July, 2018
Inspection News and Views from the American Society of Home Inspectors

Distribution Panels: Fuse Fundamentals


Fuse panels were installed through much of the 20th century, but with the introduction of circuit breaker panels in the 1960s, fuse panels had fallen out of favor by the 1970s. Many fuse panels remain in use and feature the glass-top Edison “screw-in” (plug) fuses for 120-volt branch circuits, cartridge fuses for 240-volt appliances and larger blade-type cartridge fuses in disconnects. 

Fuses are less convenient than circuit breakers. When a fuse blows, it must be replaced. There is no resetting it to restore operation, as with circuit breakers. This can be a nuisance, especially when a spare fuse is not readily available. Fuses are reliable overcurrent protection devices. Properly sized, installed and maintained, fused systems need not be replaced simply because they are not circuit breakers. 

Remember, inspecting electrical components and systems is inherently dangerous and can lead to electrocution, shock or serious burns. Do not perform electrical inspections without training and taking appropriate protective measures. 

A note on terminology: This discussion will use both technically correct and commonly used terms to describe components. If you use technical terminology in your reports, you should accompany it with explanations for your clients. For example, fuses and breakers are both overcurrent protection devices, but many homeowners would not understand the latter term. Conductors is the proper term, but most homeowners know them as wires.

The Updating Question
Fuse panels often are associated with older ungrounded wiring, a small number of receptacles and an absence of modern electrical devices, such as ground-fault circuit interrupters (GFCIs), arc-fault circuit interrupters (AFCIs) and tamper-resistant receptacles. A replacement panel with circuit breakers is a practical element of modernization of the electrical system, particularly if the service size (ampacity) is small. (Most jurisdictions will require a minimum of a 100-amp, 120- to 240-volt service on replacement installations.) 


Overfusing is the most common adverse condition in fused panels. As with circuit breakers, you determine this condition by removing the panel cover to compare the conductor (wire) size to the fuse rating. A 14-gauge copper wire with a 25-amp fuse is not adequately protected and is often said to be overfused.

Overfusing increases the potential for overheating in the circuit. Overfusing is often a homeowner error, easily corrected by replacing the 25-amp fuse with the appropriate 15-amp fuse. When you find any overfusing during an inspection, you should recommend immediate correction and make sure the client understands the safety implications. Most jurisdictions require that any work on panels with Edison-base fuses include installation of non-tamperable adapters and type-S fuses.

Compare fuse size to the wire size. In situations for which you can’t verify that they are correct, recommend that they be checked by a qualified electrician. In situations where you cannot examine fuse pullouts for fear of disconnecting power to an appliance, recommend a follow-up inspection of these fuses. (More on fuse pullouts later.)

The wire size is sometimes legible on the cable sheathing or markings on the wire insulation. Where missing or illegible, compare the wire to known wire samples, either in the panel or in samples you carry with you. Visual identification of wire size is a useful skill to develop.

Often, fuse sizes are plainly marked. Most plug fuses are color coded: blue = 15 amps (typically with a hexagonal window; larger fuses have round windows); orange = 20 amps; red = 25 amps; green = 30 amps.

Fuses for electric heaters may seem oversized when they are properly sized. A 14-gauge wire typically requires 15-amp fuses, but an electric heater may be adequately protected with 20-amp fuses. This can be true if the wire is copper and the circuit is dedicated to a permanent electric heater.

Also, dedicated circuits for air conditioners may seem overfused, but they can be acceptable. The data plate on the air conditioner lists the fuse or breaker size, as well as the minimum circuit ampacity. These are dedicated circuits with a single fixed load. General convenience circuits with several receptacles are susceptible to overloads, so the correct fuse size is necessary to protect the circuit from overheating. Dedicated circuits are unlikely to overload wiring to a water heater or air conditioner. 

Special fuse types
It is useful to understand different fuse types. This is especially true in situations where seemingly right-sized fuses blow frequently.

  • Type-T fuses are conventional Edison-base fuses. All have the same thread. A 15-amp, 20-amp, 25-amp or 30-amp fuse all fit into the fuse socket; therefore, Type T fuses are susceptible to overfusing.
  • Type-S fuses have a rejection base that will only accept the appropriate size of fuse. Each size type-S fuse has a unique thread. This helps prevent overfusing. 
  • Type-W fuses are fast-acting fuses. They have no time delay and will blow quickly if the rated amperage is exceeded. These often are used for lights and general-purpose circuits for which devices with electric motors are not commonly used.
  • Type-SL and -TL fuses are medium-duty, time-delay fuses. They will not blow immediately if there is a circuit overload. These fuses are typically used with electric motors that often draw excess current during startup. Time-delay fuses prevent nuisance fuse blowing on circuits with electric motors. These are the most common fuses used today. The Type-TS has a rejection base; the Type-TL does not.
  • Type-S Heavy-duty Time-delay and Type-T Heavy-duty Time-delay fuses are used for circuits with motors that cycle on and off frequently (for example, sump pumps). These fuses have a longer time delay than the type-SL and TL-fuses.

Mini-breakers—when a fuse is not a fuse
Mini-breakers look like fuses and screw into Edison-base receptacles, but they are actually circuit breakers. Rather than a glass top, they have a solid cap with a reset button in the center. The advantage is these can be reset when they trip and do not need to be replaced. Many jurisdictions do not permit these devices because incorrect amperage mini-breakers can be readily installed. Some panel covers will not close properly with mini-breakers installed. The button on the breaker may interfere with the cover as it is closed.


Wire sizes for large appliances
It’s a good idea to know the common wire sizes used for the major appliances. This can help you recognize when the wire is inappropriate for the appliance. The table lists common wire sizes for larger appliances. As always, there are exceptions.

If the fuse rating is too large, the circuit is not adequately protected, increasing the potential for overheating and fire. If the fuse is too small, there is a risk of nuisance fuse blowing. If the fuse is correctly sized for the appliance, but the wire is too small, there is increased potential for overheating and excessive voltage drop, causing the appliance to run incorrectly. 

The electrical inspection must not be limited to the panel. What you may not know and won’t likely discover during your inspection of the panel, is that a 10-gauge conductor may be spliced with a 14-gauge conductor somewhere along the circuit. The 14-gauge wire is not adequately protected by the 30-amp fuse. Electrical codes don’t allow this practice for obvious reasons, but it is done. Where possible in unfinished spaces, try to follow the entire length of the larger (240-volt) appliance cables to check for splices or connection points. This is not feasible if the wiring is in conduit. 

Fuses bypassed
Fuses can be bypassed by putting a penny in a fuse holder or wrapping a fuse with foil. Electricity will flow through the fuse whether it has blown or not. The purpose of the fuse is defeated and you have a very dangerous situation. This condition is typically done by uninformed occupants.

Sometimes, you can see foil around the edge of a fuse. In other cases, you may see the fuse projecting out of the fuse holder more than other fuses. This may mean that a penny has been put in the base of the fuse holder. It can also mean that there is a fuse rejecter washer installed. Be careful. If a fuse is blown but the circuit is working, you know the fuse has been bypassed. Look for signs of overheated insulation on conductors (wires) in the panel.  Blown fuses typically have an obscured window from the melted fuse material.

Fuses loose
Loose fuses are caused by one of the following:

  • poor installation
  • vibration
  • adjacent fuses over-tightened (the bus bar bends)

Many electrical inspectors automatically tighten each fuse as they inspect the panel. It’s not up to home inspectors to do so; however, if you find loose fuses, report the condition and recommend correction. Home inspectors are not supposed to correct adverse conditions in houses, although you may be tempted to simply tighten the fuse rather than to write up a loose fuse as a defect. But as a reminder, don’t do any repairs. Simply report the condition and make a recommendation.

Fuse holders loose or broken
Sometimes, the supports for the fuses are worn or broken and will not stay securely in their position in the panel. In other cases, they won’t hold the fuses securely. This is particularly true of pullouts, often associated with 240-volt circuits.

This may be caused by one of the following:

  • repetitive use over time
  • rough handling

In situations where fuse holders are loose or where there are pieces obviously broken, recommend corrections by a qualified electrical contractor. Fuse blocks with handles may be fragile and although they may look fine, they may crack or crumble if you pull them out.

Fuse blocks/pullouts 

Fuses for multi-wire circuits not linked
A multi-wire circuit typically has a red and a black wire. This is typical of 240-volt circuits. It’s also true of 120-volt circuits going to split receptacles, for example. Because the circuit has two energized conductors, two fuses are needed to disconnect both conductors to work safely on the system. Removing one fuse and having the circuit not work can be very misleading. People may think the circuit is not energized and start to work on it. The other conductor will still be energized and can cause a shock.

The solution is to make sure that both conductors in the circuit have a common disconnect (one that pulls out two fuses at the same time). Multi-wire circuits should be on fuse pullouts if the two circuits end at the same location (for example, a duplex receptacle). Where this is not done, recommend corrections.

The condition is a result of poor installation. Look for red wires at the panel. Each one should be on a pullout fuse block. If in doubt, recommend that it be checked by a qualified electrician.

Multi-wire circuits on the same bus bar
The illustration below highlights a problem with multi-wire branch circuits. Each live leg of a multi-wire branch circuit should be fed from a different bus bar. If they are fed from the same bus bar, the neutral wire may overheat when both legs are working. When wired properly, a multi-wire branch circuit with 12 amps flowing through one side and 10 amps flowing through the other will have 2 amps (12 minus 10) flowing through the neutral wire. If they’re both fed from the same bus bar, there will be 22 amps (12 plus 10) flowing through the neutral wire—not a good situation.

Fuse pullouts—to remove or not to remove?
240-volt circuits and 120-volt multi-wire branch circuits have two fuses. As discussed previously, both fuses have to be pulled to safely work on that circuit. It is often not possible to see the fuses, so you cannot tell whether the circuit is protected properly. (Sometimes, you can see the fuse through a small hole in the fuse holder.) Some home inspectors pull out the fuse blocks to look at the fuses. This is not recommended in occupied houses, since pulling the fuses disconnects the circuit and may inconvenience the occupants (for example, by changing timers).

As mentioned earlier, there is also potential of the fuse block cracking, breaking or crumbling when you pull it out. These blocks may not have been pulled for years and if you break it, you may be responsible for providing a new panel. Replacement parts for old fuse panels often are difficult to obtain.

Fuses and fuse panels are slowly being eliminated; however, you should know how to inspect them.

Note of appreciation: Thanks to Roger Hankey of Hankey and Brown Inspection Services Inc. for his thoughtful and important contributions to this article.

Carson Dunlop - Consulting engineering firm devoted to home inspection since 1978. ASHI@HOME Training Program, www.carsondunlop.com, Carson@carsondunlop.com.