What size fuse/circuit breaker do I need?

Where do I need a fuse or circuit breaker.

Regulations require every un-grounded wire except those in the engine starting circuit to have circuit protection. This can be accomplished with either a fuse or a circuit breaker. Fuses tend to be more compact and less expensive, but need to be replaced after they have been activated. Circuit breakers are more costly, but can easily be reset. For more information on which type of protection best suits your needs, click here.

Regardless of whether you choose a fuse or circuit breaker, the way you calculate sizing will be the same.  The size of your circuit protection device should always protect the wire according to its rating. Some manufacturers will specify a fuse size, which will typically be lower than the maximum fuse size for the cable. If the recommended fuse size is too high to protect the cable, then a larger cable will be required for that circuit. More information on cable size selection can be found here.

To determine the required fuse or circuit breaker size for your application, take the following steps:

Choosing fuse/circuit breaker amperage

Most Enerdrive products have a recommended fuse size listed in the installation portion of the user manual. If possible, use the recommended fuse size and ensure that the cabling is of sufficient size. If you do not have a manufacturer recommended fuse size for the product your installing, you can take the following steps to determine the size of fuse/circuit breaker you will need.

Choosing the Fuse Amperage

STEP 1: Find Maximum Amperage

Use the chart below to determine the maximum fuse/circuit breaker size for the size of cable you are using. The maximum amperage will also depend on whether the protection is for a single or bundled wire, and whether the circuit is located inside or outside an engine room. For example, a single 4 AWG cable (roughly 25mm2) located outside an engine room has a maximum fuse amperage of 150A. Choosing an amperage closer to the wire's maximum will reduce the chance of a nuisance blow, but offers less protection for the wire.

STEP 2: Find the Minimum Amperage

To find the minimum fuse amperage, multiply the amperage of the product you are using by 1.25. For example, a 40A ePOWER AC charge has a minimum amperage of 50A (40 x 1.25 =50). Choosing an amperage close to the minimum offers better protection but increases the chance of a nuisance blow.

STEP 3: Select an Amperage in the Middle

Choose an amperage close the mid-point of he maximum and minimum amperage for the circuit

STEP 4: Choose a Fuse or Circuit breaker

The chart below shows the range of sizes available for each different type of fuse. Enerdrive also offers circuit breakers in variety of different styles, including push button, surface mount, and rocker switch.

Depending on your application, you might also need to consider fuse/circuit breaker properties other than amperage rating. Some fuses and circuit breakers are designed to trip quickly (e.g. blade fuses), while others require the current to be above the rating for a length of time before tripping (ANL fuses). Circuit protection devices also have an ampere interrupting capacity that determines the maximum fault current can be interrupted without the device failing. For more information on fuse/circuit breaker properties, consult with the specification sheet provided by the manufacturer.

To view the range of fuses offered by Enerdrive, click here.

To view the range of circuit breakers offered by Enerdrive, click here.

Click the image below to enlarge

Step 5: Select a Fuse Holder or Fuse Block

If you decide to use a fuse with your system, then you will need to select a fuse holder or block as well. In-line fuse holders are typically more compact and hold a single low amperage fuse, while fuse blocks are larger but can accommodate multiple fuses at once. You may also want to purchase a fuse block/holder offers ignition protection, ingress protection, or an insulating cover depending on your application.

Enerdrive also offers its own range of fuse holders, which can be found here.