[Healeys] Wiring

Hap Polk happolk at cox.net
Sat Feb 5 21:29:01 MST 2011


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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