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120v verse 277v

120v verse 277v

Just left meetings with an Industrial lighting manufacture sale rep and He quoted “there is no difference between 120v to 277v ballast.” The difference I was concerning myself with was the operating cost. I believed the 277v version was cheaper to operate because of the reduced amps and watts. His explanation was they are both produce the same thing in the end and that what really mattered.  

thank you

RE: 120v verse 277v

Think about it... If you have a 175W bulb it doesn't matter what voltage you ultimately run the ballast with, it has to provide 175W.  You pay for the watts not the voltage.  SO he he correct.  And yes, higher voltage less amps but the same watts.

Power = Volts x Amps

You can screw with the V and the A all you want as long as the P stays the same.

Keith Cress
Flamin Systems, Inc.-

RE: 120v verse 277v

I was hoping you would jump in, thanks for the reply. I do understand what you said. I was looking at it differently thank for clearing it up.


RE: 120v verse 277v

Considering lights are usually ganged with long runs, lower power losses in the wire going to the lamps and the potential use smaller wire can give some cost advantage to 277.  Other installation considerations would have a much higher weight in the decision.

RE: 120v verse 277v

The rep is basically correct - as far as the ballast itself goes.  I doubt there is much difference in efficiency between 120V and 277V ballast.

But, for the lighting system as a whole, as OperaHouse says, 277 V could be more efficient due to lower current in the lighting circuit and thus reduced losses in the wiring.

RE: 120v verse 277v

Lower copper loss does assume that you use the same size wiring. If you were using 277V wouldn't you use lighter gauge wiring? The lighter gauge wire would probably run hotter and the total copper loss would be comparable. At least the capital cost would be lower assuming the ballasts are the same price for both voltages.

  Sometimes I only open my mouth to swap feet...

RE: 120v verse 277v

For either voltage you would typically be using 15 or 20 amp breakers and a minimum of 14 AWG or 12 AWG wire. ASt a given current your circuit length can be 2.3 times greater for 3% voltage drop on 277 V compared to 120 V for a given wire size before wire size must be increased to compensate for voltage drop.
Your watts per circuit can also be 2.3 times greater at 277 V compared to 120 V at the same current.
You save on wire, breakers and panel space.

RE: 120v verse 277v

One thing you might want to consider. I have 4800v service which is dropped to 480v to MCCs. If I want 120v,I have to drop the voltage again with another transformer. If you run 277v, it eliminates a transformer.

RE: 120v verse 277v

Be cautious, the 480 volt system may be a 3 wire, 4 wire or high resistance grounded system.  Only the 4 wire solid grounded system will support 277 volt lights without side effects.  The 3 wire or high resistance grounded system requires a 3 to 4 wire transformer to operate correctly.  480-277/480V transformer.  delta / wye.  waross is correct.  You can even get 480 volt line to line ballasts.  In Canada we use 600/347volt systems which are even more efficient.  

BTW, the ballast that runs on 277 or 120 is a multi-tap unit, meaning that it has different winding taps brought out from the transformer that are connected for each respective input voltage.  

RE: 120v verse 277v


In Canada we use 600/347volt systems which are even more efficient.

Please explain this further. 'More efficient' with reference to what, and how is the efficiency improvement brought about?

  Sometimes I only open my mouth to swap feet...

RE: 120v verse 277v


I believe the efficiency being referred to by AllorNothing was intended to describe the overall system efficiency in using the available power, not the ballasts themselves.

While it is quite true, as noted by Keith (itsmoked) that:

"Power = Volts x Amps

You can screw with the V and the A all you want as long as the P stays the same."

keep in mind that Keith was speaking of the ballast which has relatively low internal resistance.  When dealing with the overall lighting system, efficiency is greatly affected by power lost in the transmission of the total power throughout the wiring of the system.

For example, let's say the lighting system has a Total Power of 1,000 Watts delivered into it at the source.  That Total Power could be any combination of Volts x Amps that equals 1,000 Watts, just as Keith stated.  100 Volts at 10 Amps would provide the same total power as 50 Volts at 20 Amps at the input to the lighting system.  

What proportion of the Total Power will be actually used for the intended purpose (providing light) in the lighting system is 1,000 Watts MINUS the power lost in the transmission lines.  Since the power wasted in losses is equal to the Amperage squared times the Ohms (resistance) of the transmission lines (the wires), reducing the Amperage component reduces the lost (wasted) power expotentially (the Amperage squared term).  In other words, because Total Power = Volts x Amps, BUT the Wasted Power = Amps (squared) x Resistance, the proportion of the Total Power that gets lost as Wasted Power decreases expotentially as the Voltage component increases and the Amperage component decreases.

So the higher voltages quoted by AllorNothing in their post result in lower amperages, further resulting in less Wasted Power in transmitting the Total Power thoroughout the lighting system.  As you previously stated, this assumes the wire size (or more accurately, the system Resistance component) stays the same.  Changing wire size can affect overall system efficiency as well by changing the Resistance component.

Best regards,


RE: 120v verse 277v

I neglected to add, as an additional example, that this is the reason that power generating stations transmit power at very high voltages (usually greater than 20,000 Volts or more in the US) even though the typical US household uses power at 110-120 Volts.  The efficiency of transmitting power long distances increases expotentially as the Amperage component of the Power = Volts x Amps equation is reduced.


RE: 120v verse 277v

Thanks for the post. I'm always curious when unsupported claims of efficiency improvements are made.

The only one I could readily add to those already mentioned was that the switching components in an electronic ballast would possibly have lower losses at a higher supply voltage, but that I wouldn't be certain of that because the higher voltage rated switches usually have higher on-state losses.

AllOrNothing is welcome to comment further.

  Sometimes I only open my mouth to swap feet...

RE: 120v verse 277v

There is actually quite a difference.  As a rough rule of thumb, if you run 120 V circuits you need twice as many circuits to do the same job.  Each circuit has the same I2R (that's I squared R) losses.  So if you double your current, you are doubling your losses in your distribution and branch circuit system.

Do the math on a system, you will be amazed how much it will cost over the course of a year.

Plus, at today's copper prices, every foot of conductor you can save is good.

RE: 120v verse 277v

I think that was adequately covered in my post of 19 Feb 07 12:49. Using the same size conductor for half the current is poor design - copper is way too expensive to oversize conductors! I agree about the principle though.

Is the consensus that efficiency of the ballast doesn't change significantly regardless of whether it is fed at 120 / 277 / 480V, even if the efficiency of the overall installation may improve?

  Sometimes I only open my mouth to swap feet...

RE: 120v verse 277v

600 Volts Canadian is the same as 480 Volts U.S. (exchange rate joke)

RE: 120v verse 277v


I believed the 277v version was cheaper to operate because of the reduced amps and watts. His explanation was they are both produce the same thing in the end and that what really mattered.  
The ballasts themselves may or may not have the same efficiency. Even though it may in fact be only one ballast with a 120 volt tap and a 277 volt tap.
However the cost of operating must include the cost of losses in the wiring from the revenue meter to the ballasts and there will definitely be reduced costs overall, both in the initial installation and operating costs with a 277 volt system.

RE: 120v verse 277v

pooslinger I like that Canadian exchange rate joke, eh!

RE: 120v verse 277v

The multi connections ballasts are probably using a transformer inside so there'd really be little difference in the efficiency at different voltages. Maybe some I2R losses in the transformer wire but that wouldn't make much difference.

RE: 120v verse 277v

Hi 2571.  I used to work for a ballast manufacturer and thought I should weigh in here.

If you have 277V readily available for lighting use it.  There are a couple of different reasons.  If 277V is available, your installation costs with 277V will be lower because of reduced wiring and breaker count and such.  Also, since you will have fewer amps running in building wiring you will save a little there.  From an efficiency standpoint, 277V ballast will tend to have a slight edge over 120V units but not by much.

Another consideration is that manufacturers tend to build alot more 277V ballast than they do 120V units.

BTW if you are powering flourescent lighting I would look closely at electronic ballasts.  They are more expensive up front but will pay for themselves quickly as they are considerably more efficient for flourescents.

RE: 120v verse 277v

Hi itsmoked,

I have seen electronic ballast for HID lighting but don't think they are much if any more efficient than magnetic for HIDs.  Most of what I have seen electronic HID ballast for is some type of special application.

The efficiency gain in fluorescents is a result of the operating frequency and physics associated with the lamp.  Above about 10kHz to 12kHz the physics of developing a discharge in a fluorescent lamp changes.  At higher frequencies it becomes easier (i.e. less power) to cause the electrons in the fluorescent lamp to change energy state and release photons.  This phenomenon begins to plateau around 15kHz to 20kHz.  For a 4ft, 32W, T8 type lamp an electronic ballast operating at about 25kHz will produce the same amount of light as a magnetic with only about 25W instead of 32W.

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