Fuse Size Calculator
This fuse calculator makes the following simplifying assumptions:
- Wiring is installed in a 30°C (86°F) ambient temperature. Consult Section 310.15(B) of the National Electrical Code (NEC) for ampacity correction factors where the ambient temperature is other than 30°C (86°F).
- The current flow in amps is a continuous load.
- Conductors are rated 60°C (140°F), 75°C (167°F), or 90°C (194°F). If no wire temperature rating is selected, a rating of 75°C (167°F) is used by default.
- There are not more than three current-carrying conductors in the raceway or cable, including spare conductors. Consult Section 310.15(C)(1) of the NEC for adjustment factors for more than three current-carrying conductors.
Note: This calculator is designed for solar power systems. We do not recommend using it for other types of electrical systems.
How to Choose a Fuse Size
There are two ways to choose a fuse size:
- Use the manufacturer’s recommended fuse size
- Calculate the fuse size yourself
Let’s look at an example for each scenario.
1. Use the Manufacturer’s Recommended Fuse Size
Let’s say you’re designing a solar power system and you want to place a fuse between your solar charge controller and battery bank. You’re using the Renogy Wanderer 30A as your charge controller.
To choose the right fuse size in this example, simply look for the manufacturer’s recommended fuse size in the charge controller’s documentation.
You grab the little booklet that came with your charge controller and flip to the section titled “Fusing”.
Doing so, you see that Renogy recommends using a 30A fuse from controller to battery.
Because it’s recommended by the manufacturer, that’s the size you’ll use.
2. Calculate the Fuse Size Yourself
Note: Always use the fuse sizes recommended by the manufacturer when available. Only calculate fuse size yourself when there is no recommended fuse size, and even then only if you possess the appropriate working knowledge and understanding of electrical systems.
Let’s say you’re designing a solar power system and you want to place a fuse between your inverter and battery bank. (Many inverters have built-in fuses, but let’s assume in this scenario that yours doesn’t.)
You consult your inverter’s documentation but don’t find a recommended fuse size.
To calculate the right fuse size in this example, you can either:
- Use our calculator. Fill out our calculator at the top of the page and it will provide a recommended fuse size based on your inputs.
- Calculate fuse size following NEC guidelines. This is the hardest way to size a fuse. Consult the NEC and its guidelines for overcurrent protection.
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Disclaimer: Footprint Hero has made every effort to ensure that the results generated by this fuse sizing calculator are correct and valid for the provided user inputs. However, Footprint Hero neither warrants nor accepts any liability for the use of this calculator. Use of this calculator and selection of specific input parameters often require engineering judgment on a case by case basis. The user is cautioned to seek competent engineering assistance in the use of this calculator and application of results generated by it. The user is further cautioned that the results generated are based upon certain simplifying assumptions, which may or may not be valid for a specific case. Fuse sizes generated by this calculator do not supersede recommended fuse sizes provided by manufacturers. Always use the wire and fuse sizes recommended by manufacturers when available. WARNING: All electrical systems present a risk of fire and other hazards. Footprint Hero has developed this calculator for use only by individuals having the appropriate working knowledge and understanding of electrical systems. This calculator is based on industry standards including NFPA 70, National Electrical Code (NEC). It is not a substitute for calculations based directly on these or other industry standards that may apply. This calculator may contain errors of entry or calculation. Footprint Hero expressly disclaims responsibility for any use of this calculator that results in inadequate wire size or circuit protection.