# Flourescent Lighting Circuit Size

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#### jsharvey

##### Member
I've been told several different ways to calculate the size of a branch circuit for flourescent lighting. One is the amp draw of the ballast times the number of fixtures, the other is the total wattage of the tubes times the number of fixtures divided by the voltage. Can someone please tell me which way is correct. Thanks in advance.
Jim

#### tom baker

##### First Chief Moderator
Staff member
Re: Flourescent Lighting Circuit Size

A HID fixture is not 100% efficient. Take a look at NEC section 220.4(B).

#### pasha

##### Member
Re: Flourescent Lighting Circuit Size

Both method of calculation is equally acceptable, But the convenient method is the amperage one because it takes into consideration in the power factor of choke that is intended to be employed into the circuit.

#### steve66

##### Senior Member
Re: Flourescent Lighting Circuit Size

Per Tom's post you are supposed to use the actual current. However, it is often harder to get the specs. on the current draw than the lamp wattages. I usually use the lamp wattages and add an extra 25% for ballast losses and power factor. I count a 2 tube 32 Watt T8 fixture as 80 Watts, a 4 tube fixture as 160 watts, etc.

By not exceeding 1920 Watts (for a 20A circuit)another 25% factor added in for continuous loads.

Steve

#### gregwalter

##### Member
Re: Flourescent Lighting Circuit Size

use the amperage on the ballast label

#### pierre

##### Senior Member
Re: Flourescent Lighting Circuit Size

220.4(B) Inductive lighting load.

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. .

That is in Part one of Article 220.

Pierre

#### bwyllie

##### Senior Member
Re: Flourescent Lighting Circuit Size

Most manufacturer's will specify the input wattage or amperage of their fixtures(including the ballast factor).

#### steve66

##### Senior Member
Re: Flourescent Lighting Circuit Size

It is easy to tell we have both engineers and electricians sharing info. on this site (at least not one for any light fixtures I have specified as part of my job). As for the manufacturer's specs, we never know exactly which company will win the bid for the fixtures. Even if we knew who was going to manufacture the fixture, they are free to use bid out ballast suppliers, so that is another unknown.

That's why I use the lamp wattage and add %25 percent. If anyone can show me a case where this wouldn't be enough of a safety factor, I would appreciate it (and I would start using a larger factor).

Steve

#### iwire

##### Moderator
Staff member
Re: Flourescent Lighting Circuit Size

Steve I would say you are more than covered with the 25%.

[ January 29, 2004, 06:06 PM: Message edited by: iwire ]

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