Which is more efficient 3ph 208v or 3ph 480v

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JJBarnes

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A question has been raised about which is more efficienct 3ph 208v or 3ph 480v. I understand the advantages of installing 480v as far as smaller feeder size but I was always told watts are watts they both use the same amount.

Jeff
 
Wouldnt the higher voltage reduce the load therefore lower the energy use?

Look at any data plate on just about any polyvoltage motor
 
JJBarnes said:
A question has been raised about which is more efficienct 3ph 208v or 3ph 480v. I understand the advantages of installing 480v as far as smaller feeder size but I was always told watts are watts they both use the same amount.

Jeff
Click to expand...

If you look at the whole picture, there are some advantages to 480 beyond pure energy efficiency.

I don't know how most folks look at motor voltages; if I look at "same" proportions, a set of voltages would be 115-200-230-266-460-575. These are the typical voltages at which motors are rated. The supply voltages associated with these are 120-208-240-277-480-600.

Many 200-230-460 motors are not quite as capable at "208" as they are at "230-460" running on the 230 windings at 208. A motor starter, and potentially the bucket in a MCC, will be larger at 208 than at 480. But the transformer required for 120 power from a 480 system is additional. 120 comes "for free" on 208.

When I was designing textile machinery, we found 208 to NOT be an industrial option. If small machinery was going in a light commercial facility, it was quite common.

MY OPINION, not an electrician, is that if the total motor load in a facility is under perhaps 100HP (and none over maybe 30HP or so), 208 is suitable, and possibly preferable. Looking at it another way, if the motor total load is under 300A, consider 208. Your POCO likely has criteria as to what they will install, as well. That may be the deciding factor.
 
Thanks for all the replies. George, you hit on something that I Knew but didn't even think about and that's the 208v motor running on 208v vs a 460v motor running on 480v. I guess that gives you a little more bang for you buck. It will work the same with the heating elements as well.

Thanks,

Jeff
 
Most electrical losses are from current flow (IsqxR), a higher voltage means lower current for the same total power. So the higher voltage would be more efficient. Thats why utilities use as high as possible voltages for transmission.
 
zog said:
Most electrical losses are from current flow (IsqxR), a higher voltage means lower current for the same total power. So the higher voltage would be more efficient.
Click to expand...
It isn't quite as straightforward as that. The lower current also means that smaller conductors can be, and are, used. Thus, as well as the I^2 being reduced, the R term is increased.
 
I'm in the higher voltage camp. I will add that it is a generalization and not true for all cases.
 
I've spent time comparing install cost between 480 and 208 equipment. Comparative cost of switchgear, motor controls and conduit and wire sizes has made it interesting. It seems that until you get to significant horsepower or really long runs the comparisons is close, but I'm certain folks with a lot more experience have "threshold" numbers. GeorgeB's "opinion" is pretty much what I've found..
For some of you guys who work with such things as "losses",In general, ignoring equipment and install cost, would the "savings" that zog notes be offset by the losses in transformers on a 480 system when you need 208/120 transformers.
(I know it would depend on specifics, but in general)
 
Voltage is not significant to the efficiency of the motor. Power factor is significant. Balancing the phases is significant.

Most especially running near design of the motor is the biggest factor. Motors should be run at 75% to 100% FLA for peak efficiency of 85% to 95%. Size the motor hp to the load requirements. Don't overspec the motor.
 
It really depends on how you define efficiency. It can be dollars per Kva, HP per Kva, etc. Which costs more to install, including supplying other equipment in the premises, as mentioned above?

Generally, insulation is cheaper than conductor, possibly making the higher voltage cheaper to a point as ampacity requirements lessen, but then equipment cost begins to rise again with the voltage rating.

I'd say that, from the load's point of view, it doesn't really matter, as long as the load receives its design voltage and adequate current, it doesn't care. The supply system, on the other hand, does care

Voltage drop comes into play. Higher voltage means lower current for a given power level. There's theoretical efficiency, and there's realistic efficiency. You have to compare installation as well as operation costs.
 
augie47 said:
For some of you guys who work with such things as "losses",In general, ignoring equipment and install cost, would the "savings" that zog notes be offset by the losses in transformers on a 480 system when you need 208/120 transformers.
(I know it would depend on specifics, but in general)
Click to expand...
In general, you want to avoid multiple transformations.
 
Your more likely to have 480 and any loads without a transformer will cut losses. Wire size in both motor and feeders will be smaller so smaller conduit. Switch gear will be more costly. As far as running watts are I X E. You need to calculate everything to arrive at total. The transformer will be very costly so most likely 480 be the winner
 
Besoeker said:
It isn't quite as straightforward as that. The lower current also means that smaller conductors can be, and are, used. Thus, as well as the I^2 being reduced, the R term is increased.
Click to expand...

Now that no fair, changing the conditions of the question. OK, I can play that game, smaller conductors are used so they are cheaper and you save money, so more efficient.
 
Besoeker said:
It isn't quite as straightforward as that. The lower current also means that smaller conductors can be, and are, used. Thus, as well as the I^2 being reduced, the R term is increased.
Click to expand...
zog said:
Now that no fair, changing the conditions of the question. OK, I can play that game, smaller conductors are used so they are cheaper and you save money, so more efficient.
Click to expand...
Wait, it's not that simple. If were talking about small circuits, where, for example, we're going to use #12 at either voltage, then yes, the higher voltage will have lower losses.

However, if we're talking about a load large enough that the wire will be selected using the usual methods, the wire will be smaller for the higher voltage, so the loss issue is moot.

The load for either voltage could end up being closer to the ampacity limit for the size chosen, so either one could end up being the one with greater voltage-drop-caused losses.
 
LarryFine said:
Wait, it's not that simple. If were talking about small circuits, where, for example, we're going to use #12 at either voltage, then yes, the higher voltage will have lower losses.

However, if we're talking about a load large enough that the wire will be selected using the usual methods, the wire will be smaller for the higher voltage, so the loss issue is moot.

The load for either voltage could end up being closer to the ampacity limit for the size chosen, so either one could end up being the one with greater voltage-drop-caused losses.
Click to expand...


That is not the only place that a loss will show up.
 
zog said:
Now that no fair, changing the conditions of the question. OK, I can play that game, smaller conductors are used so they are cheaper and you save money, so more efficient.
Click to expand...

I would qualify that by saying that
smaller conductors are MORE efficient in cost analysis,
but
smaller conductors are LESS efficient in transferring power.

The first is dealing with money $$$,
The second is dealing with Electrical Efficiency.
 
[QUOTE
MY OPINION, not an electrician, is that if the total motor load in a facility is under perhaps 100HP (and none over maybe 30HP or so), 208 is suitable, and possibly preferable. Looking at it another way, if the motor total load is under 300A, consider 208. Your POCO likely has criteria as to what they will install, as well. That may be the deciding factor.[/QUOTE]

I agree, in my experience as an electrician. Take a typical water pump station. For 1 or 2 pumps, 20 hp or so, go with 120/208.
The electrical gear (panels) is smaller and much less expensive.
No step down transformer is needed for lighting
With a 50 hp, its better to go with 277/480, the wire is smaller and the starters are smaller.
Drawbacks
Need a lighting transfomer
480 requires much better workmanship in installation
May require a disconnect ahead of the meter

As far as which is more efficient, they are the same, but the amps on 480 is much less, wires are much smaller, and what we are concerned with is the I squared R losses.
 
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