# 3 Phase calculation HELP



## southern elec (Apr 14, 2006)

I am a residential electrician needing help with some 3 phase calculations. The questions on a test I am taking are as follows:
1) How many watts are available in a 60 amp, 208 volt, 3 phase service?
2) How many 20 amp, 120 volt, 1 phase services can you support froma 100 amp, 208 volt, 3 phase distribution panel?
3) What size conductor is required for a 100 amp, 208 volt, 3 phase service less than 100 feet in length? More than 300 ft in length?
4) What size and how many conductors are requires for a 400 amp, 480 volt, 5 wire parallel, 3 phase service?
5) What is the maximum amperage available in a 45 kva, step-down 480 volt to 208 volt transformer?
6) If you were asked to install a 72,000 watt, 208 volt, 3 phase service, how many amps would this be?

can not find the formula in the ugly's book 
i don't have the power factor


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## Speedy Petey (Sep 30, 2003)

southern elec said:


> I am a residential electrician needing help with some 3 phase calculations. The questions on a test I am taking are as follows:
> .
> .
> .
> ...


Residential or not, calculations are calculations.

How about try a code book as opposed to the Ugly's book???



BTW - At least three of those questions are erroneous at best.


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## skeeter (May 18, 2007)

there are a few examples in the back of the code book, like sp said use the code book.


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## mdshunk (Mar 13, 2005)

Southern electrician? Are you really an electrician? Recently apprenticed?


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## jbwhite (Jan 12, 2007)

I do not see any power factor involved in any of those questions

I also do not see why the Ugly's book needs bashed. In fact all of the information needed to answer the above questions are in there.


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## ABLE1 (Apr 30, 2005)

Excuse me Gentlemen!!! 

The original post was from Feb. 07. Southern Elec. either failed the test and went on to other things or passed and is making lots of profit and too busy to come here.

BTW I like the Ugly Book really good.

Les


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## Magnettica (Dec 19, 2006)

www.MikeHolt.com

!!!!

Hope this helps:


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## Speedy Petey (Sep 30, 2003)

I was not bashing the Ugly's book. It does have it's place. 
I am just so used to using the code book and Handbook I do not care for the Ugly's. I can find stuff just as fast using a code book.

Also, the OP stated he could not find something in the Ugly's book. I suggested he try the real code book. Maybe he could find what he needed there. 

No offense was meant to Ugly's fans. :sad:


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## JohnJ0906 (Jan 7, 2007)

Granted, the NEC is, and should be, the "bible" of our trade. However, I use the uglies for looking up something quick, since it is small enough to tuck in my tool bag, and the NEC is in the truck. It all depends. 
I think one of the points of education is to learn to use the codebook, so someone preparing for a test should be using the NEC.


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## mdshunk (Mar 13, 2005)

I carry an Ugly's, but I'm not exactly sure why. Crane hand signals is the last time I remember referencing it. That's not really in the NEC. I'll probably give it to the next guy that shows an interest in it. When I catch people gawking at something of mine that I hardly ever use, like tools I thought would be useful and they aren't, I just give it to them if they want it. It seems like a vendor will give me an Ugly's every 4 or 5 years anyhow, so I'm probably due for a new copy.


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## jbelectric777 (Jun 1, 2007)

*This will help ya.*

In three phase calculations just remember the square root of the number 3 (1.732)..... and include it in all your calcs. Example for your questions 1 & 2:

1) 208V X 1.732(360.25) X 60A = 21,615 or round to 21.6KVA

2) 208V X 1.732(360.25) X 100A= 36,025 or round to 36KVA
36000VA / 2400VA (20A 120V) = 15 20A Branch circuits.

Keep in mind the above calcs are basic theory only and are not taking into account continuous loads or other NEC demand factors. Also in one of your questions you used the word " Services " where I think you meant " Branch circuits " 

I always took the voltage (E) and multiplied it by 1.732, other people leave the (E) alone and increase the VA or KVA either way it works out. Good Luck on your exam..... The above is how I learned it and used on all my exams.


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