Fuse rating - exceeding manufacturer's specification

I recently had an aftermarket SPAL fan installed on my Ron Davis radiator. It comes with its own wiring harness, including an otter switch, optional override switch and an in-line blade-type fuse, with a 30A max rating. It didn’t take long to blow the fuse and several replacements.

When I put an ammeter in line with the fan circuit and switched on the fan, the surge current pegged the ammeter at over 60A momentarily before dropping back down to the manufacturer’s rating of a continuous 22A draw (which is too much, by the way, for my generator, but that’s another story). So I installed a 40A fuse, and all is now good. The otter switch turns on the fan when the kettle gets hot and turns it off when the kettle cools down. I can turn on the fan with the override switch and shut it off again. No fuse burn-out.

I plan to call the SPAL USA’s 800 number tomorrow to see what they recommend. But I’m curious if anyone else has had to deal with a similar problem. What is the risk of using a 40A fuse when the manufacturer recommends a 30A maximum (notwithstanding a >60A surge current)?



You could get what is known as a slow blow fuse of 30 Amps.

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I agree, a Slo-Blo should be just the thing.

That is what I use on my TR overdrive solenoid that pulls a momentary 22 amps at kick-in then switches to much lower hold-in current. If the internal switch ever fails (constant 22 amps) the fuse will blow and save the solenoid.

Fuses protect wires and devices. Unless your wires are rated for 40 amps you could be creating a fire hazard. I would NEVER use fuses above the suggested rating.

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Not sure about US fuses but here in Blighty a 30A fuse is rated at 17A continuous, 30A blow.
My solution was to reduce the load on the fuse on the fuse board in the dash by a separately fused direct feed from the battery to the relay for the fan. I made sure that the otter switch and the aftermarket fan’s switch were wired as triggers for the relay rather than taking the full current of the fan (correcting the PO’s shoddy wiring). I also wired in a by-pass switch and a tell-tale light to indicate when the fan is working.

Thanks for the replies. I had never heard of slow-blow fuses before, so I did a bit of research and found two 30A models:

  1. Bussman offers a slow-blow glass fuse (MDL-30), with the delay characteristics shown below:

The 30A model will pass 60A for up to 2 minutes before opening.

  1. Littelfuse makes a large-format blade-type slow-blow fuse (MAX30), with the response time shown below:


The 30A model will pass 60A for between 6 and 30 seconds.

  1. For comparison, here is the time-delay data for Littelfuse’s plain-vanilla ATO fuses:


An ordinary 30A fuse at 60A would open between 0.15-5 seconds.

The inrush current of my fan pegged my ammeter beyond 60A and blew out several ordinary 30A fuses. On the other hand, an ordinary 40A fuse survived. That suggests to me that the inrush current must have been between 60-80A. (If it were at or above 80A, the 40A fuse would have failed just like the 30A fuse.) In the table immediately above, a 40A fuse at 160% of rating (64A) would open between 0.25-50 seconds. It may be that the surge lasts just long enough to kill an ordinary 30A fuse but not long enough to kill a 40A fuse. Deductive reasoning, I admit; a better ammeter would help answer this question.

I have a call in to SPAL’s tech support line. I’m wondering what they’ll say. Will they say that an ordinary 30A fuse should work? Seems contrary to my experience. Will they agree that a 40A fuse makes sense? Seems contrary to their printed instructions. Or, will they say that the customer should use a slow-blow 30A fuse? If so, why didn’t they supply a fuse block that accepts either the Bussman or Littelfuse versions, neither of which fits the supplied fuse block?

If I have to move towards a slow-blow fuse model, I think I will choose the Littelfuse model. It seems to offer just enough delay to handle the inrush current, but will respond reasonably promptly to short-circuits. The Bussman’s up-to-2 minute delay seems a bit scary.

More to follow.






Bob, it seems to me that if the vendor supplied 30amp fuse is blowing, maybe there is a problem with the fan being defective and drawing too much current. Have you contacted the vendor?

British vs American Fuses

British did things just a little bit different than the Americans, including what the ratings marked on fuses mean. The British Standard rated fuses by the current that was guaranteed to make them blow instantly while the American (and now international) standard rates fuses by the current that they will carry forever. Fuses work on heat, so a small overload may take a long time to build up enough heat to melt the fusible element and blow the fuse. How warm the fuse was to begin with plays a large role too. So the carry rating winds up being quite a bit lower than the blow rating… the difference is roughly 2 to 1!

So, what this means to you is that, if you elect to use common American standard fuses (Bussman is one maker, there are others), you need to use a much lower rated fuse than what your manual says to use. There have been cases reported where a restored Triumph has caught fire from a short, because someone used American fuses with the Triumph recommended rating marked on them… the short did not blow the fuse but instead set the wiring harness on fire!

Here is a conversion chart, lifted from a long-ago Bussman fuse catalog:
Bussman Conversion Chart
English Type American Standard Replacement
50 amp AGC 30
35 amp AGC 25
30 amp AGC 20
25 amp AGC 15
20 amp AGC 10
10 amp AGC 7 1/2
5 amp AGC 3
Personally, I feel even this chart results in fuses rather larger than needed, so for example I use an AGC 20 to replace the original “35 amp” fuse in my TR3A. My conversion chart would go more like :
Randall’s Recommendations
English Type American Standard Replacement
50 amp AGC 25
35 amp AGC 20
30 amp AGC 15
25 amp AGC 15
20 amp AGC 10
10 amp AGC 5
5 amp AGC 3

Tim Buja and Glenn Merrell put on a good visual demonstration of the difference in fuse ratings at VTR this year, where they first put in a “25 amp” Lucas-equivalent fuse and applied enough load to blow it. Then they installed a “25 amp” American fuse and applied the same load. The fuse did not blow, but after just a minute or so the wire started visibly leaking smoke…

By the way, proper fuses with the correct British markings are available from the usual suspects (Moss, The Roadster Factory, Victoria British, etc.). The British Standard fuses are also a slightly different physical size than American standard fuses, so it’s probably best to buy the proper British fuses. American fuses fit in most British fuse holders, but not all of them.

The above is information I found when trying to find replacement fuses.



Ha!, I’m reminded of the story, of the guy that used a live .22 round as a fuse replacement for his headlights. Ended his romantic interests for a while one dark night…supposed to be a true story…

The whole point of a fuse is to protect the components it is feeding from damage/fire if something goes wrong. Putting in a 2X larger fuse completely defeats the purpose, and you might as well just replace the fuse with a piece of heavy wire.

If the fan is rated for 20A, but blows on power-up, then the correct solution is to install a SLIGHTLY larger fuse, or, better, a 20A Slo-Blo fuse. A 60A fuse will be near useless. If the fan was provided with a 30A fuse, and that fuse blows when power is applied, then there is something very wrong with that fan.

All that is ignoring the fact that a 20A fan will be near useless on an E-Type anyway, as you will be constantly discharging the battery, especially at night, in the rain, etc. where the electrical system is already running near its limit. That goes double for any car with a generator. Even with a Lucas alternator, it will be really marginal.

CoolCat (coolcatcorp.com) has EXCELLENT fans that move a TON of air, and use very little current. Their S1 fan is a direct replacement for the factory “lawnmower blade”, moves 2500 CFM, and pulls only about 8A. If that fan is not adequate to cool your engine, then something is VERY wrong with your engine and/or cooling system.

Ray L.


I finally connected with SPAL USA. I described the problem, and unprompted by me the tech support line said to use a 40A fuse. The instruction manual for the wiring harness hasn’t been updated in some time, and it should now state that a 40A fuse is suitable and should be used with their high capacity fan. I asked whether a 40A fuse would still offer adequate protection for all of the components; he said that it would.

I haven’t yet gotten enough fan-time on this new set-up to prove Ray’s point immediately above, but I expect that he is correct. When I drove it for about 15 minutes last weekend in ~70F temperatures, the fan was only on for a few minutes. The dash ammeter was definitely showing discharge. I’d hate to see what it would look like with the headlights on. Fortunately (or not) I don’t have to worry about wiper motor draw because my wipers are too feeble to use. Energy conservation at its best, at least until rebuild.

So I guess that settles that. I’m glad to have learned about slow-blow fuses and even gladder that I don’t have to modify my fan’s wiring harness to use one. On the other hand, if one of you reads a headline “Northern Virginia Driver Torched in Classic Car Fire,” please ask the Fire Marshall to check the fan wiring on my car. And then contact my wife’s lawyer.






Was just perusing some of these posts, one thing to point out…
“The whole point of a fuse is to protect the components it is feeding from damage/fire if something goes wrong” . This statement in the way it says components is incorrect.
Fuses( circuit protection) in they’re use,are designed primarily to protect the Conductor, not the component or part they are feeding. They may help protect the component from say certain over loads internal shorts etc ( at that time the part may already be bad. etc but in a designed circuit the fuse is protecting the wires from exceeding their rated ampacity based on gauge, length run time etc.
That being said , I assume when they told you you could use a 40 amp fuse that that is based on a Harness they provided to the relay and fan ?

I had the same problem with a SPAL fan that came with my Classic Jaguar radiator (Ron Davis). When I talked to the folks at CJ they explained how to wire a relay and circuit breaker into to the system using a thicker wire coming off the circuit breaker. It worked, but I did not like the loud sound of the SPAL fan and living in Texas it came on a lot. When out in California I mentioned my issue to Dave Ferguson and he said to use the CoolCat fan and to put back the fan shroud I had removed. As Ray mentioned, the CoolCat draws far less current and the system stays cooler with the shroud and CC fan in place. The SPAL lives on a shelf in my garage.

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Correct. SPAL provided all of the harness components (see the diagram at the top of this thread). And thanks for the explanation on circuit protection.

That is only true sometimes. Granted, protecting the wiring is perhaps the most common application in a car, but there are countless other applications where the fuse is added specifically to protect components, NOT wiring. In particular, the fuse is there to prevent damaging the DRIVING components, not the DRIVEN components. Power supplies and audio power amplifiers are two very common examples. Shorting the speaker terminals on many power amplifiers will blow a fuse, while there is near zero chance of any wires over-heating. The fuses prevent the output transistors/FETs from being damaged due to sustained over-current or over-heating.

Ray L.

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Ray yes I stand corrected as you said. Like fuses incorporated into components like amps etc.
I guess my main point in when used to supply power in a circuit. But your description an examples are spot on.
I guess key point was trying to make not to just consider a fuse rating for the components amp draw but what conductors you have going to it.
In the Yacht industry where we need to meet certain standards to voltage drop etc.
This comes in handy for quick ref. For D/C circuits:


they also have an app. , makes a nice
quick start when say running a new circuit etc.

if anybody