How to Replace a Fuse in a Fuse Box
Replacing a Blown Fuse
Sooner or later nearly everyone is going to have to know how to replace a blown fuse in a fuse box - fuses don't last forever and overloads or electrical shorts are always possible with the result always being that changing a fuse is required. As an electrician myself, I have not only replaced more than a few fuses but installed thousands in new construction work - some commercial buildings may have hundreds in just one school or large store.
One of the first things that is needed is a replacement fuse, but there are literally thousands of different types of fuses available. It is far beyond any single article to look at every style, type and size of fuse, but some of the more common types of fuses will be covered.
Secondly, the fuse must be replaced with a new fuse. The old one must be removed from the clips holding it in the fuse box and a new one installed. Again, every possibility cannot be covered in one article, but common fuses and how to remove and replace them will be discussed.
Types of Fuses That Might Be Encountered
The first thing in finding a replacement fuse is not necessarily the physical type or size, but the rated ampacity of the fuse - the electrical size in other words. Every fuse has a particular number of amps that it will carry; go beyond that and it will blow. Never replace a fuse with one rated at a higher ampacity to prevent blowing it; the wiring carrying the current is carefully sized to the size of the fuse and requiring it to carry more amps than it is rated for is a majorcause of electrical fires.
Many fuses are of the "time delay" or "slo-blow" type in that they will tolerate a short period of overload without blowing. This is done to accommodate motors of all types with their high current requirements as they come up to speed, but if that high level of current continues the fuse will still blow. In general it is not a bad idea to purchase time delay fuses and in many cases that is all that is available. One of the most common exceptions is for fuses in electronic equipment; quickly blowing a fuse on a circuit board that has been overloaded by a failing component can save the rest of the board.
Commonly used in older homes that were constructed prior to the introduction of circuit breakers,these fuses screw into a socket similar to a light bulb and are easily changed. There are two basic thread sizes; make sure that you have the correct size of thread on the new fuse; take the old one to the store with you when buying. The threads are considerably different and it is easy to distinguish between the two and adapters are available to convert a large socket to the smaller size. Occasionally these fuses may also be found with a switch controlling a remote motor such as an attic ventilation fan.
Some plug fuses are of the "t" type and require an adaptor to use; this is done to prevent tampering with the fuse box. If this is the case, make sure you have the adaptor as well.
Plug FusesClick thumbnail to view full-size
These are less common in home use, but are still found in many pieces of outdoor equipment. Appliances such as well pumps, outdoor air conditioner units and even roof top swamp coolers may have a cartridge type fuse in a small electrical box.
More care must be taken here in choosing a new fuse as cartridge fuses come in many, many physical sizes. They can vary from an inch long to a foot or more. They can have a groove at one end that fits into only one of the clamps so that the fuse must be installed in one direction only. Diameter can vary from as little as ¼" to several inches.
Each cartridge fuse will have a number/letter designation stamped on it, along with an ampacity. Matching both is necessary for proper replacement - the same ampacity is absolutely critical and the letter designation indicates the type of fuse, whether it is a time delay or other specialty fuse. Make sure that both match when replacing any cartridge fuse.
Different Cartridge FusesClick thumbnail to view full-size
Automobiles may have any of three different fuses; a glass tube fuse, and "blade" fuses in both regular and mini sizes. All three are again available in various amperage ratings which must be matched exactly between the blown fuse and it's replacement.
Blade fuses are typically color coded, with the color indicating amperage; replace one of these with the same color as is being removed. Blade fuses come in two physical sizes, the ATC pictured here and the smaller "mini" size. The fuse box will only accept one size, however, so make sure you have the right physical size. Blade fuses are commonly available in a variety pack such as shown here and this is a perfect way to purchase them for a car, boat or RV. This kit even came with a tester and fuse puller.
Glass tube fuses come in many, many different aperages and physical sizes. Amperages as low as 1/10 of an amp are available as they are common in electronic circuit boards where only very small amperages are used. Automobiles will use a much higher amperage, typically from 15 to 30 amps. These fuses can even be found in some Christmas tree light sets, as an "in line" fuse; a small fuse container that is part of the cord assembly.
Automotive fusesClick thumbnail to view full-size
Fuse pullers from Amazon
Primarily used for automotive fuses.
Changing the Fuse
Changing a Plug Fuse
It is generally pretty easy to tell if a plug fuse is blown; the glass window will either be blackened or the silver wire inside will be burned in half. If you are unsure, though, the article on how to check a fuse will walk you through checking the fuse.
With the right replacement fuse on hand, changing a plug fuse is a very simple task. Simply unscrew the blown fuse just as you would a light bulb and screw the new one in. In very rare cases, the fuse may be stuck; very careful use of pliers may be required to break it free. Extreme caution is the name of the game here as if you crush the fuse with pliers it is probably time to call an electrician to dig it out of the fuse box.
Changing a Cartridge Fuse
This is a little more difficult as removing the old fuse isn't quite so easy. A fuse puller is extremely handy here and are quite inexpensive. Examples of some fuse pullers are shown above, available from either Amazon or eBay.
If there is any question that power is still on at the fuse, check with a non contact voltage detector or voltmeter before proceeding. Do not attempt to remove a cartridge fuse that is still has power to it!
If a fuse puller is available, grasp the fuse near the center with the puller and pull the fuse straight out of the clips holding it. If you have waited too long to buy a fuse puller, these fuses can often be removed by prying with a screwdriver or using pliers, but be aware that squeezing too hard will crush the fuse and shatter a glass tube fuse.
Holding the replacement fuse in place, push the bottom end into the clips. A hammer handle can be used for large fuses or other tool for smaller ones if finger pressure won't do the job. Be sure to press hard on only the metal ends as the fuse can be broken by pressing hard in the center. If the fuse has a groove on one end, make sure it is not upside down as it will not fit the spring clamps if it is. With one end fitted into the clamps, push the top end in as well. The fuse should be centered as well as possible in the clamps, without having one end protruding; many clamps will not accept a fuse that isn't aligned properly.
Changing a Blade Fuse
A fuse puller is almost a necessity here as these fuses are difficult to grab with pliers and tweezers usually can't supply the force necessary to remove it. Grasp the center of the fuse with the fuse puller, then, and pull it straight out. The new fuse is simply pushed into place, again making sure that it is aligned reasonably well.
Replacing a Glass Tube Fuse
These fuses will again need a fuse puller unless the ends are very accessible and can be reached with a screwdriver to pry them out. Use of pliers will almost inevitably result in a broken fuse, with broken glass scattered throughout the fuse box.
Grasp the fuse in the center with the fuse puller and pull the entire fuse out. Insert the new fuse into the fuse puller and push it into place in the clamps using the tool. It is often easier to use the fuse puller on these small fuse than fingers
Questions & Answers
I am trying to replace a plug fuse in my electric panel. I have been using "P" type 20 amp fuses. Now all I can find is "TC" type 20 amp fuses. This new kind is referred to as a "Dual Element Fuse" and is apparently designed with a time delay. Am I correct in thinking that the "Type" ("P" vs "TC") doesn't matter, but the "Amps" are what matters? Am I safe to replace the Type P with the Type TC as long as it is the same amount of amps? Thanks very much!
© 2012 Dan Harmon