How much power registered at meter?

[JohnR66]

I have a 30watt "sping" type flourescent lamp. Current draw is .5 amp, or
60va (120 volts).

I understand that the electrical meter (rotating disk type) cannot read
reactive power, only real power drawn.
Reactive power it typically a voltage to current phase shift. The current
wave form from these CFLs is more of a high crest factor rather than a phase
shift. I wonder how the meter responds to this type of waveform?

Let me slide in another question. I have a fixture using 5 60 watt lamps.
This light is on 14 hours per day. I think we're still at 10 cents a kw/hr
around these parts, so I pay $12.60 a month to run this light (using 30 days
to calculate a month). I want to install a two 4' T8 tube fixture. I
estimate total power drawn would be 70 watts (two 32 watt tubes + 6 watts
loss in the electronic ballast. This should drop me down to only $2.94.

5 880 lumin 60 watt lamps give me 4400 lumens, what is a good lamp to use
that gives 3000K color temp, good CRI and and similar lumens?

Thanks
John

 

[Paul Hovnanian P.E.]

I have a 30watt "sping" type flourescent lamp. Current draw is .5 amp, or
60va (120 volts).

I understand that the electrical meter (rotating disk type) cannot read
reactive power, only real power drawn.
Reactive power it typically a voltage to current phase shift. The current
wave form from these CFLs is more of a high crest factor rather than a phase
shift. I wonder how the meter responds to this type of waveform?

These electromechanical meters (and most electronic metering suitable
for revenue purposes as well) work by integrating the instantaneous
product of the voltage and current waveforms over time.

Try this (run it in a spreadsheet for fun) with a 60 Hz voltage sine
wave and a 60 Hz current sine wave offset by some phase angle and plot
the resulting waveform (OK, use 50 Hz if you are on the other side of
the pond).

For a high crest factor current waveform, what you are dealing with is
series of sine wave harmonics (multiples of the fundamental frequency).
If you do the math, you will see that for a 'clean' voltage waveform,
only the fundamental component of the current waveform which is in phase
will integrate over time to produce a non zero value.

Since true watt-hour meters actually perform this
multiplication/integration, they will measure e the real power
correctly.

 

[Victor Roberts]

I have a 30watt "sping" type flourescent lamp. Current draw is .5 amp, or
60va (120 volts).

I understand that the electrical meter (rotating disk type) cannot read
reactive power, only real power drawn.
Reactive power it typically a voltage to current phase shift. The current
wave form from these CFLs is more of a high crest factor rather than a phase
shift. I wonder how the meter responds to this type of waveform?

The meter will read the actual power, as Paul has said. It
will ignore the reactive power.

Let me slide in another question. I have a fixture using 5 60 watt lamps.
This light is on 14 hours per day. I think we're still at 10 cents a kw/hr
around these parts, so I pay $12.60 a month to run this light (using 30 days
to calculate a month). I want to install a two 4' T8 tube fixture. I
estimate total power drawn would be 70 watts (two 32 watt tubes + 6 watts
loss in the electronic ballast. This should drop me down to only $2.94.

Actually, most 32-watt electronic ballasts run the lamps at
30 watts or less since the lamps are more efficient at high
frequency. 30 watts at high frequency would give a ballast
factor of about 1.0 But, you can buy electronic ballasts
with a wide range of ballast factors, so your mileage may
vary either way.

You can also buy 28-watt "energy saver" 4-foot T8 lamps
designed to run on ballasts for 32-watt lamps, and at least
one company makes a 25-watt energy saver lamp. You will
need to check the specs for your ballast to see what the
ballast factor will be for each of these lamps.

5 880 lumin 60 watt lamps give me 4400 lumens, what is a good lamp to use
that gives 3000K color temp, good CRI and and similar lumens?

To give a good estimate we would need to know the fixture
efficiency of the old incandescent lamp fixture and the new
fluorescent lamp fixture, and something about the light
distributions of each and the application. In the absence
of that data, we can proceeded with a comparison of lamps,
but you should understand this is only part of the story.

Normal 32-watt T8 lamps are rated for ~ 2850 lumens, but
there are also so-called Super T8 lamps that have ratings of
~ 3100 lumens. Two of either of these lamps would give far
more raw lamp lumens than the five 60-watt lamps you are
replacing. The 28-watt T8 lamps I found today all seem to
be rated for about 2750 lumens, so I'm assuming they use the
same technology as the 32-watt Super T8 lamps.

By using a ballast with a low ballast factor you can reduce
the power and light output further, assuming, of course,
that you don't want the additional light.

You have not told us if this fixture is being used in a
color critical application. If so, I would recommend a CRI
of no less than 82 and preferable 85, or even 90, if you can
find these in the CCT you like.

My own CCT preference for residential applications is 2700K,
but "best" CCT varies with light level and application and I
also know that personal CCT preferences are all over the
map. You're likely to get recommendations here ranging from
2700K to over 6000k.


--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.

 

[JohnR66]

The meter will read the actual power, as Paul has said. It
will ignore the reactive power.


Actually, most 32-watt electronic ballasts run the lamps at
30 watts or less since the lamps are more efficient at high
frequency. 30 watts at high frequency would give a ballast
factor of about 1.0 But, you can buy electronic ballasts
with a wide range of ballast factors, so your mileage may
vary either way.

You can also buy 28-watt "energy saver" 4-foot T8 lamps
designed to run on ballasts for 32-watt lamps, and at least
one company makes a 25-watt energy saver lamp. You will
need to check the specs for your ballast to see what the
ballast factor will be for each of these lamps.


To give a good estimate we would need to know the fixture
efficiency of the old incandescent lamp fixture and the new
fluorescent lamp fixture, and something about the light
distributions of each and the application. In the absence
of that data, we can proceeded with a comparison of lamps,
but you should understand this is only part of the story.

Normal 32-watt T8 lamps are rated for ~ 2850 lumens, but
there are also so-called Super T8 lamps that have ratings of
~ 3100 lumens. Two of either of these lamps would give far
more raw lamp lumens than the five 60-watt lamps you are
replacing. The 28-watt T8 lamps I found today all seem to
be rated for about 2750 lumens, so I'm assuming they use the
same technology as the 32-watt Super T8 lamps.

By using a ballast with a low ballast factor you can reduce
the power and light output further, assuming, of course,
that you don't want the additional light.

You have not told us if this fixture is being used in a
color critical application. If so, I would recommend a CRI
of no less than 82 and preferable 85, or even 90, if you can
find these in the CCT you like.

My own CCT preference for residential applications is 2700K,
but "best" CCT varies with light level and application and I
also know that personal CCT preferences are all over the
map. You're likely to get recommendations here ranging from
2700K to over 6000k.


--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.

Thanks to you an Paul for sharing your knowledge!

75% or more of my house is CFL. Outside lighting is 100% CFL. I'm glad the
meter is registering only the real power used.

The fixture in question is a ceiling fan with 5 bulbs. 4 are open in white
tulip glass diffusers, so most of the light gets out. The center bulb is in
a white globe. Total efficiency is not known but I see there is some loss.

I was looking at a two tube F40T12 fixture with a wrap around diffuser made
of clear acrylic with the diamond pattern. I saw some GE tubes rated 3400
lumens, CRI 70 and color temp of 3000K (I think it was). Fixture was a
Lithonia lighting. No info on its ballast. CRI is not critical, but 75 or
better would be ideal. I wish this info was clearly labeled on all brands.

I'm going check around on some 32 watt T8s fixtures today to see if I can
find something with an efficient diffuser. Some of the more decorative
fixtures seem poor in this regard.

Thanks, John

 

[Victor Roberts]

I was looking at a two tube F40T12 fixture with a wrap around diffuser made
of clear acrylic with the diamond pattern. I saw some GE tubes rated 3400
lumens, CRI 70 and color temp of 3000K (I think it was). Fixture was a
Lithonia lighting. No info on its ballast. CRI is not critical, but 75 or
better would be ideal. I wish this info was clearly labeled on all brands.

The CRI range is often coded into the lamp model number. A
"730" lap has a CRI in the 70's and a CCT of 3000. An "835"
lamp would have a CRI in the 80's and a CCT of 3500.

I'm going check around on some 32 watt T8s fixtures today to see if I can
find something with an efficient diffuser. Some of the more decorative
fixtures seem poor in this regard.

I agree on the decorative fixture efficiency.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.

 

[[email protected]]

I understand that the electrical meter (rotating disk type) cannot
read reactive power, only real power drawn. Reactive power it
typically a voltage to current phase shift. The current wave form
from these CFLs is more of a high crest factor rather than a phase
shift.

I don't know how a typical rotating-disk meter responds. I recently
got a portable elecronic kWh meter (Kill-a-watt) and it can measure
both real and reactive power. It's been interesting to compare the
readings on various things around the house.

I tried it just now on three lamps I have handy.

V A W VA PF Hz *
Electronic CFL 121.6 0.19 15 23 0.63 59.9
Magnetic CFL 122.1 0.19 12 23 0.51 59.9
Incandescent 122.1 0.84 103 102 1.0 59.9

The first one is a "Great Value" FLE14TBX/2/SW/GV CFL, rated at 120 V
60 Hz 0.24 A. I suspect this is a private-branded GE lamp. It has
been in service for a year or two. The second one is a "Lights of
America" 2614 (040) ballast and FDL13LE 981 lamp. The ballast is rated
at 120 V 60 Hz 13 W 0.23 A and the lamp is rated at 13 W. The ballast
and lamp have both been in service for 6 or 7 years. The last one is a
plain old "GE" Soft-White 100 W incandescent lamp. It has some time on
it, but not much, as it's mostly been sitting around since I've
installed CFLs in most of the house.

Standard disclaimers apply; I don't get money or other consideration
from any companies mentioned.

Matt Roberds

* The 59.9 Hz reading is pretty consistent; I suspect it's just down to
the accuracy of a $25 meter. On the other hand, I wouldn't be entirely
surprised to learn that the power company is shorting us a tenth of a
Hz and hoping nobody will notice. **

** It's a joke! At least the "nobody noticing" part. I know about
synchronous motors and other devices that use the AC line as a
frequency reference.

 

[Clive Mitchell]

* The 59.9 Hz reading is pretty consistent; I suspect it's just down to
the accuracy of a $25 meter. On the other hand, I wouldn't be
entirely
surprised to learn that the power company is shorting us a tenth of a
Hz and hoping nobody will notice. **

** It's a joke! At least the "nobody noticing" part. I know about
synchronous motors and other devices that use the AC line as a
frequency reference.

The mains frequency often drops during the day when the system is under
heavy load. In the UK they have a master clock and an atomic clock
reference so they increase the frequency slightly at night to bring the
two timing references back in line. That's why the old synchronous
motor and mains digital clocks kept such perfect time.

 

[JohnR66]

The CRI range is often coded into the lamp model number. A
"730" lap has a CRI in the 70's and a CCT of 3000. An "835"
lamp would have a CRI in the 80's and a CCT of 3500.


I agree on the decorative fixture efficiency.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.

I found a two lamp F32T8 fixture. It looks like something that would go in a
utility room. I thas a prismatic cover that wraps around the sides that
appears like it would be pretty efficient at letting more of the light out.
I bought 2,800 lumen, 78 CRI, 3000K lamps for it. I removed the center strip
that covers the ballast. It uses the small Advance brand electronic ballast.
After an hour of operation, I was impressed that the ballast was barely warm
(about human skin temp), so not much power is lost there. I'm happy with the
output. At 10 cents per kw/hr, it should pay for itself in only 4 months. I
decided to make my house 100% fluorescent now. I'm almost there.
John