Service Calc. Optional Method?

[wolfman56]

I just trashed a test today because I couldn't make sense out of: 2002 NEC Table 220.34
I understand it now because I looked it up in a reference book when I got home.
But here's a question. Is it possible to understand 2002 NEC 220.34 and Table 220.34 without having to find an explanation outside the NEC? Has anyone out there figured out this optional method without going outside the NEC for an explanation?
There was no way to pass this test without an understanding of the table. I was unlucky enough to not have a reference with that addressed that option. Right now I'm kind of feeling like the test was unfair.

 

[bphgravity]

Re: Service Calc. Optional Method?

This is a confusion table for those that have not had much practice with it.

The confusion point is that the table has already combined the square footage with the allowable volt-amps. Whereas Table 220.3 (which we are all use to) breaks it down to volt-amps per 1 square foot. The key is to start with connected load, and end up with demand load.

This is how I tesch this method.

First: In your code book, write: "/Sq.Ft" next to "Connected Load" in the table.

Next: Write "next 17" next to the second line that starts, "Over 33 to 220 VA/m....."

Then circle "Remainder" in the third line.

Now lets do an example and apply the Table:

"What is the demand load for a 20,000 sq.ft. school building with a total connected load of 680kva. using the optional method."

First you must determine "connected load / sq.ft"

680,000 va / 20,000 ft = 34 va/ft^2

First 3 at 100% = 3

Next 17 at 75% = 12.75 (17 x 75%)

Remainder at 25%= 3.5 (34 - 3 - 17 = 14 x 25%)

Add these up = 19.25

Finally, take this new demand load and multiply it back to the square footage:

19.25 va/sq.ft. X 20,000 ft. = 385,000 va.

It has been my experience that the average demand calculation is roughly 50% of the original connected load.

God luck on your next attempt!!!