Fuse Safety

Good Morning Team!

I wanted to share a story I was told about an electrical issue someone was having just a few days ago. It will be described as it was told to me and since I have done no journalistic investigation of my own, I will not be naming names. Though I will assure you this is not about one of our managed properties. It seems a local business was having some issues with an electrical circuit that kept popping or blowing 100 amp fuses. As I understand it, the fuse would be replaced and it would hold for some time, but eventually it would fail again. The person telling me the story didn’t know what steps had been taken to try and find the cause, but what I was told happened the next time it did blow, was that a 150 amp fuse was installed in the 100 amp’s place. Some of you are probably way ahead of me at this point, but bear with me as I try and explain why this is a very bad idea without some very thorough investigation.

First of all a fuse, just like a circuit breaker, is designed to limit the amount of amperage, or current, passing through it. Unlike the circuit breaker though, the fuse is a onetime use device. Once it’s done its job, it has to be replaced. When electricity flows through wiring it causes heat buildup due to the resistance, or friction, to that flow of electricity in the wiring itself. This is exacerbated by the diameter of the wire the power is passing through. The thinner the wiring diameter the more friction and therefore heat buildup, there is. Here’s a simple chart of wire size and their normal ampacity ratings. You’ll note that there are different types of wire in each gauge. The temperature rating shown indicates what the insulation of the wiring is rated to withstand. Most home wiring is NM-B and most commercial would be THW or THWN.

So using the chart above, we can determine that a 100 amp fuse would need to have at least a 3 gauge copper, in the 75 degree Celsius range, to be appropriately sized for safety. Aluminum 1 or 2 gauge, again depending on the temperature rating needed, would also be safe.  So we can clearly see that simply installing a 150 amp in place of a 100 amp fuse, without verifying the size of the existing wiring first, will seriously overload the ampacity rating of that downstream wiring leading to possible fire or other damage to the electrical system. If using copper, the wire size needed to safely deliver 150 amps would be 1/0. (Read as ‘aught’ or ‘1 aught’) Here’s graphic depiction showing the differences in wire diameter size. (AWG = American Wire Gauge)

Look at the 3 gauge size needed for 100 amps vs the 1/0 gauge – shown as 0 AWG above – needed for 150 amps. That’s a pretty significant size increase! Aluminum wiring, though not shown on the ampacity chart, would have to be 2/0 or 3/0 (‘2 aught’ or ‘3 aught’) to carry 150 amps.

As I stated in the beginning, we don’t know what was done prior to the fuse being upsized but if existing wiring size was not verified first, something very bad could occur. Also, we in the maintenance field know that often we inherit someone else’s attempted fixes. What if the 100 amp fuse was supposed to have only been a 75 amp in the first place? And there is also another possibility in that the 100 amp fuse found blown, might should have always been a 150 amp in the first place. By the way, it is okay for a wire on the leaving side of a fuse to be bigger than needed. Just don’t base your decision to increase the fuse size on that alone. You would actually need to trace out the entire circuit to determine if there is a junction box, or other connection point, where the wiring diameter does get smaller. The fuse, or breaker, is always sized for the smallest diameter wiring found in the circuit itself.

As you can see, there are an awful lot of ways to go wrong here. A high level of understanding what we are doing is required. Of course that’s the point in all our jobs where safety is concerned isn’t it? We cannot be safe if we don’t fully understand what we are doing. Never assume, always test and thoroughly investigate before attempting repairs. If you are ever unsure or have questions about next steps, always consider engaging a professional. There is no shame in asking for help. It could be shame you didn’t.

Have a great week and be safe.

Sincerely,

Ray Moore
Director of Facilities Services