lighting load

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mannyb

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Location
Florida
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Electrician
we are reusing some 2x4 light fixtures with 4 lamp T8s. They will operate with a power pack/ OS and powered by 120v/20a ckt. What would be used to determine if the 120v/20a ckt is large enough. IMO the ckt is to small and may need to add ckt and power pack to area. These are approved drawings but i don't think this will work. there a total of 22 - 2x4 4lamp t8.
 
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Jim1959

Senior Member
Location
Longmont, CO
mannyb said:
we are reusing some 2x4 light fixtures with 4 lamp T8s. They will operate with a power pack/ OS and powered by 120v/20a ckt. What would be used to determine if the 120v/20a ckt is large enough. IMO the ckt is to small and may need to add ckt and power pack to area. These are approved drswings but i dont think this will work. there a total of 22 - 2x4 4lamp t8.
Click to expand...

What is the amp rating on the ballasts?
 
Electric-Light

Electric-Light

Senior Member
mannyb said:
we are reusing some 2x4 light fixtures with 4 lamp T8s. They will operate with a power pack/ OS and powered by 120v/20a ckt. What would be used to determine if the 120v/20a ckt is large enough. IMO the ckt is to small and may need to add ckt and power pack to area. These are approved drswings but i dont think this will work. there a total of 22 - 2x4 4lamp t8.
Click to expand...


It's not enough using realistic actual load. Definitely not going to be enough using name plate values
 
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ElectricMatt

Senior Member
Location
Waco, tx
How many watts per lamp... I figured them at 40w and that many pulls 29.33a. Sounds like 2 circuits almost at max load for each.


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mannyb

Senior Member
Location
Florida
Occupation
Electrician
Jim1959 said:
What is the amp rating on the ballasts?
Click to expand...


I was wrong about lamps Its a 3 lamp T8 fixture the ballast is a TRIAD and the 120/277 line current is .74/.32. Now I take that to mean .74 amps with lamps So
22x .74 = 16.28amps Did i do that correct ? so this pretty much overloads circuit. Correct?
 
iwire

iwire

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Staff member
Location
Massachusetts
mannyb said:
I was wrong about lamps Its a 3 lamp T8 fixture the ballast is a TRIAD and the 120/277 line current is .74/.32. Now I take that to mean .74 amps with lamps So
22x .74 = 16.28amps Did i do that correct ? so this pretty much overloads circuit. Correct?
Click to expand...

Unless every light is on at the same time for three hours or more you could have 20 amps on the circuit
 
Electric-Light

Electric-Light

Senior Member
ElectricMatt said:
Just curiosity why can you not go by lamp wattage?
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HIDs, fluorescent, LEDs are ballasted lamps and do not operate directly off the utility like heaters or incandescent lamps. The ballast exerts control over the lamp operating power and consumes some power itself. You can guess but the answer isn't certain enough for load calculation.

In majority of applications 32W T8 lamps are slightly under driven to give 88% light output and given about 25 watts of high frequency power from ballast. This results in 27-30W per lamp. However, there are less common ballasts that drive the lamp to different output and have different efficiencies. If you give it 30W per 4' T8 lamp, it is enough in most cases, but going by this is just enough knowledge to be dangerous. You have to consider power factor which can be close to 1 or as low as 0.5 for residential ballasts requiring 2VA per W.
 
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Electric-Light

Electric-Light

Senior Member
ElectricMatt said:
So how would you figure your power factor into the equation? To get the most accurate calculation


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Click to expand...

The fixture should come with the amp rating. If the label is available, go with that value.
Otherwise, you would use the highest amps for the line voltage shown for the line voltage you're using.
So, if you're using 120v and there are 3 sets of lamp sockets, you use the amps shown on the ballast label for 120v 3 F32T8.

You can not size the circuit for 25w lamps, because the ballast is rated to accept 32W lamps and there's nothing to prevent the end users from using them regardless of what you install.
 
Electric-Light

Electric-Light

Senior Member
journeyman0217 said:
is the ballast stepping down the line voltage?
Click to expand...

voltage is pressure
current is flow rate

The 120 or 277v power is a constant voltage device.

An incandescent lamp will maintain a steady flow rate on its own but many lamps such as LED, HID and fluorescent needs the help of a ballast to maintain the constant flow rate. These types of lamps do not hold a steady resistance and the ballast constantly throttles the voltage to hold the required current.
 
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equatorman

Member
Location
Simpsonville, SC USA
Gentlemen, I was reading in a text book that for fluorescent lights the lamp ratings should not be used for load calculations. Ballast ratings should be used instead. I asked two guys on the job and they said they always use the lamp ratings. So I am totally confused now.
If you have a fixture with a single 120v ballast rated at .80A that takes two 4ft 40watt lamps, how would you calculate the load?
Thanks for your help. Remember I am fairly new to this.
 
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GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Ballast ratings give you the actual current.
Using lamp ratings requires you to explicit add power factor and efficiency factors.
OTOH ballast ratings are worst case of all allowed lamp configurations. That is more of an issue with electronic ballasts than with magnetic.

mobile
 
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equatorman

Member
Location
Simpsonville, SC USA
Thanks for your reply GoldDigger. So if you had a bunch of fixtures I guess you would just multiply .80A by the number of fixtures and load the breaker to no more that 80 percent. Am I correct on this?
 
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