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
Max load @ 50% safety factor
Voltage Drop per foot 70C0
Max Constant Load
Max
AWG
Open Air
Enclosed
Fuse
0
245
172
200
114
0.016
2
181
127
150
84
0.015
4
135
95
100
63
0.014
6
101
71
80
47
0.016
8
73
51
50
34
0.018
10
55
39
40
26
0.021
12
41
29
35
19
0.029
14
32
22
25
15
0.036
16
22
15
15
10
0.038
18
16
11
10
7
0.043
20
11
8
7.5
5
0.038
22
7
5
5
3
0.039
@ max safe load
Voltage drop V=R/ft*L*C
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
Amps vs. rating
110%
100h
min. hold time
135%
750ms
min. hold time
.5h
max. hold time
200%
150ms
min. hold time
5s
max. hold time
350%
80ms
min. hold time
250ms
max. hold time
600%
30ms
min. hold time
100ms
max. hold time
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
110%
4h
min. hold time
135%
1 hr
max. hold time
200%
10s
max. hold time
Source: www.optifuse.com
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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