Best Fuse Buddy Tester

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A Fuse Buddy is a plug-in fuse adapter that lets you read how much current a circuit draws without cutting a wire. You pull the fuse, plug the Fuse Buddy into the empty socket, and put the original fuse back into the Fuse Buddy so the circuit keeps its protection, as the listings describe it. The result is an amp reading on the same fuse you were already checking. This roundup compares six Electronic Specialties (ESI) models by the current limits, fuse sizes, and included parts on their own Amazon listings. The family runs from 20 amps to 30 amps, and one model is a loop for a current probe instead of a meter.

Quick answer

For the most complete package, the ESI 310 Fuse Buddy Pro Test Kit lists an LCD with a bar graph, data hold, and auto power off. For the widest fuse-size coverage, the 309A Mini Kit lists Micro, Mini, and ATO sizes with three adapters. For a standard ATC fuse socket and a 30 amp ceiling, the 306B is the straightforward choice. The 304B tops out at 20 amps. The 305M is a Mini-size 30 amp tester. The 307M does not display a reading by itself: it is a loop meant for a current probe or scope.

What matters when choosing a Fuse Buddy

Fuse size and socket type

Automotive blade fuses come in several sizes, and a Fuse Buddy has to match the socket. The 306B and 304B are named as ATC testers. The 310 kit lists ATC in its Specification Met line. The 305M is named as a MINI. The 309A lists Micro, Mini, and ATO circuits, with three adapters in the box. Open your fuse box, identify the size, and match it to the listing before you order.

Current ceiling and resolution

The ceilings are explicit. The 304B tests up to 20 amps. The 306B, 305M, 309A, and 307M list 30 amps. Resolution is 0.1 amp on the 305M and the 309A, and 0.1 amp (100 mA) with an accuracy of plus or minus 2 percent of reading on the 310. A tester with a lower ceiling than your fuse rating can be overloaded by a heavy circuit, so choose a ceiling above the largest current the circuit can draw.

Display and power

The 310 lists a deluxe LCD with a full analog bar graph, data hold, and auto power off. The 304B and 305M list battery power and an ammeter measurement type. The 309A ships with a battery. The 306B listing gives a corded electric power source and does not describe a display in its bullets, so confirm how it reports a reading before you buy. The 307M has no display of its own; it relies on your current probe.

Complete kit or single adapter

The 309A includes a battery, a clear storage pouch, the Fuse Buddy, an instruction manual, and three adapters. The 304B lists a tester, battery, and manual. The 310 is sold as a test kit. Count what you need in your own vehicle: if you work on several fuse sizes, adapters matter more than a bar graph.

How to choose

• Choose the 310 Pro kit if you want a display with a bar graph and data hold for blower motor and fuel pump amp draws, per its bullets.
• Choose the 309A Mini Kit if you work on newer vehicles with Micro or Mini fuses as well as ATO.
• Choose the 306B if your fuse box uses standard ATC fuses and you want 30 amps of range.
• Choose the 305M if the socket is a Mini fuse and you want a 0 to 30 amp range at 48 V DC.
• Choose the 304B only if no circuit you test exceeds 20 amps.
• Choose the 307M if you already own a current probe or an automotive scope and need a socket-friendly access point.

For related tools, see our DC current amp meter picks, our 12V tester light picks, and our power probe short finder picks. These cover other ways to chase a draw or a short at the fuse box.

Worked example: pick a ceiling and read the resolution

For example, a 12 V fuel pump circuit draws 6.0 A. Power is 12 V x 6.0 A = 72 W. A 20 A tester such as the 304B shows that current with 14 A to spare, and a 30 A tester has 24 A to spare. On a 0.1 A resolution, a reading of 6.0 A means the true current sits between 5.95 A and 6.05 A. The 310 lists plus or minus 2 percent of reading, which on 10 A is 0.2 A, so a 10 A draw reads between 9.8 A and 10.2 A. Now take a 25 A circuit: it exceeds the 304B’s 20 A limit by 5 A, so only the 30 A models fit. The 307M’s bullets say do not exceed 30 amperes.

Listing conflicts to know about

• The 309A spec table lists Maximum Current as 1 A, while its Current Rating and bullets say 30 A. The bullets state it can be used on circuits up to 30 Amps.
• The 304B and 305M list Item Weight as 0.01 ounces, which does not fit their stated dimensions.
• The 306B lists Corded Electric as its power source while the 304B, 305M, and 310 list battery power.

FAQ

What does a Fuse Buddy do?

It plugs into a fuse socket so you can read the current in the circuit. The listings say the original fuse is replaced in line during testing, so the circuit keeps its fuse protection.

Which Fuse Buddy fits Micro and Mini fuses?

The 309A Mini Kit lists Micro, Mini, and ATO circuits with three adapters. The 306B and 304B are named ATC testers, and the 305M carries a Mini label.

What is the highest current a Fuse Buddy here can test?

Four listings state 30 amps: the 306B, 305M, 309A, and 307M. The 304B states 20 amps. The 310 listing gives no ceiling in its bullets.

Does the 307M show a reading?

No. Its bullets describe a loop for a current probe or automotive scope. You clamp the probe over the loop to read the amperage.

Why does the 309A list a maximum of 1 A?

The spec table conflicts with the bullets and the Current Rating field, both of which say 30 amps. Ask the seller before you rely on either figure.

How accurate is the 310?

The listing states 0.1 A (100 mA) resolution and plus or minus 2 percent of reading. Other models in this group state resolution only or give no accuracy figure.

Can I use a Fuse Buddy for a fuel pump?

Several bullets cite fuel pump amperage checks, including the 306B, 304B, and 310. Match the fuse size and keep the draw below the model’s ceiling.

Bottom line

Match the fuse size first, then the current ceiling. The 309A covers the most fuse sizes, the 310 offers the fullest display, the 306B and 305M are straightforward 30 amp testers for ATC and Mini sockets, the 304B suits only light circuits, and the 307M is for people who already own a current probe. Resolve the listing conflicts with the seller before you rely on a stated figure.