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MG MGB Technical - Blade Fuses

I am replacing the 4-gang Lucas fuse box in my 74B with a modern 6-gang blade type box since the box needs to be replaced anyway. The extra two fuse locations will accommodate the two line fuses. I know the original glass fuses are 17a-35a blow. What value should the blade fuses be? Has anyone done this?
C W Geiger

Hi.

I understand that the ratings marked on blade types are the 'continuous' rating, so it is a choice of 15A or 20A.

Take a look at :

http://www.pp.wmich.edu/stores/pdf/111874.pdf

I guess that 3A blades would be adequate for the approximately 1A drawn by the sidelights.

HTH.. Don
Don

Major question here is what do you want to have at the end of the line on those two fuses?

As you know, the fuses are there to protect gadjets. Use what ever size fuse to protect what's in that circut.
glg

My recommendation is to do what the factory did and use a common rating for all the original fused circuits, the only exception being the fuse for the sequential seat-belt system which was used on North American spec cars for part of one year. The remainder of the fuses protect the wiring, not the components it supplies, and the wiring is more than capable of taking the 17A or 15A continuous rating. If a short does develop the fuse will blow long before there is any damage to wiring, and keeping to a common rating means you only need to carry one or two spares of that rating, like the original fusebox allowed for, instead of a range of spares for each different rating. The only other time I have used different ratings was for uprated headlights with one fuse per filament of a rating double the current draw of each filament as there was also a main supply fuse for the relays of double that.
Paul Hunt

"As you know, the fuses are there to protect gadjets."

Although a commonly held belief, this is not so. As Paul points out, fuses are there to protect the WIRE. Ideally, when designing a wiring system, the wires are sized to handle the loads, and then the fuses are selected to protect the wire. For example, if the load is 7 amps, you might choose a 16 ga wire, rated at 10 amps, and then use a 10 amp fuse to protect the wire.

When it comes to sizing fuses for a modification, you have two options:

1. Determine the load on the fuse and then select a fuse apropriately (ensuring that the wire in place is adequate to handle that load).

2. Start with small fuses and replace them with larger sizes if they blow.

Since you have no choice as to wire sizes unless you are replacing the harness, I would use option 2, Starting with 10 amp, and never going above 20 amp. If you find that you are blowing 20 amp fuses, some more work is in order to find out why and correct it.

On the other hand, you may want to follow Paul's advise and use only one size. In that case, I think 15 amp would be suitable. I would NOT recomend a blanket replacement with 20 amp fuses. Most of the wire in an MGB is rated at either 5.75 amps or 8 amps, with a very small amount rated at 17.5 amps. Only some of the main power feed wires are rated above that.
Dan Masters

{CW Geiger ~ you wouldn't happen to be from Long Beach? Just wondered, as I used to know a Geiger family whose father (in the 50s) was very big into boating ~ I went to school with one of the boys and his dad was superintendent or asst. super of Long Beach schools as I recall. If you are not related, no biggie, just curious.

To the rest of you, I would have emailed offline were it possible, sorry to butt in}
Bob Muenchausen

Thanks for your help guys - I put in 20 amp fuses and everything is working fine - I will try 15 amp and if everything is still fine will go with them instead - btw I didn't replace the in-line seat belt fuse which was disabled years ago anyway - nect I would like to fuse the headlights as they have no fuse whatsoever - anyone with suggestion as best place to locate an in-line fuse for the headlights? Thanks again

Bob: I live in Simi Valley but grew up in PA in the 50's - most of my relatives are still back east
pmcguy

Hi all.

I certainly agree that fuses don't need to be 'fine tuned' to a precise figure, but I personally feel that 15A is too high for the thin wiring of the sidelight circuits used in my 80 BGT. In the event of an accident or other incident resulting in a sidelight wiring short I suspect that loom damage may occur before a 15A fuse blew.

As a general rule the best place to position a fuse is as close to the source of current as possible, eg near the light switch rather than near the headlamp unit.
This will protect against wiring shorts in as much of the length of the wire as possible.

Don
Don

I am now using 15 amps all around and everything is still fine but I plan to try yet even smaller values as some of the wires in the factory harness are quite small. I would rather have a fuse blow than a wire melt.

Don
I agree. On my 74B the wire on the fuse box (from the light switch) that feeds BOTH side/tail lamps (fuses 1 and 2)looks to be about 18 gauge - there is no way that wire can handle the total fuse rating of 34 amps (17 amps each). 34 amps would melt that wire long before a fuse would blow. I plan to try 5 or 10 amp fuses in these circuits. I would recommend that everyone might consider replacing the factory values for these two fuses.
pmcguy

pmcguy

You might want to consider putting in relays for the lights. And they are available with "built in" fuze holders for the blade type fuses.

FWIW

Larry
Larry Hallanger

Well then, I mis read and mis-understood the question at hand. I sit corredted.
glg

This thread was discussed between 09/02/2004 and 10/02/2004

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