10 Fluorescents on one Switch

[Dan]

I incorrectly posted the following note in the Tips section yesterday:

I have a need to turn on 10 fluorescent fixtures with a pair of 3-way switches. These fixtures have four 40 watt tubes each. What is the actual amperage draw from such a fixture? Can I do this safely on 20 amp rated switches?

Wannabee responded that the circuit would handle 2400 watts and therefore, based on the bulb wattage total, the circuit could handle this okay. However, don't you have to take into account the power loss draw of the ballast? If each ballast pulls another amp, wouldn't I have another 10 amps and overload the circuit. Wannabee, I appreciate your response, but just want to make sure about the ballast issue.

Thanks,
Dan

[6pack]

Effiency maybe a factor with age, otherwise no of no such added calculation. Are you thinking ballast amperage rating PLUS tube draw in your thinking?? See what others say.

[Ron]

The ballast definitely has losses that you must consider. I didn't realize that they sell new fixtures with 40W bulbs anymore, just the 34W in the T12 Configuration.
In any case, in your example you have 4x40*10=1600W worth of lamps.
An ballast manufactured by Advance R4S40-A-TP-AC, has a 172 input watt rating for (4) 40W lamps.
So 172*10=1720W In many occupancies this will be continuous load, so 1720*1.25=2150W

Since a 120V 20A breaker is rated at 2400W, you would be fine.

[mdshunk]

Unless the fixtures have already been purchased, it's becoming more and more standard to use the F32T8 lamps in troffers and such. You get many more lumens per watt (brighter light) and less "fade off" as the lamps age. I havn't installed any new T12 fixtures in quite a while, except for fixture replacements. The energy cost savings gained by using the T8 lamps is a fairly decent ROI when spec'ing out new fixtures.

[6pack]

sorry bout the calculation. comes out to 1 amp TOTAL for the question you had asked not 1 amp for each ballast. Always held circuit limit to 10 lumens anyways as mentioned.
Curious now as I have never seen one ballast handling 4 tubes. Either new design or always around. Troffers i saw always had 2 ballasts.
Since were in commercial forum here are then all lighting loads to be calculated at 125%? Seems like somewhat of a gray area?(continuos load) Curious

[KJBeck]

When calculating the number of permissible flourescent fixtures on a branch circuit...... It is not necessary to calculate the lamp wattage, ballast wattage, etc., when determining total load. Every ballast, no matter who the manufacturer, must indicate it's amperage draw on the label. And, as the NEC states...
220.4 (B) - For circuits supplying lighting units that have ballasts, transformers, or autotransformers, the computed load shall be based on the total ampere ratings of such units and not on the total watts of the lamps

The 125% is used when calculating the required conductor ampacity or the required Overcurrent Protection for the given load on the branch ckt or feeder. In other words, "This is your load, how big of wire or how big of ckt bkr. do you need?" It is also very important to note that there is also an exception in the NEC: [ Branch: 210.19 A (1) exception1 or 210.20 A exception1 ---- Feeder: 215.2 A (1) exception1 or 215.3 exception1] if the assembly, including the overcurrent devices protecting the (branch circuits/feeders) is listed for operation at 100%, the allowable ampacity of the (branch circuit/feeder) conductors shall be permitted to be not less than the noncontinuous load plus the continuous load.

What this is eluding to is motor duty-cycle. When doing motor calculations, it is necessary to factor in the motor's duty cycle when calculating feeder & branch ckt ampacities & overcurrent protection. If a motor is rated anything other than continuous duty, it is, in other words, "not rated for operation at 100%" Knowing this, we now see that when caclulating the previously mentioned flourescent lighting load, it is not necessarry to factor in the 125%.

There is, however, one restriction in the NEC which mandates what size ckt brkr you must use to protect small wires.

240.4 (D) Small Conductors. Unless specifically permitted in 240.4(E) through (G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper;...

If you used 20A rated switches, chances are you were using #12 wire landed on a 20A ckt. bkr. In a commercial application where conduit is typically used, several current-carrying conductors are ran in the same conduit. Therefore, table 310.15 (B)(2)(a) in the NEC tells us that we must derate the 20A by a factor of 20%. The #12 wire on a 20A bkr is now only good for 16A. Divide this 16A by the amperage draw of each fixture to determine the number of fixtures that are allowed on that circuit.

Remember, even if you aren't required to derate, it's probably not a bad idea. The closer you get to the full interrupt rating of the breaker, the more chances threre are for nuissance tripping to occurr.

Kevin J Beck

[Ron]

Kevin, although I'm nit picking, I just wanted clarify for other readers that the 20% derating as required in 310.15 (B)(2)(a) is for 4-6 current carrying conductors. More conductors requires more derating. In addition, 110.14(C) would allow you to derate from the 90 deg column, which is 25A for a #12.

[Wgoodrich]

It should be much more simple in your switch design. If this is a 20 amp switch on a branch circuit that is rated 20 amp max then you are covered regardless of the load. The breaker is designed to trip before you max out the switch being rated switch same as branch circuit size.

However if you had a 20 amp circuit and only 10 amp load say you could install a 15 amp rated switch on a 20 amp rated circuit. This is when the calculations mentioned would need to be done.

20 amp switch on properly installed 20 amp branch circuit you are in like flint.

Wg