359 Volt equation

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DanZ

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Living in: Orient, OH ; Working in Central Ohio
I got a fax from our PM today detailing what they wanted to do for an electrical service. The PDFs are too large to attach here, but I think I can sum up what they have for their single line. The are coming out of an existing wire way with a 200A 3 phase meter, than running into a 2 1/2" PVC conduit with (4) 250MCM Cu & (1) #2 GND. This breaks off into a 250A, 3 phase 4 wire panel with a 125A 3 phase 4 wire panel.

This alone is not bad, but what I put on the prints is a 400A meter with (4) 500 kcmil Cu W/ (1) 2/0 GND, running into a 4" conduit with (2) sets of (4) 3/0 Cu with (1) #4 GND, feeding into a wireway that branches off into two 200A panels.

The load calc the EC is showing is 76,756VA/359 Volts=213A @ 3ph 4 wire 208V

My load calc is 38,388VA+38,386VA=76,774VA/240 Volts=319.9A x 125%=399.9A

The EC's math works out, but I've never heard of a 359V service before. Is there some piece of the puzzle I don't have, or is the EC making up numbers to get his bid to be the lowest? Uh, you don't have to speculate on why, I just want to find out if there's a piece of the puzzle I'm missing.
 
359 volts is approximately the 3 phase equivalent for a 208 3 phase power supply. He is apparently taking your total kw and figuring the load at 208v 3 phase. This assumes the loads are evenly distributed between phases.
for 3 phase I=P/E x 1.732
 
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hence the word "approximately" apparently his EC likes to "round down":smile:
 
DanZ said:
The load calc the EC is showing is 76,756VA/359 Volts=213A @ 3ph 4 wire 208V
Click to expand...

359 is not 'volts' it is a short cut to figure the current @208Y/120, here is the long way.


76,756/208= 369 amps

369/1.73 = 213 amps
 
DanZ said:
The load calc the EC is showing is 76,756VA/359 Volts=213A @ 3ph 4 wire 208V

My load calc is 38,388VA+38,386VA=76,774VA/240 Volts=319.9A x 125%=399.9A
Click to expand...
You forgot to divide your VA by 1.73.
 
Or,

Or,

iwire said:
359 is not 'volts' it is a short cut to figure the current @208Y/120, here is the long way.


76,756/208= 369 amps

369/1.73 = 213 amps
Click to expand...


Or we could say,

Iline = 76.8KVA/3(phases)x120V = 213A (balanced)

That is, the apparent power is the summation of Iphase x Vphase.
 
Thanks guys! Let me see if I have this down. Watts (or VA) / Nominal Maximum voltage = Amps / Power factor (1.73 for 3 phase) = Total Amps, right?

Or, in this case:
76,774w/208V=369.1A/1.73=213.4A

Which would need a service and meter bigger than the 200A service the EC is spec'ing, right?

What about over sizing? Using the 125% for the total (that's how i was taught, so I'm not sure where it came from) but I do that the 2008 NEC 210.19(A)(1) calls out for oversizing continuous loads by 125%, so I plan my 20A breaker max at 1800w. I've been told by a EI that I had to oversize my calcs by 125% as a safety factor, too. There was no section cited.
 
DanZ said:
Thanks guys! Let me see if I have this down. Watts (or VA) / Nominal Maximum voltage = Amps / Power factor (1.73 for 3 phase) = Total Amps, right?

Or, in this case:
76,774w/208V=369.1A/1.73=213.4A

Which would need a service and meter bigger than the 200A service the EC is spec'ing, right?

What about over sizing? Using the 125% for the total (that's how i was taught, so I'm not sure where it came from) but I do that the 2008 NEC 210.19(A)(1) calls out for oversizing continuous loads by 125%, so I plan my 20A breaker max at 1800w. I've been told by a EI that I had to oversize my calcs by 125% as a safety factor, too. There was no section cited.
Click to expand...
watts and VA are not the same. watts = VA * power factor and VA = watts / pf
so VA / V = A and for watts W / pf / V = A

3 phase VA / phase V / sqrt(3) = A per phase
1 phase VA / V = A per phase

so you have 76,774_VA / 208_V /sqrt(3) = 213_A
 
DanZ said:
Which would need a service and meter bigger than the 200A service the EC is spec'ing, right?

What about over sizing? Using the 125% for the total (that's how i was taught, so I'm not sure where it came from) but I do that the 2008 NEC 210.19(A)(1) calls out for oversizing continuous loads by 125%, so I plan my 20A breaker max at 1800w. I've been told by a EI that I had to oversize my calcs by 125% as a safety factor, too. There was no section cited.
Click to expand...
I'm not sure where your load calculations came from. Have they been diversified? Is all of this load continuous?

Multiple motors have the largest motor at 125%. Then, only the continuous loads are at 125%.

They may spec some oversizing for future growth.

Add: If the 213 amps accounts for diversification, continuous loads, and future expansion, then a 200 amp service is too small.
 
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mivey said:
Have they been diversified?
Click to expand...
I never thought anyone would ask me if electrical calculations were diversified!

It's not all continuous load, but I was taught to calculate it that way (20A breaker = 1800W). I calculated the load by using equipment loads where known (dental equipment, fans, lighting, hvac) the duplex receptacles are calculated as 180W each (10 per 20A circuit, that's how I was taught). I was also taught, and told by inspectors, that my panel calcs had to show the 125% "safety factor" load.

The 213A does not include any future expansion.
 
DanZ said:
I never thought anyone would ask me if electrical calculations were diversified!

It's not all continuous load, but I was taught to calculate it that way (20A breaker = 1800W). I calculated the load by using equipment loads where known (dental equipment, fans, lighting, hvac) the duplex receptacles are calculated as 180W each (10 per 20A circuit, that's how I was taught). I was also taught, and told by inspectors, that my panel calcs had to show the 125% "safety factor" load.

The 213A does not include any future expansion.
Click to expand...
Why would you never expect to talk about diversification?

Perhaps your PM has had conversations with someone who knows more about the loads in question and how to calculate them. Have you talked to your PM about the difference in the two results?
 
DanZ said:
It's not all continuous load, but I was taught to calculate it that way (20A breaker = 1800W). .
Click to expand...

Of course that will work but it is not required by the NEC.

There is plenty of 'fluff' built into the rules of Articles 220. To keep the service size down take the continuous load at 125% and the non-continuous load at 100%.

Read the definition of continuous load in Article 100 then redo your calculations and there is a darn good chance that you will come in at 200 or under.
 
> The load calc the EC is showing is 76,756VA/359 Volts
> =213A @ 3ph 4 wire 208V

Is this what is being said?
Power / Volts = Amps
(77KVA / 1.73) / 208 = 213A @ 3ph 4 wire 208V

Short cut equations are not always clear.
 
glene77is said:
> The load calc the EC is showing is 76,756VA/359 Volts
> =213A @ 3ph 4 wire 208V

Is this what is being said?
Power / Volts = Amps
(77KVA / 1.73) / 208 = 213A @ 3ph 4 wire 208V

Short cut equations are not always clear.
Click to expand...
The end result is the same, but your equation is odd (see my post #12 as a reference).

What is (77KVA / 1.73) supposed to represent?
 
Glen,

I thought I'd better clarify my question. You made the effort to post something that had already been posted. I am wondering if you are raising a new point.

You had this equation: Power / Volts = Amps
and then this: (77KVA / 1.73) / 208 = 213A

Are you saying there is some significance in grouping (77KVA / 1.73) as a part of power (referring to your 1st equation)?
 
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