there is no thermal reason for a bus bar.

Take rather more than the highest rated current, let's say 40kA, and use an approximation that over-estimates the dual exponential (8us rise 20us fall) of the official
spec for the unit   by a simple rectangular current pulse duration of perhaps 20 microseconds at this higher level,  and bung it back into the adiabatic forumula, to estimate a conservative cable size.

K²A²   >   I²t  (units joules per ohm)


re-arranges as to give the area A in mm2 as


A²   >   I²t/K²


(K is a constant set by the heat capacity of copper- temp rise per joule), the electrical resistance of a unit cube of copper, and the maximum temperature rise we are happy with,)

for copper under pvc with a cold start, K  is perhaps 120 ish, could be up to 140.  so K²  is in the region of 10000 to perhaps 20000 ish

So I²  might be 1600 million, t might be 20 millionths  of a second so the top of the fraction is perhaps 32000

divide that by a K² of at least 10000 and A becomes square root of  3 and a bit,  or a touch under 2mm2


In reality, just because it does not have time to get hot at 40,000 A does not mean you should use the bare minimum - we need to keep inductance down, and avoid damage by magnetoconvulsion - so a bit of extra 'meat' is desirable. Personally I prefer flat braid. I imagine the Wylex accountant prefers plain round wire however.


regards Mike

there is no thermal reason for a bus bar.

Take rather more than the highest rated current, let's say 40kA, and use an approximation that over-estimates the dual exponential (8us rise 20us fall) of the official
spec for the unit   by a simple rectangular current pulse duration of perhaps 20 microseconds at this higher level,  and bung it back into the adiabatic forumula, to estimate a conservative cable size.

K²A²   >   I²t  (units joules per ohm)


re-arranges as to give the area A in mm2 as


A²   >   I²t/K²


(K is a constant set by the heat capacity of copper- temp rise per joule), the electrical resistance of a unit cube of copper, and the maximum temperature rise we are happy with,)

for copper under pvc with a cold start, K  is perhaps 120 ish, could be up to 140.  so K²  is in the region of 10000 to perhaps 20000 ish

So I²  might be 1600 million, t might be 20 millionths  of a second so the top of the fraction is perhaps 32000

divide that by a K² of at least 10000 and A becomes square root of  3 and a bit,  or a touch under 2mm2


In reality, just because it does not have time to get hot at 40,000 A does not mean you should use the bare minimum - we need to keep inductance down, and avoid damage by magnetoconvulsion - so a bit of extra 'meat' is desirable. Personally I prefer flat braid. I imagine the Wylex accountant prefers plain round wire however.


regards Mike