We pretty much beat up the Healey wiring sizing, fusing and connecting
subject, but for those who want even more stuff, take a look at wire sizing
at http://www.simetric.co.uk/siwire_elect.htm
Fabricators vary as to how much wire load rating safety factor is best. My
personal maximum amperage limits for 150C rated insulated wire at an assumed
for design purposes stress-case 70C under hood temperature are as follows.
Standard Automobile Wire Capacities
Gauge | Max constant amps in open air | Max constant amps enclosed | Max
fuse | Max load @ 50% safety factor | Voltage drop per foot at 70 degrees
Centigrade at max safe load.
AWG 0 | 245 | 172 | 200 | 114 | 0.016
AWG 2 | 181 | 127 | 150 | 84 | 0.015
AWG 4 | 135 | 95 | 100 | 63 | 0.014
AWG 6 | 101 | 71 | 80 | 47 | 0.016
AWG 8 | 73 | 51 | 50 | 34 | 0.018
AWG 10 | 55 | 39 | 40 | 26 | 0.021
AWG 12 | 41 | 29 | 35 | 19 | 0.029
AWG 14 | 32 | 22 | 25 | 15 | 0.036
AWG 16 | 22 | 15 | 15 | 10 | 0.038
AWG 18 | 16 | 11 | 10 | 7 | 0.043
AWG 20 | 11 | 8 | 7.5 | 5 | 0.038
AWG 22 | 7 | 5 | 5 | 3 | 0.039
AWG 20 and 22 are not recommended for car wiring.
Open air = single wire in an air stream
Enclosed = all other conditions
Bill Lawrence kindly suggested checking out the Wiki article about
automobile fuses. That article referenced an OptiFuse site,
http://www.optifuse.com/fusesautoblade_reg.php which gives the following
fuse hold/blow standards.
ATO/ATC Fuse Design Standards SAE J1284
Time Characteristics
Amp load versus amp rating | hold time min | hold time max
110% | 100 h min | na
135% | 750 ms min | 0.5 h max
200% | 150 ms min | 5 s max
350% | 80 ms min | 250 ms max
600% | 30 ms min | 100 ms max
So, blade fuses in car service survive at 110% of rating, survive short
surges but otherwise cook at 135%, blow at 200%, and almost always blow
within 1 second somewhere around 250%.
A Lucas 35 amp fuse holds at 17.5 amps. Using 17.5 as the Lucass USA
equivalent rating, the Lucas 17.5 amp blows at 200%, and fast blows at
228%, quite similar to the ATO/ATC standard blade fuse.
USA glass ACG BUSS or LittleFuse glass fuse substitutes for Lucas fuses have
somewhat different characteristics with a shorter 110% overload hold time.
Again, per OptiFuse,
FSA/FSA-P Fuses < x 1 < (6.3 x 32mm) 3AG
Fast Acting/Glass Construction
12A-30A 32VAC
Time characteristics
Amp load versus amp rating | hold time
110% | 4 h min
135% | 1 h max
200% | 10s max
So, substitute a Lucas 35 amp with a US ACG 15 or 20 amp. Let your personal
experience be your guide. From the above starting at 15 amps but carrying 20
amps just in case should work but, frankly either should be sufficient to
protect the Healeys wiring from dead short burn risks.
Hap Polk
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