy, fuseholder rating, coordination, dimensions, and required listings or approvals.
Fuse Sizing Formulas
The calculator first converts the entered load to amperes. It then multiplies that current by the selected preliminary sizing factor and chooses the first listed nominal rating at or above the result.
Current conversion
- DC or single-phase VA
- I = S ÷ V
- Three-phase VA
- I = S ÷ (√3 × VLL)
- Single-phase watts
- I = P ÷ (V × PF × η)
- Three-phase watts
- I = P ÷ (√3 × VLL × PF × η)
Variable definitions
- ICurrent in amperes
- PReal power in watts
- SApparent power in volt-amperes
- VDC, single-phase, or line-to-line voltage as selected
- PFPower factor as a decimal from greater than 0 through 1
- ηEfficiency as a decimal
Worked Fuse Sizing Examples
16 A continuous load
Assumption: a preliminary 125% factor is selected.
16 A × 1.25 = 20 A.
The first listed nominal rating is 20 A. Confirm that the conductor and equipment permit that rating.
5 kW, 240 V single-phase load
Assumptions: PF = 1.00 and efficiency = 100%.
5,000 W ÷ 240 V = 20.83 A. At 125%, the preliminary minimum is 26.04 A.
The first listed nominal rating is 30 A, subject to all conductor, equipment, and application limits.
15 kVA, 480 V three-phase load
15,000 VA ÷ (√3 × 480 V) = 18.04 A.
At 125%, the preliminary minimum is 22.55 A, so the first listed rating is 25 A.
Transformer or equipment-specific rules may produce a different permitted protective-device rating.
Fuse Selection Checklist
1. Confirm the protected item
- Branch-circuit conductor
- Equipment or component
- Motor, transformer, HVAC, PV, or electronic load
2. Verify electrical ratings
- Nominal ampere rating
- AC or DC voltage rating
- Interrupting rating above available fault current
3. Verify operating behavior
- Fast-acting or time-delay response
- Starting current, inrush, or pulse energy
- Ambient temperature and enclosure conditions
4. Verify installation constraints
- Fuse class and rejection features
- Fuseholder rating and dimensions
- Coordination, current limitation, and approvals
Fast-acting vs time-delay fuses
| Characteristic | Fast-acting | Time-delay |
|---|---|---|
| Typical behavior | Opens quickly during sustained overcurrent. | Allows defined temporary inrush before opening. |
| Common concern | Nuisance opening on startup or charging current. | Excess delay if applied where rapid component protection is required. |
| Selection basis | Use the equipment instructions and the exact manufacturer time-current data for the chosen fuse family. | |
Common fuse selection mistakes
- Replacing a nuisance-opening fuse with a larger rating without identifying the cause.
- Assuming an AC fuse is automatically suitable for the same DC voltage.
- Ignoring conductor ampacity, equipment maximum overcurrent protection, or terminal limits.
- Confusing nominal ampere rating with fuse class, interrupting rating, or response speed.
Engineering Basis and Sources
Scope: preliminary current conversion, declared sizing factor, nearest listed nominal rating, and fuse-selection checks.
This page uses an original calculation workflow and a compact summary of the standard nominal rating sequence in NEC 240.6, including the additional fuse-only ratings. It does not reproduce manufacturer product tables or time-current datasets.
- NFPA 70®, National Electrical Code®Supports overcurrent-protection principles, standard rating concepts, and application limitations. Use the locally adopted edition.
- ICC public NEC provisions excerpt — Section 240.6Provides a publicly viewable excerpt of the standard ampere rating sequence and the additional fuse-only ratings used by this reference.
- Littelfuse Fuseology Design GuideSupports the broader fuse-selection factors beyond nominal current, including voltage, ambient conditions, pulse energy, time-current behavior, and interrupting rating.
Dataset and Review Details
- Publisher
- Turn2Engineering
- Page type
- Educational calculator and transformed reference
- Canonical units
- Amperes, volts, watts
- Calculation method
- Power-to-current conversion, declared factor, first qualifying listed rating
- Known exclusions
- Manufacturer product tables, time-current curves, product availability, and project approval
- Rights status
- Original calculations and standards summary; CSV export disabled
Fuse Size Chart FAQs
Using a simplified 125% preliminary factor gives 20 A, so 20 A is the first matching listed rating. The conductor, equipment, fuse class, voltage, interrupting rating, and application rules must still permit it.
No. The next standard rating can exceed a conductor or equipment limit, and application-specific rules may prohibit upward selection. Confirm the governing limit before choosing a larger fuse.
Not automatically. A fuse must have an appropriate voltage and interrupting rating for the circuit and must be specifically suitable for the AC or DC application.
Fast-acting fuses respond quickly to sustained overcurrent, while time-delay fuses tolerate specified temporary inrush. Preserve the response type required by the equipment and verify the exact time-current curve.
That depends on the circuit and device. Branch-circuit overcurrent protection must protect conductors within the applicable rules, while supplemental or internal fuses may be selected to protect specific equipment components.
Use the Chart as a Starting Point
Calculate the load current, apply the stated preliminary factor, and identify the first listed nominal rating. Before installation, verify every conductor, equipment, voltage, interrupting, response-time, environmental, and code constraint for the actual circuit.