120/208Y Cheaper?

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mbrooke

mbrooke

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Is a 120/208Y distribution system cheaper when 480-120/208 step down transformer losses are taken into account?
 
Do you mean is a 208Y/120 system less expensive than a 480Y/277 volts system that utilizes transformers to create the 208Y/120 volt?
 
I think you're going to have to provide a lot more information on the givens and assumptions we are to make. I'm reading this like you're asking if owning a car is cheaper than uber if you factor in leaving a tip.
 
bwat said:
I think you're going to have to provide a lot more information on the givens and assumptions we are to make. I'm reading this like you're asking if owning a car is cheaper than uber if you factor in leaving a tip.
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That's a good example. Per mile driven, it is cheaper to own your own car in the long run. But if you only have to travel ten miles once, it's cheaper to hire an Uber.

Likewise, per kilowatt of load in your building, it is cheaper with the economy of scale to use higher voltage loads and have a higher voltage service, and only have the step-down transformer(s) that serves the minority of loads that are required to be 120V. Many of the big-ticket loads like lighting and HVAC, are available in 277V or 480V, and can run off the higher premises voltage. The bigger your building, the more likely you'll want these loads to use 277/480V
 
winnie said:
I'd be shocked if a building that needs 2 MW of service only has 120 or 208V loads.

I think part of the equation is that HVAC can be 480V, lighting can be 277V, etc.

Jon
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Right, but you can get units at 120/208, especially if packaged or roof top.
 
mbrooke said:
Basically a 2,500 or 3000 amps 480 volt service vs a 5000 or 6000 amp 120/208Y service.

I'm wondering if the saving in wire will offset all the idiling losses.
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Many utilities have a maximum kVA for which they will give you a 120/208V service, even if they allow a service size of the same amps at 277/480V. So even if if has an advantage to use the lower service voltage, the utility may not provide a multi-thousand amp service at 120/208V in practice.
 
mbrooke said:
Right, but you can get units at 120/208, especially if packaged or roof top.
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Wiring and equipment sizes would be smaller at 480v which would come into play in regards to the HVAC.

I would guess typically it’s always best to use the highest voltage available. However if the building has a lot of 208v loads, like large shops, maintenance garages, restaurants it may not make sense. Dealing with a lot of step down transformers can lead to other things , fire rating of room, exhaust fans in electrical rooms, plus square footage which all would effect cost as well.

Alot of chain retail and restaurants are 208v 3phase in the 400A to 1200amp range, if 480v makes the service smaller and cheaper they would be all over it to save money.

I would be curious to see rough cost comparison for your situation, it’s a good service size to compare
 
zbang said:
I wasn't thining about service size, I'm asking about the loads- are all the loads 208/120 or can/are some of them 480? And are those less expensive than 208v equipment?
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All the loads can be 120/208, or the HVAC and lighting can be 277/480.
 
mbrooke said:
5000 amp vs 2500 amp calculated service.
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IMO the 480Y/277 would be better for this. Someone once told me the crossover point was about 500kva of load where the 480 service would become more economical, that was many years ago so I'm unsure if that number still is good.
 
If all of your loads _must_ be 120/208V, then my hunch is that the losses in properly sized wiring would be lower than the losses in the transformers. During installation the wiring might be more expensive, but over the life of a project the cost of electricity generally dominates the total cost of ownership and efficiency wins. So if your loads _must_ be 120/208V then my hunch is that it would be preferable to get the 120/208 directly from the utility.

But HVAC and lighting can be either 120/208 or 277/480 (and probably 240/416).

In this case I think the balance between choosing 120/208 from the utility vs higher voltage and stepping down really depends upon the types of load served.

For loads that can happily either be 120/208 or 277/480, I believe that the balance favors the higher voltage.

A motor is essentially a transformer, and the difference between a motors intended for 208V and 480V is negligible. Same cost, same mass of copper and iron, same efficiency. In fact many three phase motors are rated 208-230V/460V where you can use them in either voltage range simply by changing connections in the motor. So there is no difference in the raw motors for using 120/208 or 277/480.

But the wiring to the motor and the controls for the motor will be very different in cost at the two voltages. For runs where voltage drop is a concern, supplying the same kW at 120/208 requires 5X the conductor as at 277/480. Big building means long runs, favoring higher voltage.

Things like VFDs are less expensive per HP in the higher voltage class. IGBT voltage drop is roughly '2 diode drops' so the higher voltage VFDs might even be more efficient, though I've not researched this recently.

My point being that if loads which can be 277/480 make up the bulk of your load, it would not make sense to get the 120/208 versions of these loads so that you can take service at 120/208. But if 120/208V loads make up the bulk of your load then a 120/208V service makes more sense.

-Jon
 
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