Every fuse has markings that tell you three things: how much current it can handle, what voltage it’s rated for, and how quickly it reacts to an overload. Once you know what to look for, reading a fuse takes seconds. The specifics vary depending on whether you’re looking at a glass tube fuse, a car’s blade fuse, or an industrial cartridge, but the core information is the same across all types.
The Three Numbers That Matter
The most important marking on any fuse is its amperage rating, which tells you the maximum current the fuse can carry before it blows. This is printed as a number followed by “A” (for example, 20A means 20 amps). On smaller fuses, the “A” may be omitted, leaving just the number.
Next is the voltage rating, which indicates the maximum voltage the fuse can safely interrupt. You’ll see this written as something like 250V or 550 Vac. A fuse rated at 250V can be used in any circuit at or below that voltage, but never above it. Using a fuse in a higher-voltage circuit than it’s rated for can cause it to arc internally instead of cleanly breaking the circuit.
Some industrial fuses also display an interrupting rating (such as 80 kA), which describes the maximum fault current the fuse can safely stop. This matters in commercial electrical panels where short circuits can produce enormous surges. For household and automotive fuses, you generally won’t see this marking.
Speed Codes: Slow-Blow vs. Fast-Acting
A single letter or pair of letters on the fuse body tells you how quickly it responds to an overcurrent. These codes come from German abbreviations and appear on most small cartridge fuses:
- FF: Very fast acting
- F: Fast acting
- M: Medium acting
- T: Slow acting (also called slow-blow)
- TT: Very slow acting
Fast-acting fuses (F or FF) blow almost instantly when current exceeds their rating. They protect sensitive electronics that can’t tolerate even brief surges. Slow-blow fuses (T or TT) are designed to handle short current spikes, like the momentary surge when a motor starts up, without blowing. They only trip when the overcurrent is sustained. If you replace a slow-blow fuse with a fast-acting one, it will likely blow immediately under normal operation. If you put a slow-blow fuse where a fast-acting one belongs, you lose the quick protection that circuit needs.
Breaking Capacity Markings
Some fuses carry an “H” or “L” on the body. H means high breaking capacity, and L means low breaking capacity. This describes how large a fault current the fuse can safely interrupt without rupturing or arcing. High breaking capacity fuses are used in circuits where a short could produce very large currents, such as near the main power supply. For most household applications, you won’t need to worry about this marking, but if you see it, match it when replacing the fuse.
Reading Automotive Blade Fuses by Color
Automotive blade fuses skip the fine print and use a standardized color code instead. Each color corresponds to a specific amperage, and the number is also stamped on top of the fuse. The most common ratings you’ll encounter in a car’s fuse box:
- Red: 10A (interior lights, small accessories)
- Blue: 15A (windshield wipers, heated seats)
- Yellow: 20A (power sockets, audio systems)
- Green: 30A (heavy-duty accessories like sunroofs)
This color system applies across Micro, Mini, and standard ATO blade fuses. The physical size of the fuse varies between these types, but the color-to-amperage mapping stays the same. Your vehicle’s owner manual or the inside of the fuse box cover will have a diagram showing which fuse position controls which circuit, so you can quickly find the one you need to check.
Plug Fuses in Older Homes
If your home has a fuse panel instead of a circuit breaker box, you’ll find screw-in plug fuses. The original style is the Edison base, which has a standard screw thread like a light bulb. The amperage is printed on the face of the fuse, and a small window lets you see the metal strip inside.
The problem with Edison base fuses is that any amperage fuse fits the same socket. This made it easy for homeowners to swap a blown 15A fuse for a 30A one, a dangerous practice called overfusing that can overheat wiring and start fires. Type S fuses solve this problem with a tamper-resistant adapter system. Each adapter has a unique thread size matched to a specific amperage. Once you screw a 15A adapter into the socket, only a 15A Type S fuse will fit. You physically cannot force a higher-rated fuse into that adapter. If your panel uses Type S fuses, always pair the fuse with its dedicated adapter and never try to bypass the system.
How to Tell if a Fuse Is Blown
Before you grab a replacement, you need to confirm the fuse has actually failed. The method depends on the fuse type.
Glass Tube Fuses
Hold the fuse up to a light and look at the thin metal wire running between the two metal caps. A good fuse has a continuous, unbroken wire. A blown fuse will have a visible break in the wire, or the inside of the glass will be darkened or coated with metallic residue from the wire vaporizing. If the glass is blackened, the fuse blew from a significant overcurrent or short circuit.
Blade Fuses
Automotive blade fuses have a visible metal strip running in a U-shape at the top of the colored plastic body. Pull the fuse out (a small plastic fuse puller is usually clipped inside the fuse box) and look at that strip. If it’s broken, melted, or if the plastic around it shows burn marks, the fuse is blown.
Ceramic Cartridge Fuses
These are trickier because you can’t see through the body. Look for bulging, cracks in the ceramic, or burn marks around the metal end caps. If the body looks normal, you’ll need a multimeter to confirm whether the fuse is good.
Testing a Fuse With a Multimeter
When you can’t visually confirm a fuse’s condition, a multimeter gives you a definitive answer. Turn off the power to the circuit first, then remove the fuse from its holder. Testing a fuse while it’s still installed can give false readings because current can flow through parallel paths in the circuit.
Set your multimeter to the continuity setting, which is usually marked with a diode symbol or the word “CONT.” Touch one probe to each end of the fuse. It doesn’t matter which probe goes on which end. If the multimeter beeps, the fuse is good. No beep means the fuse is blown.
You can also test using resistance mode, marked with the omega symbol (Ω). A good fuse shows very low resistance, near zero ohms. A blown fuse displays “OL” (overload) on the screen, meaning infinite resistance. There’s no current path left because the internal wire has broken.
Replacing a Fuse Safely
The single most important rule: replace a blown fuse with one that has the exact same amperage rating. Installing a higher-rated fuse doesn’t fix the problem. It just allows more current to flow through wiring that isn’t designed to handle it, which can melt insulation and start fires. If a new fuse of the correct rating blows again quickly, that points to a deeper electrical problem in the circuit, not a fuse that’s too small.
Match the voltage rating and speed code as well. In a car, turn off the ignition before pulling or inserting fuses. In a home fuse panel, stand on a dry surface, use one hand, and avoid touching the metal threads of the fuse as you screw it in. Keep a small assortment of spare fuses in your car’s glove box and near your home’s fuse panel so you aren’t hunting for the right one in the dark.