RV Electricity – Don’t blow your fuse!

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Dear Mike,
As always, many, many thanks for all you do explaining RV electrical systems and for your very important Stray Voltage Patrol. I am also very excited and eager about the tests you are about to run. I have a question – a small tidbit – that might make an item for one of your “tips” or, if consequential (which I doubt), for an article someday.

I read in something last year (can’t remember what) that it was wise to place a “slow-blow,” 10-amp fuse inline to the water pump. I dutifully made a note last year and filed it away. Now it is time to replace my water pump. So, after ordering the new pump, I started searching for a 12-volt, 10-amp slow-blow inline fuse to install when I wire the new water pump. I have been completely unsuccessful in finding such an item. I find many 120-volt, 10-amp fuses but no 12-volt, 10-amp fuses. Therefore, these questions:

  1. Is there such a fuse (as above)?
  2. Is it advised or even necessary?
  3. If yes to (2) above, where might I find this type of item?

Again, thank you for all you do.  —James Stiner 

Dear James,
That’s a great question that stumps a lot of electricians, so here’s how it works. As you’ve ascertained there are two basic ratings for fuses, amperage and voltage, as well as the operating speed (fast-blow or slow-blow). So let’s look at the first part of the question. When a fuse is in the circuit it doesn’t know anything about the voltage on the circuit, only the amperage of the current flowing through it. See my little diagram below of a complete circuit operating normally.


At this point in time, the fuse doesn’t know or care if there’s 12 volts or 120 volts powering the circuit. It only knows if there’s 1 or 5 or 10 or 20 amperes of current flowing through itself. And once that current exceeds a set amount (in your case, 10 amps) for a given amount of time, the little metal element inside of the fuse will heat up enough to “blow” or open up. And when it opens up, the current in the circuit stops flowing and the light goes out.

So why is there any type of voltage rating on a fuse at all? Well, at the exact instant the fuse opens up there’s a spark that occurs when the molten metal separates. Usually it’s a pretty small spark at 12 volts. But you can imagine that a fuse opening up on a higher voltage circuit would make a bigger spark. In fact it can get so big that the current going through the fuse will continue to flow like an arc welder until something burns out. That’s why fuses built to work on 12-volt circuits have a different architecture than ones designed for 120 volts, or 12,000 volts or even 500,000 volts.

When you have to interrupt half a million volts of electrical potential in a big power line you’d better make sure that the disconnect can withstand and even snuff out the resulting lightning bolt. Click on the picture to the left to see what happens when a pair of in-line 250,000-volt disconnect/fuses fail by one of them not opening up in time, and you get a really big electrical arc. Kids, don’t try this at home.

That suggests that you can use a fuse with a higher voltage rating than the electrical circuit it’s working in, but never a lower voltage fuse. What this means for you on your water pump circuit is that you can use a fuse rated for 120 volts on a 12-volt circuit, but you can’t use a fuse rated for 12 volts on a 120-volt (or 500,000-volt) circuit. So any 120-volt-rated fuse with the same form factor and amperage rating (10 amps) should be fine.

But what’s the difference between a slow-blow or fast-blow fuse? Well, a fast-blow fuse is designed with very little extra mass in the melting element so it will blow rapidly with maybe 125% of the current rating. In fact, some current-limiting fuses used in big motor systems will blow in less than one line cycle, so they’ll open up in a small faction of a second to limit over-current damage downstream of themselves.

However, slow-blow fuses have more mass and sometimes a little pool of solder with a spring to open the circuit after a few seconds of current that’s several times over their nominal rating. So a 10-amp/slow-blow fuse might survive 4 or 5 seconds with 20-amperes of current flowing though it. This will allow things like motors to get up to full speed (when they typically draw a lot more current for a few seconds) rather than randomly blow if the grease in the motor bearing is a little cold, or there’s an extra momentary load on the motor.

The general rule of thumb is that most electronic circuits that need fast protection in the event of a component failure (short circuit) require a fast-blow fuse. But motor circuits are often listed to use a slow-blow fuse to stop random fuse blowing on start up. In any case, you’ll want to stick with the manufacturers’ recommendation in most instances.

So if you’re not experiencing any nuisance tripping on your water pump motor with a 10-amp fast-blow fuse, then you really don’t want to swap in a 10-amp slow-blow fuse which offers less protection. And you never want to substitute a fuse with a higher nominal rating (15 or 20 amps) in a circuit designed for a 10-amp fuse, since it could allow the wiring to overheat, melt and possibly catch on fire.

If in doubt, always obtain and follow the manufacturers’ recommendation for correct fuse types and sizes.

Let’s play safe out there….

Mike Sokol is an electrical and professional sound expert with 40 years in the industry. Visit NoShockZone.orgfor more electrical safety tips. His excellent bookRV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

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George C
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George C

Some water pump configurations account for borderline high amperage draws by using a relay to control the pump. Our Class A has a 10A fused circuit to the pump, but that line only controls a 30-40A relay that directly powers the pump. This may be due to the pump draw, concerns about aftermarket pumps drawing more, or because the pump power circuit also has 4 indicator lights throughout the coach that illuminate when the pump is on. So, maybe the OP doesn’t need a different fuse, just a relay in the circuit.

Bill Lampkin
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Bill Lampkin

Mike, For your residential refrig test, please include a compressor-type retrofit system like those made by JC Refrigeration. Since its a retrofit, no new refrig ‘box’ is required, and it runs on 1amp. or so they say. There are thousands of fire-prone ammonia-based refrig out there that are aging and could use the compressor-based refrig system. Much more cost effective than a new residential fridge. BTW, I have no skin in with JC, just an interested RVer with an older, 4-door Norcold (that works fine for now).

http://jc-refrigeration.com/products/

Mike Sokol
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Mike Sokol

Thanks, I’ll give them a call and see if they’ll ship me one to test.