Fuses and circuit breakers are designed to prevent circuit overload. What's the danger of circuit overload? Fire, in a word. Indeed, as you already know from the toaster in your kitchen, wiring can get hot. While that's okay for the toaster, you certainly don't want red-hot wires glowing inside the walls of your house!
Most wiring in pre-1970s houses can safely carry a sustained load of 15 amps without overheating, while circuits in many (but not all) newer homes can safely carry 20 amps.
So what's the danger of fuses? Well, for hundreds of different reasons, we use much more electricity than did our parents or grandparents. Indeed, 50 years ago many houses were wired with 60 amps of total capacity--and fuse boxes often only had room for four 15-amp circuits and a single 30-amp 220-volt circuit for a stove or water heater. Today, by contrast, most new houses have 150 or 200 amps of capacity, with dozes of individual circuits.
When a 15-amp fuse blows, it can be mighty tempting to screw in a 20-amp fuse as a replacement instead of hiring an electrician to wire a new circuit. And when that 20-amp fuse blows, it can be equally tempting to screw in a 30-amp fuse. Having done so, you've created a fire hazard.
So to get back to your question:
- If you find a fuse box stuffed with 30 amp fuses, or even all 20 amp fuses, chances are the fuses have been blowing frequently--and the occupants have installed heavier fuses than are safe. This house definitely needs additional wiring!
- Even if you find a newer 150 or 200-amp circuit breaker box, you're not entirely out of the woods. If you discover only five or six circuit breakers in the box, this means that little or no new wiring has been added over the years, even though the capacity exists to add it. (Most electrical codes, for example, require outlets every 12 feet or within six feet of each doorway. But many older houses have only one outlet per room.)
- If the main fuse (or circuit breaker) is rated for 100 amps, which was normal residential capacity from the early 1950s to the mid-1960, you've probably got adequate overall capacity if most of the major appliances in the house (stove, water heater, dryer) are gas rather than electric.
- A house with 60-amp service, on the other hand, almost always needs upgraded electrical service.
- And in some (but not many) older houses you'll find a 100 or 150-amp fuse box with a dozen to 20 or more individual circuits, each protected by a 15 or 20 amp fuse. This is a sign that the house was rewired years ago by an owner who was playing it safe, and planning ahead.
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