TCC curves at different voltages

[Grouch1980]

Hi all,
I have to place two TCC curves onto one graph... 3 upstream 2500 amp fuses at 120/208 volts, with a downstream 3 pole, 1200 amp circuit breaker at 265/460 volts. There is a step up transformer between them. My question is, can both curves be placed onto the graph, or does one curve (doesn't matter which) have to be converted to the voltage of the other device?

Thanks!

 

[Ingenieur]

look at it like this
if you have 1200 A on the 480 side what will you have on the 208 side?
480/208 x 1200 ~ 2770 A

if you are trying to coordinate the fuse/cb for a 480 fault you need a common v base

yiu need to account for xfmr Z
calc your max and min fault i
max at cb
min at end of feeder

pick a few points between, adjust for v base and plot on cb curve
draw the curve by hand
should get you close

 

[ron]

As mentioned by Ingenieur, yes they go together on the same graph. He just said it in a more detailed way.

Either pick 208V or 480V as the base, and shift the other TCC to accommodate the transformer ratio.

You will draw the CB instantaneous characteristic as far out to the right as its calculated short circuit current adjusted for the base voltage you are drawing at (if it wasn't 480V)