• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Which US Fuse To Replace the 17/35 Lucas Fuse?

Share this page

78MGB

78MGB

Jedi Hopeful
Offline
Blew a fuse and one in the MGB trying to get power to the wire that needs constant power to the radio. (another thread) Popped the fuse box open and I see two Lucas 17/35's and two 30amp (a US brand) fuses. Have read where some replace the four with (US brand) fuses of 20 amps, 30 amps and 35 amps. Please explain your answer so this electrically challenged person can understand it.

About the only thing electrical that I understand is the Lucas switchs have only three positions, dim, flicker, and off.

Thanks for the help.
 
A 20 amp USA rated fuse will carry 20 amps continuously & blow at 35 amps in a short time. A 17/35 amp Lucas fuse will carry 17 amps continuously & blow at 35 amps in a short time. The 20 amp US fuse is pretty much equivalent to the 17/25 amp Lucas fuse. Just different rating systems.
D
 
So, I NEED to pull the 30's and go with the 20's. Guess I will replace all four. Thanks and sorry if this is a retread thread.
 
I use 20A "slow blow" fuse as a replacement. This would seem to be closer to the UK spec 17/35 fuse.

If you just pop in a normal or a fast blow 20A fuse (sometimes marked with an F) it will blow closer to its rated or stated value (ie ~20 Amps). This could mean that any kind of intermittent spike or surge in current like you might see when the “cooling fans” are kicking in - could blow it a fast blow fuse almost instantly. This is probably why you find so many cars with 30 or 35 Amp fuses in place of the 17/35’s.

In the case of a 20A slow blow fuse, like the 17/35A fuse it can see up to twice its rated/stated value for a minute or so before blowing.
 
[ QUOTE ]

If you just pop in a normal or a fast blow 20A fuse (sometimes marked with an F) it will blow closer to its rated or stated value (ie ~20 Amps). This could mean that any kind of intermittent spike or surge in current like you might see when the “cooling fans” are kicking in - could blow it a fast blow fuse almost instantly. This is probably why you find so many cars with 30 or 35 Amp fuses in place of the 17/35’s.


[/ QUOTE ]
Hi Bret,
I don't think I agree. The typical 20 amp "AGC" type fuses, from .1 to 30 amp ratings, are rated to blow:

At 110% (22 amps) in four hours.

At 135% (27 amps) in one hour

At 200% (40 amps) in two minutes

This is not all that different from the Lucas fuses.

I would think that two minutes is plenty of time for a fan to "kick in".
D
Hi again Bret,
I screwed up. you are much closer to the truth than I was.
A 20 amp AGC fuse will blow in about:
100 seconds at 27 amps
10 seconds at 30 amps
1 second at 40 amps
My apology to you,
D
 
Then 20 amp AGC it is. What is the difference between the AGC and SFE fuses? The AGC says it can replace the SFE, but I did not see the same on the SFE package.


Thanks again.
 
[ QUOTE ]
Hi Bret,
I don't think I agree. The typical 20 amp "AGC" type fuses, from .1 to 30 amp ratings, are rated to blow:

At 110% (22 amps) in four hours.

At 135% (27 amps) in one hour

At 200% (40 amps) in two minutes

This is not all that different from the Lucas fuses.

I would think that two minutes is plenty of time for a fan to "kick in".
D

[/ QUOTE ]

Hey Dave,

Actually we are both correct. But just so you know “AGC” could be used to describe either a fast blow or a slow blow fuse. While the operating range might be on a par between the two they are two totally different fuses serving two different applications.

Fast Blow Fuses: would be ideal in regulated DC voltage circuits. Where there will be little or no fluctuation will ever be seen. Computers, Home electronics etc.
Slow Blow Fuses: are a little more robust in that and can handle electrical surges regularly assonated with un-regulated circuits. Like Automotive electronics or AC circuits.

Moreover I’ve seen the numbers you give above and admit that they can apply to both fuse types. The difference is that a slow blow fuse has time lag characteristics that allow it to handle surges commonly found in automotive circuits (like the windings of an electric cooling fan motor) as accessories are turned on & off. While a slow blow can withstand strong surges that would fry a slow blow fuse – it can also blow quickly in short circuit conditions.

“These fuses are designed for circuits in equipment having high inductive or capacitive surges, or heavy starting currents such as magnets, solenoids, motors, lamp circuits, etc.”

In other words: I use 20 AMP "slow blow" fuses.
 
Thanks Dave,

I noticed one of the fuses I replaced tonight was a tad shorter than the rest. The 20amp AGC's work great so far and do not think I will have any problems. As far as the radio, I connected the memory to the clock. So, now have sound and more confidence in the fuse box.

Thanks to all for the electrical class. Not so intimidated by it now. Had a couple of "Lucas moments" with this adventure, but all is working...for now. Thanks again!!!
 
OK, I have to ask another "dumb question". I follow all the above on slow blow vs. fast blow fuses. however, which is which? I think the AGC is a fast blow and the SFE is a slow blow (S stands for 'slow'....) Is this the case in fact? I am currently using AGC fuses. Thanks in advance.
 
AG_ & SFE markings are an indication that the fuse is a glass and its fuse size.

AG = All Glass
SFE = Society of Fuse Engineers

Example:
AGC & SFE 20 fuse = ¼” x 1-1/4”.
AGW & SFE 7.5 & SFE 9 = ¼” x 7/8”

Fast Acting vs. Slow Blow: Speaking in Little fuse lingo:
A fast acting fuse could be one of the following: 3AG - 311/312
A slow blow fuse could be one of the following: 3AG – 313

There are other examples from other manufactures, but I think you get the idea.

Anyway here are a couple of links with PDF files that might confuse you further or help.
AGC & SFE Slo-Blo Fast Acting

But the easy way to avoid a headache would be check the packaging to sure is when you purchase the fuses. If it doesn't say "Slow Blow" (or Slo-Blo) – then I would assume it’s a normal or a fast blow fuse.

Good luck!
 
Back
Top