how to calculate low voltage lighting
- Thread starter d1reyad
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- Chapel Hill, NC
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- Retired Electrical Contractor
20 cans at 50 watt is 1000 watts. The transformers have a PF but it would not affect it much in this case.
- Location
- New Jersey
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- Journeyman Electrician
According to your calculation you're drawing 4.1 amps @ 12 volts or .41 amps at 120 volts (10:1 ratio). As Dennis mentioned the transformer will not operate at unity PF so you could be cautious and figure .5 amps @ 120 volts for your calculation. You could get a more accurate calculation by knowing the transformer loss and using that instead.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I am not sure what formula you want. Are you thinking that 4 amps at 12v means that 20 fixtures would be 80 amps??? As trevor stated- simplest rule is that watts are watts so it is 50 watts at 120V = .4 amps for each one so 20 * .4 = 8 amps.
20 fixtures at 50 watt= 1000 watts. 1000/120= 8 amps
Again the actually usage will be a bit higher for the PF of the transformer.
Say your trany has a PF of .9. If the Watts is 50 then the VA (apparent or total power) would be 50/.9= 55.5 VA (true wattage used). The higher the PF the more real your wattage will be. Thus a .95 PF with 50 watt bulbs would be 53 VA.
As you can see even at 6 watts more for each can (20 cans) you would only have 120 watts more to the load.
Please correct me if I am wrong-- I know you will.....
or
20 fixtures at 50 watt= 1000 watts. 1000/120= 8 amps
Again the actually usage will be a bit higher for the PF of the transformer.
Say your trany has a PF of .9. If the Watts is 50 then the VA (apparent or total power) would be 50/.9= 55.5 VA (true wattage used). The higher the PF the more real your wattage will be. Thus a .95 PF with 50 watt bulbs would be 53 VA.
As you can see even at 6 watts more for each can (20 cans) you would only have 120 watts more to the load.
Please correct me if I am wrong-- I know you will.....
- Location
- Massachusetts
Just keep in mind that the maximum power per LV circuit is 300 watts so you need multiple transformers or one transformer with multiple fused outputs.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I am not sure what formula you want. Are you thinking that 4 amps at 12v means that 20 fixtures would be 80 amps??? As trevor stated- simplest rule is that watts are watts so it is 50 watts at 120V = .4 amps for each one so 20 * .4 = 8 amps.
or
20 fixtures at 50 watt= 1000 watts. 1000/120= 8 amps
Again the actually usage will be a bit higher for the PF of the transformer.
Say your trany has a PF of .9. If the Watts is 50 then the VA (apparent or total power) would be 50/.9= 55.5 VA (true wattage used). The higher the PF the more real your wattage will be. Thus a .95 PF with 50 watt bulbs would be 53 VA.
As you can see even at 6 watts more for each can (20 cans) you would only have 120 watts more to the load.
Please correct me if I am wrong-- I know you will.....
Nothing to correct. Even with a somewhat poor power factor IMO you can safely put 24 of these fixtures on 1-15 amp, 120 volt circuit.
.5 amps * 24 = 12 amps
15 amps * 80% (continuous) = 12 amps
That helps i have another question may be you guys can answer it for me this is an example and i know the NEC don't allow it If i have 75W bulb at 120v total of 28 Can 75*28=2100W at 120V if i ran a 12AWG to my switch box and fuse it at 20A then split my light on Two switches 14 light per switch now using 14AWG splice to my 12AWG for light wip my 14AWG should not pull more than 9amp off each switch which mean my 14AWG Tap should be safe i can be wrong on this so correct if i am
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
That is one of the longest sentences I ever read. :grin:
You cannot tap with 14 gauges of a 12 gauge unless you protect the circuit at 15 amps. Would it be safe?-- probably, but not compliant.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
broadgage
Senior Member
- Location
- London, England
In practice it would be fine, and in some countries it would be permitted.
It is however a clear violation of the NEC which in all common circumstances requires at least #12 on a 20 amp circuit. Therefore not to be considered, even though not actually dangerous.
Electric-Light
Senior Member
How efficient are 120 to 12v transformers?
90% sounds about reasonable and power factor of maybe 0.8 or so depending on how heavily loaded it is.
watts/0.9 = watts input
watts input/0.8 = VA input.
VA/120 = amps.
What does the code say? If one uses a 1kVA transformer, do you have to assume secondary size utilization factor of 100% in case more stuff is added? Transformer should have the input current at full load labeled.
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