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#83622 - 02/10/03 06:55 PM Calculation rounding  
Steve T  Offline
Member
Joined: Feb 2001
Posts: 306
Oak Park, IL, USA
Are there any formal rules as to rounding when doing calculations?

In a practice workbook, the calculations are rounded to the tenths. When I do the calculations precisely (to the thousandths) my fractional answer is below .5, but the book's fractional calculation is .5 exactly.

This changes the answer I get versus what the book says is the answer.

I don't see anything in the NEC that calculations must be rouded to the tenths before applying 220-3(b).


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#83623 - 02/10/03 08:30 PM Re: Calculation rounding  
Chris Rudolph  Offline
Member
Joined: May 2001
Posts: 160
Winter Park,Fl USA
I think the amount of rounding up or down depends on the magnitude of the answer.
For example if an answer is 120.6 volts,it would be ok to round up to 121.If the answer is 0.555 amps,I would round it up to 0.56.I guess a lot depends on the required accuracy.
Chris


#83624 - 02/10/03 08:43 PM Re: Calculation rounding  
Joe Tedesco  Offline
Member
Joined: Oct 2000
Posts: 2,749
Boston, Massachusetts USA
This is what Annex D in the 2002 NEC says:

Quote
Fractions of an Ampere. Except where the computations result in a major fraction of an ampere (0.5 or larger), such fractions are permitted to be dropped.


Joe Tedesco, NEC Consultant

#83625 - 02/20/03 10:48 PM Re: Calculation rounding  
Steve T  Offline
Member
Joined: Feb 2001
Posts: 306
Oak Park, IL, USA
The exact question is this--

Calculate the minimum size Type THW copper conductors required to serve the following motor loads from a 480/277, 3-phase, 4-wire service: Two 40-horsepower, 460 volt, 3-phase synchronous type, 90 percent P.F. and one 10-horsepower squirrel-cage, 460 volt, 3-phase motor at 100 percent P.F.

A. No. 3
B. No. 2
C. No. 1
D. No. 1/0


#83626 - 02/21/03 08:11 PM Re: Calculation rounding  
Roger  Offline
Member
Joined: May 2002
Posts: 1,716
N.C.
Steve, I'm a little confused as to where 220.3(B) comes in to play with motor feeders.

The answer to the question would be "B" #2

Table 430.150 shows 41 amps for your 40 hp
and 14 amps for the 10 hp.

41 amps with a 1.1 mutiplier = 45.1

largest motor x 125%
45.1 x 1.25 = 56.38

56.38 + 45.1 + 14 = 115.48

115 amps, .48 could be dropped, but, if you know the test is going to force you to round to the tenth, you would have to raise the .48 to .5 and take you to 116 amps meaning #1 THW.

Now, in reality if you converted 116 amps at 460 v to watts, you could then divide by your true voltage of 480 x 1.732 and get down to 111 amps.

Sorry about the rambling. [Linked Image]


Roger



[This message has been edited by Roger (edited 02-21-2003).]


#83627 - 02/21/03 10:53 PM Re: Calculation rounding  
electure  Offline


Member
Joined: Dec 2000
Posts: 4,259
Fullerton, CA USA
A motor with a 100% PF?
My friends would like to buy one...S


#83628 - 02/21/03 11:27 PM Re: Calculation rounding  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
Er, synchronous {versus induction} motors can run at 1.0 or leading PF, but require a DC excitation source and a wound rotor.


#83629 - 02/21/03 11:43 PM Re: Calculation rounding  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
Roger,
You used the 575 volt amp for the 40 hp. The 480 volt current is 52. Also what is the 1.1 multiplier for?
I come up with 1.25(52) + 52 + 14 = 131 amps. This will require a #1 THW copper feeder.
Don


Don(resqcapt19)

#83630 - 02/22/03 12:11 AM Re: Calculation rounding  
electure  Offline


Member
Joined: Dec 2000
Posts: 4,259
Fullerton, CA USA
Geeze, Bjarney.
The last synchronous motor I saw was 30 yrs. ago.(and then was 30 yrs old)
You must admit they're in not common use.
Giant Compressor.


#83631 - 02/22/03 10:24 AM Re: Calculation rounding  
Roger  Offline
Member
Joined: May 2002
Posts: 1,716
N.C.
Don, wouldn't we use the Sychronous-Type Unity Power Factor coloums and then apply the multiplier of the foot note?

Roger


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