All -

I am going to an electricians course to augment my field experience with theory and "book learning". We are currently using "Stallcup's Journeyman Electricians Study Guide based on the 2202 natioal electrical code"

In chapter 10 there are several examples for doing comercial load calculations and in Appendix A there are several blank calculation sheets for residential and commercial.

The example on page 10-19 (figure 10-20) shows "Calculating largest motor load" (halfway down the second column) as being 9540VA x 25%.

It gives NEC references of 220.14 and 430.24. NEC 220.14 basically refers you to 430.24. The way I read 430.24 is that I should use the motor with the largest full load current and multiply that by 125% and then add the sum of full load current rating for any other motors that may be on that circuit.

My questions are:

Am I reading this properly, and if so, where does the 25% figure come from in the examples ?

If I am not reading this properly, could some one give me a simple (I'm a retired Marine - grin) explanation of what I should be understanding, and again, where does the 25% in the example come from?

Thanks so much.

Steve

Why don't you call him or sendan email and ask for clarification?

Probably should look for an errata list too as in the NEC errata list for the NEC on www.nfpa.org
http://www.grayboyinc.com/

"The way I read 430.24 is that I should use the motor with the largest full load current and multiply that by 125% and then add the sum of full load current rating for any other motors that may be on that circuit."

Me too.

Joe -

I sent an email to kcarroll@nfpa.org but haven't received a reply yet (not expecting one until next week thoug) and thought I would tap the vast experience/knowledge here as well.

Steve

Contact the author of that book here:
http://www.grayboyinc.com/

The NFPA link was for you to get the Eratta in the 2005 NEC and there are many.

I took a trip over to the NFPA site and I am slightly confused. Took a second to locate the errata for the 2005 NEC. My concern is the document is labeled as the "final" errata and is dated Dec 24 2004. The document is also only two very well spaced pages and could be condensed into half a page. Am I missing a more recent document that fits this quote:

"The NFPA link was for you to get the Errata in the 2005 NEC and there are many."

Or do I indeed have the correct document?

Dave

Look here for direct link to 2005 NEC
http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=70

At the bottom see the errata link.

Taking 25% of the largest motor and adding that to the sum of ALL MOTORS is the same as taking 125% of the largest motor and adding that to the sum of ALL OTHER MOTORS.

Except that Stallcup's method is easier and less confusing...maybe NEC might think about re-wording the 220 section.

Ok so I'm in the right place. I must have the correct errata for the 2005 NEC.

Thanks Joe


Dave

The requirement of the 125% of the largest motor could be performed in two ways:
Say of motor of 100 amps

1. 100 x 1.25 = 125 amps

OR

2. 100 x 25% = 25 amps
then add the original 100 amps plus the 25 amps, and you will have 125 amps

I love this stuff... when it works

Pierre