Is 480v more "efficient" than 208v?

[donw]

I've seen discussions similar to this, but now I'm facing the decision. I have a private school (25000 s.f.) that I designed using 208/120v 3-phase. At the time, changing the voltage was not an option...but now, the utility co. is saying they will be replacing the transformer any way, and we have a choice. I know there are greater I squared R losses associated with the lower voltage, but how much does this affect actual energy costs. We're in Arizona, so air conditioning will be the heaviest load. All the A/C and lighting will be new. Of course, I'd have to add transformers for the 120/208v recepts, etc.

 

[roger]

Besides just looking at the electrical side of it, there is also material and resource efficiency to consider, such as smaller raceways and conductors if you go with the 277/480 for instance.

Roger

 

[sparky59]

480

480

Most larger buildings use 277/480 for the lights and a/c. If it was me i would probably go with it.

 

[donw]

I was afraid you all would say that. How much long term savings would you expect? And would the cost of transformers not balance the savings in reduced wire sizing?
Thanks.

 

[LarryFine]

To me, 25K sq.ft. is large enough, and will present a large enough load in HVAC and lighting alone, that I'd go high.

You'll probably save enough in copper to cover the transformers.

 

[donw]

So upfront costs are a wash (or favor 480V, some.) What about long term? What kind of savings can they expect over time? I can't seem to find any studies anywhere.

 

[LarryFine]

I guess the best way to answer this is to say that a building will condume a certain numkber of KWH to get a certain amount of work done. Let's say we're talking about air conditioning.

The thermostat will be set to whatever setting is required to make the setter comfortable. The higher the voltage reaching the unit, the less time (smaller duty cycle) the unit will have to run to maintain temperature.

When we're talking about incandescent lighting and the such, the savings can be realized by being able to use a lower-wattage lamp to generate a given amount of illumination. I'm not sure about discharge lighting.

My point is that the savings should appear over time, and may not be something that can be seen in one point in time. However, I suggest working out the costs both ways before deciding. We're mostly opinion experts here.

 

[iwire]

Let's say we're talking about air conditioning.

The thermostat will be set to whatever setting is required to make the setter comfortable. The higher the voltage reaching the unit, the less time (smaller duty cycle) the unit will have to run to maintain temperature.

I have to disagree here.

3 tons of cooling from a 208 RTU is the same as 3 Tons of cooling from a 480 unit.

 

[LarryFine]

I have to disagree here.

3 tons of cooling from a 208 RTU is the same as 3 Tons of cooling from a 480 unit.

My bad. I wasn't referring to the two different system voltages, but to less voltage drop, and less I^2R loss, and to the voltage reaching the terminals being closer to nominal.

In other words, I feel that a 480v load is more likely to recceive 480 volts than a 208v load is to receive 208 volts. Of course, proper design should minimize the difference, but I'm talking in the theoretical sense. "Efficiency."

I could be wrong; it wouldn't be the first time. Second, maybe.

 

[hmspe]

I've done schools in Arizona both ways. I'd stay with 120/208 for a school this size. I2R in the wires will be higher, but that will be balanced by the transformer losses if you go 480. On my designs the circuit count wouldn't change -- I do one lighting circuit per room. From what I've seen the costs of the extra equipment for 480 are about equal to the costs for the larger conductors, etc., at 208. I agree with Bob -- 3 tons is 3 tons. Small tonnage heat pumps (one per room) are cheaper at 208 than 480. ... and in schools there's never room for transformers.

Martin

 

[LarryFine]

25K sq.ft. is considered small?

 

[iwire]

25K sq.ft. is considered small?

IMO yes, that is small for commercial type construction.

If we assume two floors that is about a 100' x 100' building.

With a centrally located electric room all the circuits (Feeders and Branches) would be fairly short.

 

[necnotevenclose]

480V is a great application for office, medical, and other multi-use facilities because you are usually able to have a smaller size main distribution system and you can add a ton of load + HVAC usually operates better at a higher voltage than 208V. However, for schools I would also suggest utilizing 208V/3-ph. The size of your distribution may be larger but like it was previously stated schools usually don't have room for transformers.

 

[LarryFine]

The high-shcool I went to was what is known as campus-style, with4 classrooms per side (8 total) of each building. The fluorescent lighting was 277, which gives a lot more light per circuit.

 

[infinity]

There is a point where the amount of load will dictate which system is cheaper. If I remember correctly it's about 500 KVA. Above 500 KVA the savings generated by using smaller conductors, less conduits, and less circuits outweighs the higher cost of some of the 480 volt equipment, transformers, etc.

 

[donw]

The school has an odd shape - like a horseshoe, single story. The horseshoe is 40' "thick". The bottom of it is 170' long; one side is 300' long and the other is 244' long. The current design has a 1600A service with about 1100A calculated load. The service entrance is on the long side (near the kitchen and multi-use room), and there is a 600A distribution panel on the other wing. They are also wanting to bump 6 of the 5-ton A/C units up to 7.5-tons which could run on either 208 or 480V.

Hey Martin, I know you!
Don

 

[bob]

You need to admit this is a guess from the start. I would use the total lighting load and the A/C loads for the calculations. Then add loads that are intermittent through out the day. All lighting will be on about 10 hrs per day during the school week. Some will be on during the weekend. The A/C will be available 24/7. The I?R losses are based on wire lengths. So you will have to estimate the circuit lengths. If you make the assumption that the lighting load will be 1800 watts (15 amps) per circuit at 120 volts, using table 9 and #12 cu, R = 2 ohms/ 1000 ft. The watt loss = (15? amps) x 2 = 450 watts/1000 ft. Making the assumption that the load is balanced, which is likely not true, the loss in the neutral is small. Using the same loads a 277 volts the watts loss/ 1000 ft is (6.5? amps) x 2 = 85 watts, a savings of 365 watts. Assume that the school is open 9 months a year at full load, the yearly estimated savings is 365 w x 10 hrs x 5 days x 4.3 weeks x 9 mo = 706 kwh / 1000 ck ft. Additional saving would be added for weekend hours used. The A/C loads would be calculated and added to this total.
You need an estimate for material and labor to install the equipment. The cost of a lower ampacity main and panels may help offset the cost of transformers for the 120 volt loads.

 

[fido]

Correct me if I am wrong on this, but a kwh is a kwh. At the higher voltage for the same load, would you not use less kwh, therefore have a less expensive electric bill for any given load? I recently came across this exact situation on a job and gave the owner the choice. He went with the 277/480.

 

[infinity]

Correct me if I am wrong on this, but a kwh is a kwh. At the higher voltage for the same load, would you not use less kwh, therefore have a less expensive electric bill for any given load? I recently came across this exact situation on a job and gave the owner the choice. He went with the 277/480.

I agree. The decision should be made based on the amount of the load not the size of the building.

 

[thinfool]

but a kwh is a kwh.

Watts is watts!

Properly wired equipment uses the same energy regardless of voltage.

I was in a motor shop one day, 30+ years ago, and saw a 100hp motor...the nameplate said 16 amps....I couldn't understand....till I noticed the voltage...I kinda remember that it was 2400 or 4160. A first for me.