# Power Consumption

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#### T.C

##### Member
This will sound like a stupid question to most. In my workbook I am calculating total power in a parallel circuit. On one part of the circuit, it shows a ballast labeled to be rated at 20W, but attached to the ballast it shows two lamps labeled to be rated at 50W each.

Would I be correct in deciding that regardless of what it is showing for lamps, the power draw for this section of the circuit cannot be be more than the power rating of the ballast (20W)?

#### peter d

##### Senior Member
You always use the rated CURRENT of the ballast based on the lamp wattage that is being used, because most ballasts can light a variety of lamps.

#### T.C

##### Member
okay

okay

so to calculate the current drawn on this part of the circuit, I should use the total wattage that the two lamps are rated for? (100W) based on a 120v circuit .833A ?

will the lamps be able to draw 100W if on a 20W ballast?

I don't know if there is a way for me to post the diagram that I am referring to if it would simplify things

#### peter d

##### Senior Member
I'm confused about what you are trying to determine. A ballast that is only rated for 20 watts would not have lamps rated for 50 watts. Something is not right about this, or you are explaining it incorrectly.

#### T.C

##### Member
Ya, I know

Ya, I know

I apologize for any confusion. I also realize that it does not make any sense. Unfortunately, this textbook is full of typos. More importantly, the workbook has already displayed a tendency to give unrealistic diagrams, I guess in order to make us pay attention to detail. As I said, I wish that I could post a scan of the page to illustrate my question.

That being said, I did describe it exactly as it is presented to me.

So I suppose that the question I should be asking is that can it draw more than 20W on a 20W ballast even if some idiot might have put two 50W lamps on it.

#### Energy-Miser

##### Senior Member
T.C said:
I apologize for any confusion. I also realize that it does not make any sense. Unfortunately, this textbook is full of typos. More importantly, the workbook has already displayed a tendency to give unrealistic diagrams, I guess in order to make us pay attention to detail. As I said, I wish that I could post a scan of the page to illustrate my question.

That being said, I did describe it exactly as it is presented to me.

So I suppose that the question I should be asking is that can it draw more than 20W on a 20W ballast even if some idiot might have put two 50W lamps on it.
They may mean that the ballast itself cosumes 20w when fully loaded. Although it seems like too much for a modern ballast powering a total of 100 watts of load. That seems to me the most reasonable assumption, unless you are dealing with typos as you suggested. e/m.

#### iwire

##### Moderator
Staff member
It also sounds wrong to me but I would ignore the lamps, it may be a trick.

It is worth a look at 2005 - 220.18(B)

#### T.C

##### Member
ballast consume?

ballast consume?

I wasn't aware that a ballast actually consumes power in and of itself?

#### iwire

##### Moderator
Staff member
T.C said:
I wasn't aware that a ballast actually consumes power in and of itself?
Yes it does.

Any electrical item that gets warm during use consumes power including the conductors themselves.

#### Energy-Miser

##### Senior Member
T.C said:
I wasn't aware that a ballast actually consumes power in and of itself?
Yes, T.C, as Bob has already said, even conductors, due to the fact that they have a small non-zero resistance, do consume power, when a current passes through them. The old sytle ballasts (of the electromagnetic type) used to consume a lot of power and warm up quite a bit. The electronic ballasts use less, and thus are more efficient to use. Incidentally, their outpout is at frequencies that are much higher than the 60 hz line frequency, which based on the literature I have seen, causes the lamps they drive to run more efficiently as well. e/m

#### T.C

##### Member
thanks guys

thanks guys

I am apparently not getting a clear picture.

does that mean that in order to make my circuit calculations and achieve total power, I should just use the 20W that the ballast is rated for this branch of the circuit?

#### iwire

##### Moderator
Staff member
T.C said:
does that mean that in order to make my circuit calculations and achieve total power, I should just use the 20W that the ballast is rated for this branch of the circuit?
That is what the NEC requires in 220.18(B), it seems your test would be based on NEC calculations.

#### Energy-Miser

##### Senior Member
iwire said:
That is what the NEC requires in 220.18(B), it seems your test would be based on NEC calculations.
I looked up 220.18, there is no (B) in it. Which year NEC are you referring? I am looking at the 2002. e/m.

#### iwire

##### Moderator
Staff member
Energy-Miser said:
I looked up 220.18, there is no (B) in it. Which year NEC are you referring? I am looking at the 2002. e/m.
2005, but in 2002 you will find the same words in 220.4(B)

#### T.C

##### Member
or is it total 120w for this branch (or fixture)?
I think I just figured out how to attach a scan of it

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#### iwire

##### Moderator
Staff member
Looking at that drawing I don't know how I would treat it. :-?

#### T.C

##### Member
:grin: LOL. That's why I just don't understand what they are wanting. But I am back to my original confusion. Is the total at this load to be 20W, 100W or 120W? Does it appear that the ballast will consume as well as the lamps in order to do my load calculation?

#### rattus

##### Senior Member
Could it be?

Could it be?

Maybe the fluorescent tubes are rated in terms of their equivalent light output. For example, I am looking at a 60W (equivalent) compact fluorescent which produces 900 lumens and uses only 14W.

And perhaps the 20W ballast is rated in terms of actual power delivered. Then it would serve two 40W tubes. Don't quote me on these numbers, I am just guessing at the moment.

Yes, the ballast will dissipate some power, but I don't know how much.

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#### Energy-Miser

##### Senior Member
iwire said:
2005, but in 2002 you will find the same words in 220.4(B)
Yes saw it thanks. "... the computed load shall be based on the total ampere rating of such units and not on the total watts of the lamps."

#### Energy-Miser

##### Senior Member
rattus said:
Maybe the fluorescent tubes are rated in terms of their equivalent light output. For example, I am looking at a 60W (equivalent) compact fluorescent which produces 900 lumens and uses only 14W.

And perhaps the 20W ballast is rated in terms of actual power delivered. Then it would serve two 40W tubes. Don't quote me on these numbers, I am just guessing at the moment.

Yes, the ballast will dissipate some power, but I don't know how much.
I think something is wrong with that diagram. Don't think they mean lumen equivalent of a 50 w incandescent as you see sometimes on the CFL packages, or on PL styles. It maybe that the ballast consumes 20 watts, while putting out 100 watts. JMO FWIW. e/m

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