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mrhud 11-11-2008 09:02 AM

3ph vs 1ph
I have 3 phase power to my house and am about to replace a/c units. I would like to know if new 3ph units will save money in the long run or should I just go with 1ph. Mainly concerned with electricity consumption, not price of equipment.

Billy_Bob 11-11-2008 10:05 AM

You are very smart to be looking at the cost of electricity and not the cost of the equipment!

It is unusual for a house to have 3 phase, but since you said A/C units (plural), I will assume you do in fact have 3 phase power.

Anyway this is an interesting question. In general electric motors run more efficiently on 3 phase than 1 phase, however there has been a lot of energy saving engineering work done in the 1 phase A/C product area. So some new 1 phase A/C units may just outperform 3 phase units?

The energy efficiency is in the SEER rating. I would go by that.

I suggest you check all the major A/C manufacturers and maybe call them. Ask what their most efficient units are. Let us know what they say.

20 SEER...

frenchelectrican 11-12-2008 01:17 AM

Genrally the 5 tonner or smaller useally run on single phase power source the crossover line useally start about 7.5 tonner depending on the set up.

Normally with older 3 system typically are wired in Delta format and you may see two fuse box or two breaker boxes depending on the set up and the smaller box typically have 3 system in there while larger box typically is 1 in there and the voltage on all three leg on delta system on resdentail is 240 volts but a dangbat good gotcha is wild leg which it will result 208 volts if you read from B to Netual.

Per NEC code the wild leg will marked in orange colour however with older delta system don't be suprised to find in red as well.

For newer home which it have 3 typically will have wye connection the voltage will read 208v all hot to hot conductors while line to netural is 120 volt.

Not very many home have 3 in there now { most POCO will not genrally give 3 unless the home is very large or specal critia it have to meet by POCO regulations }

If have more question just holler here one of us will chime in with it.


220/221 11-12-2008 02:41 PM

I KNOW the answer to this one. This comes up all the time here in AZ.

In the 60's, the first AC units seemed to be all 3 phase. Maybe because they were used commercially first. Homes built then would have 3 phase delta run to them strictly to accomdate the AC circuit. Typically they would run the high leg into a Delta breaker that picked up the high leg via a lug and the 120V legs via the bus.

In almost every single case here, it is much better to go with a single phase unit.

A) Single phase units will have a higher SEER

B) If you need to upgrade your electrical service, a 3 phase will cost an extra $1K for less space. If your service is new and good enough to handle whatever might be thrown at it in the years to come you can stick with 3 phase.

C) The Delta breakers are getting harder and harder to find. If your breaker gets old and tired, good luck.

Single phase is the way to go.

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