# correct amount of circuits

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#### normbac

##### Senior Member
72 light fixtures ballast 280 watts #10 cu wire

72 x 280 watts = 20,160 x125% = 25,200 *277V = 90.97A * 30 amp circuits = 3.03
Question is does the .03 mean that I will need a fourth circuit or is rounding down permissible I am trying to use three 30a rated switches to control 3 groups of 24 lights

TIA & Merry Xmas

#### yired29

##### Senior Member
72 light fixtures ballast 280 watts #10 cu wire

72 x 280 watts = 20,160 x125% = 25,200 *277V = 90.97A * 30 amp circuits = 3.03
Question is does the .03 mean that I will need a fourth circuit or is rounding down permissible I am trying to use three 30a rated switches to control 3 groups of 24 lights

TIA & Merry Xmas

IMHO You would need to have 4 OCPD you can't have a fraction of a OCPD. There are no provisions for rounding the number of OCPD like there is for ampere 220.5 (B).

#### normbac

##### Senior Member
IMHO You would need to have 4 OCPD you can't have a fraction of a OCPD. There are no provisions for rounding the number of OCPD like there is for ampere 220.5 (B).
It was wishful thinking

#### satcom

##### Senior Member
72 light fixtures ballast 280 watts #10 cu wire

72 x 280 watts = 20,160 x125% = 25,200 *277V = 90.97A * 30 amp circuits = 3.03
Question is does the .03 mean that I will need a fourth circuit or is rounding down permissible I am trying to use three 30a rated switches to control 3 groups of 24 lights

TIA & Merry Xmas

You may have more then the .03 to worry about with that type of load. Are they electronic ballast?

#### yired29

##### Senior Member
72 light fixtures ballast 280 watts #10 cu wire

72 x 280 watts = 20,160 x125% = 25,200 *277V = 90.97A * 30 amp circuits = 3.03
Question is does the .03 mean that I will need a fourth circuit or is rounding down permissible I am trying to use three 30a rated switches to control 3 groups of 24 lights

TIA & Merry Xmas

But if we do the math like this I believe it would work.

72 / 3 = 24
24 x 280 = 6720
6720 / 277 = 24.26
24.26 x 1.25 = 30.325
We could round 30.325 down to 30 amps.
This way there could be only three circuits. One MWBC

If I missed something please correct me.

#### yired29

##### Senior Member
You may have more then the .03 to worry about with that type of load. Are they electronic ballast?

Good point to be 100% accurate we would need nameplate values.

#### normbac

##### Senior Member
You may have more then the .03 to worry about with that type of load. Are they electronic ballast?
I assumed they were electronic ballast why does this affect the outcome?

#### normbac

##### Senior Member
Good point to be 100% accurate we would need nameplate values.
I decided to go with four circuits but what are you referring to on name plate is that actual bulb wattage which would be 54w x 6

#### yired29

##### Senior Member
I assumed they were electronic ballast why does this affect the outcome?

This is not a feeder you could do the calculations based on the individual circuits. For 24 fixtures per circuit.

#### yired29

##### Senior Member
I decided to go with four circuits but what are you referring to on name plate is that actual bulb wattage which would be 54w x 6

54 x 6 = 324watts not 280watts.

The ballast amperage rating is not necessarly the same as the bulb wattage.

Ballast,Electronic,F17T8 Lamp
Item # 3G817
Fluorescent Electronic Ballast, Lamp F17T8, Number of Lamps 2, Input Voltage 277 Volts, Line Current 0.15 Amp, Lamp Length 24 Inches

2- 17 watt lamps = 34 watts
34 / 277 = .122 amps

Ballast says .15 amps

#### normbac

##### Senior Member
54 x 6 = 324watts not 280watts.

The ballast amperage rating is not necessarly the same as the bulb wattage.

Ballast,Electronic,F17T8 Lamp
Item # 3G817
Fluorescent Electronic Ballast, Lamp F17T8, Number of Lamps 2, Input Voltage 277 Volts, Line Current 0.15 Amp, Lamp Length 24 Inches

2- 17 watt lamps = 34 watts
34 / 277 = .122 amps

Ballast says .15 amps
Always thought suppose to use ballast wattage? they are T5 HO 6 lamp

#### Dennis Alwon

##### Moderator
Staff member
Norm I think you can run 3 circuits. Each fixture is 280 watts at 277 volts is virtually 1 amp apiece. On a 30 amp circuit you can have 24 amps for continuous load. 24 *3= 72.

#### Dennis Alwon

##### Moderator
Staff member
Also look at 220.5(B)

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.
(B) Fractions of an Ampere. Where calculations result in a fraction of an ampere that is less than 0.5, such fractions shall be permitted to be dropped.

#### Paelectrician

##### Member
We were taught you can never round them down, even if it cam out to 3.1 you would need 4 circuits

#### Dennis Alwon

##### Moderator
Staff member
We were taught you can never round them down, even if it cam out to 3.1 you would need 4 circuits
Did you read article 220.5(B)? It clearly states that you can esp. with an 80% factor already in the equation.

#### Paelectrician

##### Member
Did you read article 220.5(B)? It clearly states that you can esp. with an 80% factor already in the equation.

no, but will look that up, thanks. Learn something new everyday!

#### Dennis Alwon

##### Moderator
Staff member
no, but will look that up, thanks. Learn something new everyday!
I posted it a few post above.

#### Paelectrician

##### Member
Yeah, my dumb ass kinda read the question and skipped right to the end!

#### yired29

##### Senior Member
Did you read article 220.5(B)? It clearly states that you can esp. with an 80% factor already in the equation.

There are two different subjects here. You can round down for amperage 220.5 (B) but not for the number of circuits.

If you had a calculation that required anything over a whole number when it comes to circuit calculations you need to use the next whole number.

If you had a calculation that required anything over .49999 amps when it comes to amperage calculations you need to use the next whole number if less than .5 you can round down.

I don't think I'm missing something.

#### infinity

##### Moderator
Staff member
Do this fixtures fall under 210.23(B) and have heavy duty lampholders?

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