Banjorear
Well-known member
Curious how many high output T-8's I can daisy chain together on the same 15A run?
How many T-8 lights can I link together on the same 15A run?
- Thread starter Banjorear
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pattenp
Well-known member
You need to get the amp draw off the label of the ballast. The total amps should be no more than 12 amps for a 15 amp circuit. That's 80%. Example, if the fixtures pull 1 amp each then 12 fixtures is max on a 15A circuit.
Edit: what size T8 fixture? 48" 2 lamp, 4 lamp?
Edit: what size T8 fixture? 48" 2 lamp, 4 lamp?
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pattenp
Well-known member
I looked at a couple of F48T8HO ballast for 2 lamps and they use right at .7 amps. So that would be 17 4ft 2 lamp T8HO fixtures.
OK, makes sense. I know just enough about electricity to get myself into trouble.
I use the formula Volts x Amps = Watts or 120V x 15A = 1800Watts. If you want to run at 80% capacity that would be 1440 watts.
So it would support 14 100 watt fixtures, or 24 60 watt fixtures. I believe this is the same logic pattenp is using, just stated a little differently. In his example I assume the .7 amp lamps consume 84 watts to arrive at the 17 fixture capacity.
One thing I have wondered with this formula is with the new LED and CFL lamps it seems you can run quite a few more fixtures than with incandescent. As long as you are not switching back to higher wattage lamps you should be okay?
So it would support 14 100 watt fixtures, or 24 60 watt fixtures. I believe this is the same logic pattenp is using, just stated a little differently. In his example I assume the .7 amp lamps consume 84 watts to arrive at the 17 fixture capacity.
One thing I have wondered with this formula is with the new LED and CFL lamps it seems you can run quite a few more fixtures than with incandescent. As long as you are not switching back to higher wattage lamps you should be okay?
pattenp
Well-known member
With fluorescent tube fixtures you use the line amperage from the ballast tag. You do not use the lamp wattage to figure the amp load. The lamp wattage will get you in the ball park but is not the true wattage that the fixture will use. The ballast amperage is usually less than the calculated amperage from the the tube wattage. The F48T8HO tube wattage is 44W or the total tube wattage is 88W for the 2 lamp fixture.
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Thanks pattenp, I did not know that about fluorescent ballasts. I suppose in general it's a good idea to measure circuit load in amps since watts is more of light bulb measurement and does not apply readily to other devices like garage door openers.
In my case my lights are on the same circuit as the garage door opener so assuming a 3/4 hp opener draws 5 amps (not sure what it draws) I only have 10 left over and just 7 with 20% cushion factor. Thanks again!
In my case my lights are on the same circuit as the garage door opener so assuming a 3/4 hp opener draws 5 amps (not sure what it draws) I only have 10 left over and just 7 with 20% cushion factor. Thanks again!
2ManyProjects
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Close, but no cigar.
The thing is, your "Volts x Amps = Watts" formula only holds for DC circuits, or AC circuits whose loads are PURELY resistive. As soon as you start dealing with reactive (i.e., capacitive and/or inductive) loads, Volt-Amps and Watts become two different (and divergent) things.
In theory, that is correct. But to be a bit pedantic about it, you really ought to use the maximum load for which the fixtures are rated (typically, as labeled on the fixture itself, such as, for example "100 Watt Incandescent") when calculating how many fixtures may be safely run off a common circuit. The reason for this is, even if YOU plan to run only low-draw CFL or LED replacement lamps in those fixtures, you cannot be certain that someone else won't come along and replace them with old-style incandescents.
No. You are fundamentally misunderstanding things.
Watts are watts and amps are amps, regardless of the type of load. They are two completely different units of measurement, which apply to two completely different things. Both Watts and Volt-Amps are a measure of power. Amperes are a measure of current, ONLY.
[And BTW... Even run-of-the-mill light bulbs are (slightly) inductive in practice; but usually SO slightly that the difference between VA and Watts can be safely ignored.]
The issue is, from a load-carrying (and hence, safety) standpoint, a given piece of wire doesn't care one whit what the voltage potential of the circuit (as a whole) is -- or, by extension, how much power is being delivered by that circuit. What it does care about it the total amount of CURRENT flowing through that wire. It is that current which, when it runs up against the resistance of the wire itself (and yes, ALL wires have at least SOME inherent series resistance), will produce heat. It is that heat which is the enemy.
WHOA, NELLIE!!!
You do know what they say about "***-u-ME", right? A typical "3/4 HP" electric motor will draw MUCH more than 5 Amps @ 120V -- probably at least double that. See, for example, http://www.engineeringtoolbox.com/electrical-motor-hp-amps-d_1455.html.
More to the point, when if comes to properly sizing a circuit to support that motor, it is the MAXIMUM possible draw which is important. This maximum draw typically occurs at the instant the motor is first started; but it can also be approached if/when the motor gets bogged down by a heavy load (say, for example, your 8-year-old kid trying to "ride" the garage door as the GDO struggles to lift it). This value is sometimes referred to as Locked Rotor Amps, or "LRA" for short (which is what you should find on the motor's data plate). THAT is the value you MUST allow for when sizing the circuit.
Bottom Line: If your GDO really does sport a 3/4 HP motor, then it should not share a circuit with ANYTHING.
The potential "saving grace" here is, your so-called "3/4 HP opener" probably isn't REALLY anywhere close to 3/4 horsepower. When if comes time to to plaster flashy graphics on the box, vendors lie through their teeth on this sort of stuff all the time, particularly when a "bigger" number might promote more sales. You need to check the actual data plate on either the unit or the motor itself, in order to know what you REALLY have.
Thanks for the clarifications 2ManyProjects.
The info on the opener says 5 amps so that's what I'm using in my load calcs for this circuit. It would be nice to have a separate circuit but that's not happening any time soon. Our box is in a weird spot in a finished area, I think it would cost quite a bit to add circuits. Maybe if I really need more power I could look into adding a subpanel or something some day but for now the opener will need to share a 15amp circuit with lights. Thanks again!
The info on the opener says 5 amps so that's what I'm using in my load calcs for this circuit. It would be nice to have a separate circuit but that's not happening any time soon. Our box is in a weird spot in a finished area, I think it would cost quite a bit to add circuits. Maybe if I really need more power I could look into adding a subpanel or something some day but for now the opener will need to share a 15amp circuit with lights. Thanks again!
Kevin C
Well-known member
One point you may want to consider it the power factor of the light your running. Commercial lights will have a power factor that is closer to unity.
Standard residential lights will often have a power factor that is strongly inductive (.6 power factor).
What that means? The peak current draw will be out of phase with the peak voltage for many residential lights (lagging).
For a given wattage of power consumed, the peak current will be higher than with a resistive load.
That's a good reason to use line amperage off the side of the ballast.
You can get ballasts with power factor correction. For a given wattage the peak current draw will be less, the actual power consumed will be very similar.
If you were to run at 80% of a circuits rated capacity based on wattage, the peak current would exceed the rating of the breaker. Most times that is not enough to trip a breaker, but you will get an annoying buzzing sound from it.
Standard residential lights will often have a power factor that is strongly inductive (.6 power factor).
What that means? The peak current draw will be out of phase with the peak voltage for many residential lights (lagging).
For a given wattage of power consumed, the peak current will be higher than with a resistive load.
That's a good reason to use line amperage off the side of the ballast.
You can get ballasts with power factor correction. For a given wattage the peak current draw will be less, the actual power consumed will be very similar.
If you were to run at 80% of a circuits rated capacity based on wattage, the peak current would exceed the rating of the breaker. Most times that is not enough to trip a breaker, but you will get an annoying buzzing sound from it.
2ManyProjects
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I'm not debating the photographic evidence; but SOMETHING is not adding up there. I strongly suspect that either Sears is fibbing about the "3/4 HP" figure, or that the "5.0 AMPS" figure refers only to constant running load, as opposed to start-up load and/or LRA, or both.
In which case, I would be VERY conservative about adding loads to that circuit. You mentioned "5 or 6 fixtures". If you limit those to twin-tube F32T8 fluorescents, that would be another 3 Amps or so (based on "typical" values) -- and you just might skate by. Is there anything ELSE also on that same circuit?
pattenp
Well-known member
Why are you saying Sears is fibbing about the 3/4HP? Unless I'm having brain freeze 3/4HP is 4.66A @ 120V. 1HP = 746W. I'm sure it's just running load.
Kevin C
Well-known member
View media item 34159
Typical consumer ballast... 1.02 amps for a double bulb, 4' T8 @ 32 watts.
For your 15 amp circuit: At 80% you have a total of 12 amps of continuous load capacity. 5 amps for the opener leaves 7 amps for the lights. Assuming that's all you have on that circuit that gets you seven fixtures ( not nit picking the .02 per fixture).
The power factor on this ballast is .567, and the actual wattage is 56.5 watts. 90.96 lumens per watt using Phillips 32 watt bulbs (F32T8/TL841/ALTO~ 5125 lumens from both bulbs).
That gets you 35,875 total lumens.
With such a low power factor, the actual current draw is almost 2 x higher for a given wattage... Since the peak current draw is not in phase with the voltage, the actual power consumed is less (56 watts).
With a perfect power factor (1), you should be able to run 14 of those lights. That's why power factor matters.
BTW... The numbers are not perfect but should be pretty close.
Typical consumer ballast... 1.02 amps for a double bulb, 4' T8 @ 32 watts.
For your 15 amp circuit: At 80% you have a total of 12 amps of continuous load capacity. 5 amps for the opener leaves 7 amps for the lights. Assuming that's all you have on that circuit that gets you seven fixtures ( not nit picking the .02 per fixture).
The power factor on this ballast is .567, and the actual wattage is 56.5 watts. 90.96 lumens per watt using Phillips 32 watt bulbs (F32T8/TL841/ALTO~ 5125 lumens from both bulbs).
That gets you 35,875 total lumens.
With such a low power factor, the actual current draw is almost 2 x higher for a given wattage... Since the peak current draw is not in phase with the voltage, the actual power consumed is less (56 watts).
With a perfect power factor (1), you should be able to run 14 of those lights. That's why power factor matters.
BTW... The numbers are not perfect but should be pretty close.
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Kevin C
Well-known member
http://www.espentech.com/product_info2.php?cPath=51&product_name=VE232MVHIP
Ballast #
VE232MVHIP
This ballast with a power factor of .98 puts out almost the same amount of light but only draws .5 amps!!!! That would let you install 14, 4' double bulb T8 lamps and have the same current draw. This is known as mark5767's perfect ballast.
The actual power draw would go up by about 2X... No free light or actual efficiency gain, just better control of the power factor. Whats annoying is a lot of this info is not easy to come by.
www.energystar.gov/ia/partners/manuf_res/.../rlf_platform_database.xls
Link to government spread sheet ballast data.. That and I had to open a light up to get the rest of the info.
Bottom Line:
Using the lamps with power factor correction you can get 67,564 lumens from the same 7 amps of current. That is 1.88 times more light
Ballast #
VE232MVHIP
This ballast with a power factor of .98 puts out almost the same amount of light but only draws .5 amps!!!! That would let you install 14, 4' double bulb T8 lamps and have the same current draw. This is known as mark5767's perfect ballast.
The actual power draw would go up by about 2X... No free light or actual efficiency gain, just better control of the power factor. Whats annoying is a lot of this info is not easy to come by.
www.energystar.gov/ia/partners/manuf_res/.../rlf_platform_database.xls
Link to government spread sheet ballast data.. That and I had to open a light up to get the rest of the info.
Bottom Line:
Using the lamps with power factor correction you can get 67,564 lumens from the same 7 amps of current. That is 1.88 times more light
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2ManyProjects
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Well first, I did say "suspect" and "either/or", as I'm really not sure what is going on with that particular device. But as for why I suspect the figures may be bogus (or at least not telling the whole story)...
Purely in terms of unit-conversion, sure. But when was the last time you saw a 100%-efficient electric motor? And of course, there remains the issue of the start-up/LRA load, which would presumably be several times that.
That would be more plausible. But if so, I remain a little surprised that this would be the ONLY current rating on the data plate. Then too, putting "3/4 HP" on the data plate is also a little puzzling. If it were just a matter of some flashy "advertising" label, I would be quicker to dismiss it. OTOH, if that data plate were for the motor itself (as opposed to the unit as a whole), I'd take it a bit more seriously. As it stands, its "credibility" is somewhere between those two extremes. And overarching ALL of this would be my reluctance to underestimate the power of the Sears marketing department, particularly in the context of their dealings with a "captive" OEM vendor (who could presumably be rather easily strong-armed into putting whatever numbers on that plate that Sears told them to put on that plate).
Bottom line:
Kevin C
Well-known member
UL listed and 5 amps on the same label says UL approved when it underwent approval testing at a UL LAB. The factory is audited at least once a year for conformance.
During the testing, the operating current is verified.
During the testing, the operating current is verified.
pattenp
Well-known member
I'm not seeing why make this so over the top complicated and throwing in so much extraneous information. The label states 5A it does not list LRA... it's just a garage door opener. The listed 5A is quite reasonable with the opener having a 3/4HP motor. 
Well first, I did say "suspect" and "either/or", as I'm really not sure what is going on with that particular device. But as for why I suspect the figures may be bogus (or at least not telling the whole story)...
Purely in terms of unit-conversion, sure. But when was the last time you saw a 100%-efficient electric motor? And of course, there remains the issue of the start-up/LRA load, which would presumably be several times that.
That would be more plausible. But if so, I remain a little surprised that this would be the ONLY current rating on the data plate. Then too, putting "3/4 HP" on the data plate is also a little puzzling. If it were just a matter of some flashy "advertising" label, I would be quicker to dismiss it. OTOH, if that data plate were for the motor itself (as opposed to the unit as a whole), I'd take it a bit more seriously. As it stands, its "credibility" is somewhere between those two extremes. And overarching ALL of this would be my reluctance to underestimate the power of the Sears marketing department, particularly in the context of their dealings with a "captive" OEM vendor (who could presumably be rather easily strong-armed into putting whatever numbers on that plate that Sears told them to put on that plate).
Bottom line: