Calculation

[xformer]

Question. How many 120V 20A circuits are needed for an office building with a 135KVA lighting load on a 120V/208V system?
I came up with the following solution:
135000 x 1.25 = ( Continuous)
168750 / 208 = 811 amps
811 amps / 20 = 41 Circuits

Correct or am I missing a step ?

 

[charlie b]

Why did you divide by 208, if the lighting circuits are 120 volts? Are you really dealing with 208 volt light fixtures? If not, then I would change the math as follows:

• 168,750 VA / 120 volts = 1406 amps.
• 1406 amps / (20 amps per circuit) = 71 circuits.

Please note that a circuit should not be loaded beyond 16 amps of continuous load. But you have already accounted for this factor, by using the 125% in your first step.

 

[xformer]

Been a while since I have done something of this nature and I think I confused myself on the single phase loads originating from a 3 phase panel. Even though the load is 120V doesnt 120 x 1.73 = 208 have to be used in the calculation? Or would the 208 be used to calculate the amperage on the feeder?

 

[charlie b]

Let's approach this from the other direction. Presuming the lights are fed by 120 volt, single phase circuits, then a 20 amp circuit loaded to 80% would supply a load of 120 volts times 20 amps times 0.8, or 1,920 VA. You have 135,000 VA of lighting, so that would mean that you need 135,000/1920, or 70.3 circuits. Round that up to 71.

If you want to know the load on the feeder that serves the three-phase, 208 volt panel that has all of those circuits, then take 135,000 / (208 * 1.732), and you will get 375 amps.

 

[Ingenieur]

Each ckt max 16A x 120 = 1920 VA
135000/1920 = 71
figure at least 80

 

[xformer]

Many Thanks Charlie. Have a good one.

 

[precise8128]

Let's approach this from the other direction. Presuming the lights are fed by 120 volt, single phase circuits, then a 20 amp circuit loaded to 80% would supply a load of 120 volts times 20 amps times 0.8, or 1,920 VA. You have 135,000 VA of lighting, so that would mean that you need 135,000/1920, or 70.3 circuits. Round that up to 71.

If you want to know the load on the feeder that serves the three-phase, 208 volt panel that has all of those circuits, then take 135,000 / (208 * 1.732), and you will get 375 amps.

would the 80% rule apply to this? and these would not be a continuous correct?

 

[iwire]

would the 80% rule apply to this? and these would not be a continuous correct?

I would treat office lighting as a continuous load.

When I design office lighting circuits I shoot for about 10 to 12 amps on each which leaves some capacity to add a few fixtures if needed.

 

[Sahib]

When I design office lighting circuits I shoot for some capacity to add a few fixtures if needed.

Applying ANSI nominal voltage rating 115V instead 120V is also helpful in that direction.

 

[Smart $]

Applying ANSI nominal voltage rating 115V instead 120V is also helpful in that direction.

Not in regards to Code compliance.

220.5 Calculations.

(A) Voltages. Unless other voltages are specified, for pur-
poses of calculating branch-circuit and feeder loads, nomi-
nal system voltages of 120, 120/240, 208Y/120, 240, 347,
480Y/277, 480, 600Y/347, and 600 volts shall be used.

(B) Fractions of an Ampere. Calculations shall...

 

[Sahib]

smart$: The code clearly states ''(A) Voltages. Unless other voltages are specified, for purposes of calculating'' and ANSI gives nominal voltage 115V for 120V. So no code violation by specifying 115V instead of 120V.

 

[mivey]

ANSI uses 120 volts as the nominal basis in their standard. 115 was in the past.

 

[Smart $]

... So no code violation by specifying 115V instead of 120V.

It is a violation if the nominal voltage is 120 volts.

Ditto on mivey's response.

And for those that may think it, voltage drop does not change the nominal voltage.

 

[Sahib]

ANSI uses 120 volts as the nominal basis in their standard. 115 was in the past.

The ANSI equipment utilization voltages are not changed and it is actually less than 115 V:

http://www.powerqualityworld.com/2011/04/ansi-c84-1-voltage-ratings-60-hertz.html

 

[Sahib]

It is a violation if the nominal voltage is 120 volts.

Ditto on mivey's response.

And for those that may think it, voltage drop does not change the nominal voltage.

In design, the designer can specify the utilization voltage of an equipment and calculate VA accordingly and the code allows it:

''(A) Voltages. Unless other voltages are specified, for purposes of calculating''

 

[don_resqcapt19]

In design, the designer can specify the utilization voltage of an equipment and calculate VA accordingly and the code allows it:

In my opinion that section only permits the designer to specify the voltage, where non-standard voltage systems are being used.

220.5 Calculations
(A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.

 

[mivey]

The ANSI equipment utilization voltages are not changed and it is actually less than 115 V:

http://www.powerqualityworld.com/2011/04/ansi-c84-1-voltage-ratings-60-hertz.html

Let's see...reading the ANSI standard you referenced...

Yep, there it is: 120 volt nominal base.

Thanks for validating what I said.

 

[Smart $]

Let's see...reading the ANSI standard you referenced...

Yep, there it is: 120 volt nominal base.

Thanks for validating what I said.

An open mouth, insert foot moment for Sahib...

 

[Ingenieur]

An open mouth, insert foot moment for Sahib...
E
View attachment 14597

could you post the notes and Range A and B definitions?

 

[Smart $]

could you post the notes and Range A and B definitions?

I don't have them, and I don't have the standard. That table was a graphic on the page linked.

Perhaps those are references to Notes A and B to earlier graphic??? See linked page.