Junior Member
Posted July 30, 2018
4/0 grounded cable is rated at 43kA for 15cycles. What is the minimum number of cables per phase required to safely ground a 2500kVA 5.75%Z transformer on the 480V side?
A-1
B-2
C-3
D-4
Looking for help on this. XFMR fault current is 52,297 on the 480V side assuming 3phase? my guess is 2 cables per phase.
480*1.732 = 831.36
2500000 / 831.36 = 3007.12
3007.12 / 0.0575 = 52297
NETA Level IV
Posted August 9, 2018
Wouldn't you need to know some more information about the system (i.e. what is the upstream protective device and clearing time, etc.)?
Originally Posted by samair99
Junior Member
Posted November 19, 2018
Your calculations are correct, I came up with the same answer. For this specific question I do not think that clearing time will affect the answer. The clearing time will change the PPE requirement if you are doing an arc flash analysis.
Junior Member
Posted November 30, 2018
Pro Subscriber
The correct answer is B - 2 cables per phase. The 52,297Amps of available fault current you calculated is correct. The equation to calculate the maximum available fault current is I[availablesc] = (KVA x 100)/(1.732 x KV[lowside] x %Z) = (2500 x 100) / (1.732 x 0.480 x 5.75) = 52,298Amps. The rating of 43kA for 15 cycles for 4/0 personal protective ground cable is right out of Table 1 of ASTM Standard F855-09 for Protective Ground Cable ratings (See Grade 5). It is very important to note that Table 1 applies only to electrical systems with X/R ratios less than or equal to 1.8. Table 2 applies to systems with X/R ratios greater than 1.8. Since they gave you the 43kA rating in this problem, that's what you had to use to solve this problem. In the real world, where X/R ratios are rarely less than or equal to 1.8, I personally use the ratings that are given in Table 2 since the X/R ratio is greater than 1.8 in most electrical systems.
Junior Member
Posted January 15, 2020
You also have to assume the xfmr 480 side is the low and it's a typical Y.
because if its a DELTA the answer will be different.
Junior Member
Posted December 7, 2020
How would the answer be different if it's Delta?
Junior Member
Posted December 21, 2020
I was thinking such as to ground an ungrounded delta system. Like center tap!
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