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Old 07-12-2011, 06:36 PM   #1
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Convert 120/208Y 3ph to 230 1ph


I have a 120/208Y 3ph service and need some 230V 1ph (2 hot legs and ground, 30-40 amps) to run a compressor motor. I understand that a simple buck-boost transformer can be used to boost power since a neutral isn't required. I also understand that two hot legs from the 208 are wired to the transformer line side to get 230 load.

I remember from school that 3ph is 120 degrees apart and 1ph 230 is 180 degrees. That type transformer doesn't change phases. My question is if the 1ph 230V motor will still run properly with the different phases? I can't seem to get a straight answer from a few electricans I asked.

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Old 07-12-2011, 07:09 PM   #2
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Convert 120/208Y 3ph to 230 1ph


The phase angles are determined with respect to ground/neutral. Without a third reference point, it's not possible to define a phase relationship between two hot legs, since you only have ONE signal to observe (the signal being the voltage between the two hot legs). So two phases of a 3-phase service are completely indistinguishable from two legs of a single phase service - they are both strictly single phase, until a neutral or a third phase is added to the mix.

Now with that said, why don't you just replace the compressor motor with a 3-phase motor?

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Old 07-12-2011, 07:17 PM   #3
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Convert 120/208Y 3ph to 230 1ph


The single phase motor with two power leads for 240 volts has no phasing problem if connected directly to any two legs (phase lines) of the 3 phase 208 volt system. But 208 volts may or may not be insufficient to run the motor with.

A simple (single phase) transformer with a 208 volt two terminal primary and a 240 volt secondary will work perfectly well connected across any two phase lines. You would need to be sure that the transformer wattage is sufficient to handle the starting current of the motor.

The significance of considering installing a 3 phase motor is that a transformer of appropriate wattage is not cheap.
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Old 07-12-2011, 07:17 PM   #4
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Convert 120/208Y 3ph to 230 1ph


There's a very good possibility that a compressor motor marked 230 volts will operate on 208 without any modifications.

Here's what I'd do; go ahead and hook it up to 208 and see what happens. If it runs ok and the current at high pressure is not more than what is stated on the nameplate, then you're good. Bear in mind that it might not start if the temperature is cold. Like below 35.

If it doesn't run on 208, simply add a buck-boost transformer. It can be placed anywhere in the line from the panel to the starter (or pressure switch, if no starter). If you boost 24 volts, you'll have somewhere around 229. A 1KVA 240/24 transformer will give you 41.6 amps. a 0.75 will give 31.2 amps.

I would choose a buck-boost over a 208 to 240 transformer mainly because of cost. The buck-boost will be a LOT less than the other one.

Rob
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Old 07-12-2011, 10:10 PM   #5
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Convert 120/208Y 3ph to 230 1ph


I will just run the motour on 208 network and see if that do hold up as Micromind mention and if you do need boost the voltage the Buck/Boost transfomer is far much cheaper than standard transfomer.

Basically the B/B is sorta like autotransfomer to increase or decrease voltage by small step { all it depending on the conferation there is so many diffrent conferation so if you do get B/B make sure you follow the connection diagram otherwise have a electrician assist you on this one }

Merci,
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Old 07-13-2011, 12:38 AM   #6
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Convert 120/208Y 3ph to 230 1ph


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Originally Posted by AllanJ View Post
A simple (single phase) transformer with a 208 volt two terminal primary and a 240 volt secondary will work perfectly well connected across any two phase lines. You would need to be sure that the transformer wattage is sufficient to handle the starting current of the motor.
A buck-boost transformer would be the preferable solution for this application. Rather than handling the entire wattage of the load, the boost transformer would be handling about 13% of it. Much smaller and cheaper. Still not as good as just running the motor on the existing supply, or using a 3ph motor though.
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Old 07-15-2011, 06:41 PM   #7
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Convert 120/208Y 3ph to 230 1ph


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Originally Posted by mpoulton View Post
The phase angles are determined with respect to ground/neutral. Without a third reference point, it's not possible to define a phase relationship between two hot legs, since you only have ONE signal to observe (the signal being the voltage between the two hot legs). So two phases of a 3-phase service are completely indistinguishable from two legs of a single phase service - they are both strictly single phase, until a neutral or a third phase is added to the mix.

Now with that said, why don't you just replace the compressor motor with a 3-phase motor?
Interesting point about the reference to neutral. When I find time I'd like to study up on motors. I thought a 1 ph. 240 motor required 180 degree between phases to function properly. What about a Delta 3ph system that doesn't have a neutral? That was what actually got me to question phase timing and sequencing. For example switching any two legs on any 3 ph. reverses a motor's rotation.

Normally I probably would just replace it with a 208 motor. I really like 208/120Y but some good deals were passed up on used equipment due to voltage variations. I think a buck-boost transformer with extra capacity is the best long term solution for flexibility and cost.
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As far as just trying it on 208; The manufacturer told me it will void the warrantee.
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Old 07-15-2011, 07:13 PM   #8
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Convert 120/208Y 3ph to 230 1ph


You are thinking about house power which is derived from a center-tapped transformer. The output of the transformer is 240 volts single phase but the center tap divides this in half and gives, with reference to the center tap, two 120 volt sources which are 180 out of phase. 240 volt devices do not have a connection to the center tap so it is just plain 240 volt single phase.
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Old 07-15-2011, 08:17 PM   #9
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Convert 120/208Y 3ph to 230 1ph


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Interesting point about the reference to neutral. When I find time I'd like to study up on motors. I thought a 1 ph. 240 motor required 180 degree between phases to function properly. What about a Delta 3ph system that doesn't have a neutral?
"Phase angle" is a relationship between two different signals. In order to define a phase relationship, you must have at least two independent signals to measure. A single phase motor is fed with only two wires, and thus there is only one signal present (the voltage between those two wires), so the concept of a phase relationship is meaningless in that context. As soon as a third wire is involved (whether it's a neutral or a third phase), three signals are present: the voltages between each of the three wires. Then a phase relationship can be defined between them.
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Old 07-22-2011, 11:29 AM   #10
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Convert 120/208Y 3ph to 230 1ph


I got my hands on a 2005 NEC Handbook, some product catalogs and did quite a bit of study.

The unit (cold water pressure washer) is rated 30A at 230V. I want an indoor receptacle as well as one outside. They will not be used simultaneously.

* I plan to use a 50A 2p breaker to feed the transformer circuit from a 3ph 120/208 panel.

* Run two #6 THHN and ground in 3/4 EMT from panel to a non-fused 60A 2 blade disconnect mounted on wall.

* I understand transformers vibrate a little so I'll use 3/4 FMC from disconnect to transformer.

* ACME T111683 Buck Boost transformer wired as autotransformer for 208-230. Supply rated 46.1A. Load rated 41.67A. Mounted on wall with some sort of rubber insulation to absorb vibration.

* Run same size FMC from trans. to a 100A 6 position Main-Lug subpanel on wall. (Both FMC less than 6 feet)

* Use two 2p 30A breakers to feed seperate circuits and receptacles .

* Run #10 THHN in 1/2 EMT to feed the two receptacles 2 hot legs and ground. The EMT to the outside receptacle will go to an inside metal box then fed with PVC through a basement wall and underground outside to some sort of wp housing.


I have several questions:

Do I need GFCI protection outside? The reason I ask is that a 230V GFCI breakers require a neutral in the panel and I don't get that with this particular transformer. I can't find a GFCI receptacle. Although it's for a pressure washer the receptacle area won't be exposed to the washer stream but the housing will be exposed to rain.

According to the code and Acme, I can use up to a 60A breaker for that unit. If I do that then I need # 4 THHN. The code allows 2 wires in 3/4 EMT but with all the connections I want to run a seperate ground from the panel. I could use 1" EMT but don't have a 1" bender and #4 is more expensive. Will the 50A be ok with 46.1 A supply side?

I noticed in all diagrams that fused disconnects have the fuses on the load side of the blades. Am I OK using the 50A breaker in the seperate panel, about 20 feet away on the supply side of the non-fused disconnect?
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Old 07-22-2011, 05:09 PM   #11
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Convert 120/208Y 3ph to 230 1ph


GFCI protection is not required because it's not a 120V circuit, but it's a good idea. If it were my installation, I would want GFCI (or at least GFPE). You don't NEED a neutral connection for that - the neutral connection at the breaker is only required if you're using a load with a neutral connection. Otherwise, you can just ignore it.
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Old 07-22-2011, 07:09 PM   #12
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Convert 120/208Y 3ph to 230 1ph


Quote:
Originally Posted by pf22 View Post
I got my hands on a 2005 NEC Handbook, some product catalogs and did quite a bit of study.

The unit (cold water pressure washer) is rated 30A at 230V. I want an indoor receptacle as well as one outside. They will not be used simultaneously.

* I plan to use a 50A 2p breaker to feed the transformer circuit from a 3ph 120/208 panel.

* Run two #6 THHN and ground in 3/4 EMT from panel to a non-fused 60A 2 blade disconnect mounted on wall.

* I understand transformers vibrate a little so I'll use 3/4 FMC from disconnect to transformer.

* ACME T111683 Buck Boost transformer wired as autotransformer for 208-230. Supply rated 46.1A. Load rated 41.67A. Mounted on wall with some sort of rubber insulation to absorb vibration.

* Run same size FMC from trans. to a 100A 6 position Main-Lug subpanel on wall. (Both FMC less than 6 feet)

* Use two 2p 30A breakers to feed seperate circuits and receptacles .

* Run #10 THHN in 1/2 EMT to feed the two receptacles 2 hot legs and ground. The EMT to the outside receptacle will go to an inside metal box then fed with PVC through a basement wall and underground outside to some sort of wp housing.


I have several questions:

Do I need GFCI protection outside? The reason I ask is that a 230V GFCI breakers require a neutral in the panel and I don't get that with this particular transformer. I can't find a GFCI receptacle. Although it's for a pressure washer the receptacle area won't be exposed to the washer stream but the housing will be exposed to rain.

According to the code and Acme, I can use up to a 60A breaker for that unit. If I do that then I need # 4 THHN. The code allows 2 wires in 3/4 EMT but with all the connections I want to run a seperate ground from the panel. I could use 1" EMT but don't have a 1" bender and #4 is more expensive. Will the 50A be ok with 46.1 A supply side?

I noticed in all diagrams that fused disconnects have the fuses on the load side of the blades. Am I OK using the 50A breaker in the seperate panel, about 20 feet away on the supply side of the non-fused disconnect?
This set-up will work well. If it were me, I'd very likely put the transformer directly above the panel, and save the cost/complexity of using sealtite. It could be a local thing around here, but I usually run rigid or EMT directly to a solidly mounted transformer. Just use a chase nipple from the top of the panel to the bottom of the transformer, and make your splices in the panel.

I also don't see the need for a disconnect. I've installed hundreds of transformers of all sizes (even substation size ones) and have never installed a disconnect at the transformer.

Remove the neutral bus from the panel. When the voltage is boosted line-to-line, it is also boosted line -to-neutral; depending on how the transformer is connected.

If you do it the way you've described though, it'll work just fine.

Only 120 volt receptacles are required to have GFI protection.

A 50 amp breaker in the main panel will be fine. You can use a smaller breaker that specified, but not larger.

A non-fused disconnect it perfectly fine; in this case the circuit is protected by the breaker in the main panel.

Rob

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