3 phase Vs single phase package unit

[gevans]

A good customer has asked me to replace a 3 ton package unit on his store.
The current unit is 3 phase. I can get the same unit in single phase for less money.

How do you figure electric usage for 3 phase Vs. single phase? If he saves some money up front, how long until he loses more to the single phase system?

Separate question: how big a deal is it to convert to single phase from the panels to the disconnects? We would of course be hiring a licensed electrician.

Are there any other advantages to 3 phase, other than the lower electric cost? I know those systems don't require run capacitors, so there are fewer parts involved. Anything else?

[bwalley]

stick with the 3 phase, it will be cheaper to operate and by the time you change out the disconnect and breaker, how much are you really saving?

[IntelliPak130]

Are there any other advantages to 3 phase, other than the lower electric cost? I know those systems don't require run capacitors, so there are fewer parts involved. Anything else?



Three phase motors/compressors are more efficient than a single phase motors/compressors and consumes less current of the same HP rating. Using less Amps = Lower operating costs

Engineeringtoolbox.com - Take a look at the comparison graph.
http://www.engineeringtoolbox.com/ampere-phase-d_449.html

[acwizard]

Three phase motors/compressors are more efficient than a single phase motors/compressors and consumes less current of the same HP rating. Using less Amps = Lower operating costs

Engineeringtoolbox.com - Take a look at the comparison graph.
http://www.engineeringtoolbox.com/ampere-phase-d_449.html

First of all a watt or kilowatt is the same regardless of three phase or single phase. Yes 3 phase units will draw less amps for the same tonnage because the power is being evenly divided between 3 legs instead of two. The only reasons to install a 3 phase is the savings in the installation of the wiring , in the fact that the wire size will be smaller. Three phase motors have less problems starting because each phase voltage is 120 electrical degrees apart and the 3 phase currents are 120 degrees apart.The current lags behind the voltage in each phase is identical.Single phase motors need a start winding to assist the motor to turn along with capacitors and start relays . These components shift the angle between the run winding and start winding to begin motor rotation. Three phase motors have fewer problems because of its simpler design and higher starting torque.

[VTP99]

First of all a watt or kilowatt is the same regardless of three phase or single phase. Yes 3 phase units will draw less amps for the same tonnage because the power is being evenly divided between 3 legs instead of two. The only reasons to install a 3 phase is the savings in the installation of the wiring , in the fact that the wire size will be smaller. Three phase motors have less problems starting because each phase voltage is 120 electrical degrees apart and the 3 phase currents are 120 degrees apart.The current lags behind the voltage in each phase is identical.Single phase motors need a start winding to assist the motor to turn along with capacitors and start relays . These components shift the angle between the run winding and start winding to begin motor rotation. Three phase motors have fewer problems because of its simpler design and higher starting torque.

:ditto: All though the savings in wire may be off set by the cost of the third. :grin2:

[GA FRANK]

I suggest staying with three phase,equipment really should be less expensive.
If you go with single phase , if voltage is same just drop one leg of three
phase and use same distribution set-up you have now.

[VTP99]

I suggest staying with three phase,equipment really should be less expensive.
If you go with single phase , if voltage is same just drop one leg of three
phase and use same distribution set-up you have now.

If he drops one leg of the three phase his amperage will go up. Might want to check wire and breaker size at that point.
Reread post # 4

[GA FRANK]

VTP99 you are so correct,but usually this small of a unit has more service
capacity than is needed.

[94blowncobra]

Dont flame me cause i dont know.

But, can you get away with putting in a single phase unit and just use the same breaker and disconnect? By code i mean! and of corse if the wire size and breaker amps are ok.

Im curious cause ive seen this done before, esp at C-stores.

[VTP99]

Dont flame me cause i dont know.

But, can you get away with putting in a single phase unit and just use the same breaker and disconnect? By code i mean! and of corse if the wire size and breaker amps are ok.

Im curious cause ive seen this done before, esp at C-stores.

I don't see a problem but then i'm not the NEC. I have changed 3 phase units back to 3 phase that were switched to 1 phase.

[just_opinion]

Dont flame me cause i dont know.

But, can you get away with putting in a single phase unit and just use the same breaker and disconnect? By code i mean! and of corse if the wire size and breaker amps are ok.

Im curious cause ive seen this done before, esp at C-stores.

It is not the fault of the C-store ( C if for convenient) nor the fault of the owner of the store, that has these incorrect 3-phase to 1-phase conversion. But instead, it is because of the incompetency of the SO-called technician.

[ga-hvac-tech]

Dont flame me cause i dont know.

But, can you get away with putting in a single phase unit and just use the same breaker and disconnect? By code i mean! and of corse if the wire size and breaker amps are ok.

Im curious cause ive seen this done before, esp at C-stores.

Theoretically this will work... but I would not do it myself. If the motors burned up... who gets to pay for the replacements?

[94blowncobra]

It is not the fault of the C-store ( C if for convenient) nor the fault of the owner of the store, that has these incorrect 3-phase to 1-phase conversion. But instead, it is because of the incompetency of the SO-called technician.

Ok,,,,and???

[gevans]

There are two wire runs, one for the 10KW heat and another for the a/c. I'd have to use the 10KW wire for the unit and pull new wire for the 10KW if I switched to single phase.
I just wondered if there was some sorta formula to figure out the difference in power cost from single to 3 phase. It should be the same actual power usage, but not the same cost.
I'm pushing him in the direction of keeping the 3 phase. Makes life easier for us both.

[bwalley]

There are two wire runs, one for the 10KW heat and another for the a/c. I'd have to use the 10KW wire for the unit and pull new wire for the 10KW if I switched to single phase.
I just wondered if there was some sorta formula to figure out the difference in power cost from single to 3 phase. It should be the same actual power usage, but not the same cost.
I'm pushing him in the direction of keeping the 3 phase. Makes life easier for us both.

We use 3 phase for a reason, what size circuit is feeding the unit now?

what size circuit do you need for the new unit?

If the wire is large enough for the new unit, all you have to do is change the breaker to DP single phase, you can't just drop 1 leg off of the 3 pole breaker, and change out the disconnect, you might legally be able to use a 3 phase disconnect on the unit and drop the one leg you are no longer using, not sure, but I wouldn't do it.

Seems to me this job should have been dne already, but tire kickers and cheap people always drag their feet.

[gevans]

"tire kickers and cheap people"

You nailed it.

I haven't heard back from him yet. I'm thinking he will go with the three phase or another company.
Either way I'm happy.

[just_opinion]

We use 3 phase for a reason, what size circuit is feeding the unit now?

what size circuit do you need for the new unit?

If the wire is large enough for the new unit, all you have to do is change the breaker to DP single phase, you can't just drop 1 leg off of the 3 pole breaker, and change out the disconnect, you might legally be able to use a 3 phase disconnect on the unit and drop the one leg you are no longer using, not sure, but I wouldn't do it.

Seems to me this job should have been dne already, but tire kickers and cheap people always drag their feet.

Once again people, it is NOT as simple as changing out wires to match the load.
Do we consider the main load going to the building ? I know that it is only one unit and it MAY not make a difference. But what if there are 2, then 3, then 4 etc.
Next thing you know it, the dog gone meter blows aparts and you are liable for it. Or who knows - you are the last guy who install the last unit with a single phase and your luck just runs out. You will get the whole mess as the main load melts down and burns the place.

Once again look beyond the wires.

[acwizard]

There are two wire runs, one for the 10KW heat and another for the a/c. I'd have to use the 10KW wire for the unit and pull new wire for the 10KW if I switched to single phase.
I just wondered if there was some sorta formula to figure out the difference in power cost from single to 3 phase. It should be the same actual power usage, but not the same cost.
I'm pushing him in the direction of keeping the 3 phase. Makes life easier for us both.

Your actual cost will be the same since the single phase unit is deriving it power from the 3 phase load center. You would only see a possible savings if your building had a single phase service vs 3 phase do to the demand charges that are associated with 3 phase services . The real problem would be balancing the legs of the load center evenly.A voltage drop which could occur on 2 legs would cost the owner of the building on all other 3 phase loads served from that load center. Single phase loads. Kw = volts x amps x power factor/ 1000. Three phase loads, Kw= volts x amps x power factor x 1.73/ 1000

[VTP99]

Once again people, it is NOT as simple as changing out wires to match the load.
Do we consider the main load going to the building ? I know that it is only one unit and it MAY not make a difference. But what if there are 2, then 3, then 4 etc.
Next thing you know it, the dog gone meter blows aparts and you are liable for it. Or who knows - you are the last guy who install the last unit with a single phase and your luck just runs out. You will get the whole mess as the main load melts down and burns the place.

Once again look beyond the wires.

Yes put your amprobe on the line side of the main breaker and check your load for each phase. Each phase should be less then 80% of that main breaker amperage and somewhat balanced. Also going to 1 Phase will play with your balance. :whistle:

[zionair]

stick with the 3 phase, it will be cheaper to operate and by the time you change out the disconnect and breaker, how much are you really saving?

Just go with 3 phase~
You won't be saving much after all that extra work :.02:

[Bigfan]

it's only a 3 ton unit - how much can you be saving on the single phase equipment cost. can it possibly be worth the extra electrical costs, not to mention the possible headaches involved. save yourself the trouble and stay with the 3 phase.

[bwalley]

Once again people, it is NOT as simple as changing out wires to match the load.
Do we consider the main load going to the building ? I know that it is only one unit and it MAY not make a difference. But what if there are 2, then 3, then 4 etc.
Next thing you know it, the dog gone meter blows aparts and you are liable for it. Or who knows - you are the last guy who install the last unit with a single phase and your luck just runs out. You will get the whole mess as the main load melts down and burns the place.

Once again look beyond the wires.

Changing one unit from 3 phase to single phase is not going to cause that much of a load imbalanace if any, it will not hurt the meter, the main load will not melt down and burn the place down, the sky isn't going to fall.

[just_opinion]

Changing one unit from 3 phase to single phase is not going to cause that much of a load imbalanace if any, it will not hurt the meter, the main load will not melt down and burn the place down, the sky isn't going to fall.

Please read my post again. Start it at 2nd sentence. :whistle:

Now I know who that tech is doing work at THAT C-store

[bwalley]

Please read my post again. Start it at 2nd sentence. :whistle:

Now I know who that tech is doing work at THAT C-store

even changing out 4 or 5 units from 3 phase to single phase will not cause the meter to blow up, or any of the other catastrophic failures you mentioned, the sky isn't falling.

[just_opinion]

even changing out 4 or 5 units from 3 phase to single phase will not cause the meter to blow up, or any of the other catastrophic failures you mentioned, the sky isn't falling.

Got U. :whistle: