120/208 load calcs

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coast2coastelec

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I have been asked to do load calcs for a retail store.
All lighting runs off of 120v
There is one 50 amp Air conditioning unit that utilizes all three phases.
Is it correct to calculate my lighting like this

EX: Circuit 1 (8 50w light bulbs).
Load equals 400watts

The city is explaining to me that I need to use 1.732 as a multiplying factor for my load calcs because of the 208V. But why would I use that in my calculation for my 120v lighting fixtures?

Any help would be greatly appreciated.
 
Re: 120/208 load calcs

First, add up the loads, using the process in NEC Article 220. The loads will be in terms of VA. Next divide by 208 times the square root of three (that?s the 1.732) to get the equivalent amps. Please note that as long as you have the loads in VA, the 1.732 does not come into play any earlier in the calculation process.
 
Re: 120/208 load calcs

Put the "line to neutral" voltage out of your head. It's a three phase 208 service, for your load calc. The three phase causes the 1.732 to come into play.
 
Re: 120/208 load calcs

Here's a really stupid question,

3 Phase Panel
Phase A to ground =120
Phase B to ground =120
Phase C to ground =120

Phase A to B = 208 (roughly)
Phase A to C = 208
Phase B to C = 208

How and why do I get 208 and not 240?
I'm not experienced with 3 phase stuff.

Thanks! :D
 
Re: 120/208 load calcs

How and why do I get 208 and not 240?
I'm not experienced with 3 phase stuff.
Click to expand...
Mro, the three voltages are "out-of-phase" with each other by one-third of a cycle (120 degrees) and cannot be added arithmetically.

At the instant that the A phase is at maximum value, the B and C phases are considerably below maximum.

:)

Ed

3ph1.gif


[ March 09, 2005, 07:55 AM: Message edited by: Ed MacLaren ]
 
Re: 120/208 load calcs

the formula for phase to phase voltage in a wye is V(to neutral)X square root of three or 1.732
 
Re: 120/208 load calcs

mro,

To get a better idea of the "why", take a look at Ed's sine wave diagram which shows the instantaneous (not RMS) values of the three voltages.

The line to line voltages are obtained by SUBTRACTING one phase voltage from another. For example,

Vac(t) = Va(t) - Vc(t)

(this will be the 208Vrms wave)

Now find the points that these two waves cross, Subtracting one from the other identifies the zero crossing points of Vac(t).

Now find the points that Va(t) and Vc(t) are equal and opposite. Performing algebraic subtraction at these points yields the negative and positive peak voltages.

Perform this albebraic subtraction for several other points in between and you will see another sine wave emerge--in this case the 208Vrms which will plot out to be 294Vpeak.

Ed, maybe you have a diagram showing only two of the phase voltages and the corresponding line to line voltage?

BTW, the stupid questions are those not asked!

[ March 09, 2005, 10:47 AM: Message edited by: rattus ]
 
Re: 120/208 load calcs

mro,

The confuser in this is that the voltages on the single phase part of a center tapped 3phase DELTA do add line-N + line-N = line-line. 120+120=240 There is however a higher voltage line-N (208V)in that panel on every third lug. All combinations of Line-Line are 240V

The graph of any single phase ac voltage is only one of the lines on the graph posted above. L-L graph would have 240V amplitude, but L-N has only 120V amplitude.

Jim

Jim
 
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