Sparky Joe

Ran into an interesting situation yesterday;

A 150kVA transformer, primary is 200A 480V, secondary is 400A 208V.

We fed it with 3/0, and secondary was 2 parallel 3/0's

Now normally 400A means 500kcmil which requires a 1/0 GEC. We used 2 200A conductors and a #2 GEC.

We did it in parallel for the obvious reasons, 3/0 is less than half the price of 500's. But realizing the savings on the GEC was unforseen. The way we figured it is that the cross sectional area of 2 3/0's is around 335kcmil and the seperation between a #2 and 1/0 GEC is 350kcmil.

We haven't been inspected yet(and don't know if we will be), but I'm pretty sure we are right. BTW the #2 was also a benefit due to less waste because that's what's required for the 400A EGC(although we also used a #2 EGC on the primary side).

Now finally for my question;
How can this be right?........2 3/0's is almost half the cmil's of a 500 but carry's the same ampacity. Thinking about it it all sorta makes sense, but then doesn't; higher resistance wire=smaller fault current wire, but at the same time 400A OCPD=400A.

Any comments......?
-Joe

mdshunk

Joe... I have a couple of comments, but first a quick question... where did your GEC's terminate? Building steel? Water line? Rod electrode?

Sparky Joe

Building steel, as normal on an interior transformer(because it's normally closest)

mahlere

check your math....chapter 9 table 5 has the area of 3/0 as 0.2679

mulitply that by 2 and you get 0.5358 which is larger than the 0.5242 for 350mcm. So you go up to the next size 400MCM (0.5836) which requires a 1/0.

Least that's what I was taught. But doesn't mean it's right.

Sparky Joe

Table 5?
"dimensions of insulated conductors..."

How about table 8 chapter 9 dude.
"Conductor Properties"

3/0 = 167800 cmil's

x 2 = 335600 cmil's

mahlere

then you should be alright..

Sparky Joe

Perhaps then this post is just a heads up if such an installation is confronted.
3/0's, though twice as many will probably take half the time as landing 500's, running two 2"'s is probably less than one 4", then of course the copper price will be less.

I was just wondering why? 2 200 amp conductors can carry the same current as one conductor nearly 4 times the size?

I'm probably asking in the wrong place, but mostly I'm probably pointing out my bafflement on this silly realization

mahlere

go ask over at Mikeholt.com there are guys there who live and breed code. They'll actually be willing and able to explain it to you.