I have a customer who has a 3 phase service with no neutral for a well pump. He wants to power his greenhouse with it but there is no neutral. It's a 240 delta with 120, 208, 120. My only solution is to use a transformer to provide a neutral. The well pump providers have told him a single phase transformer will work but I guess I don't get it. Does anyone have a better idea? The customer has a line on a surplus Square D EE37S3H single phase transformer he wants to use but I don't think it will work. If it will work, can someone please enlighten me?
Providing neutral for 3 phase
- Thread starter joelj
- Start date
- Status
ptonsparky
Tom
- Occupation
- EC - retired
He only has three conductors coming in from the POCO? My first thought is to bring in the fourth wire. That needs to be there.
If there is no neutral conductor and you measure 120, 208, 120, it likely is a grounded service. 250.24(C) requires a grounded conductor to be brought to the service equipment.
However, a grounded conductor is not required to be brought to any equipment on the load side of the disconnecting means, unless there is need of such at the time of installation (though there is at least one exception to this general rule).
So the questions here are is the service compliant? ...and are you pulling the power from service equipment or load-side equipment? Next up would be the difficulty of pulling a grounded conductor vs. installing a SDS XFMR?
The SDS XFMR would be 240V primary, 120/240V secondary and establish a neutral to use for loads that it supplies.
Last edited:
I have actually run a wire down the riser and connected to the messenger to provide the neutral at the service. He originally had just a ground. I confirmed that the messenger was connected to the POCOs transformer and was in fact a neutral. Before I was involved, the customer ran 3 wires and a ground about 300 ft to the building he wants to power. He was going to try to pull an additional wire but says that it is not possible. He says that he won't have any 3 phase loads in the building. I thought of just using the 2 120 legs and using the 3rd wire for the neutral but I was worried about the balance of the loads. I read in another post that the POCO sort of expects this with the 240 delta and it may not be a problem. Any thoughts?
Sounds like you solved the 1st part of the problem. As others have pointed out, NEC requires a grounded conductor to be installed if you are supplied by a grounded service (you might want to look at 250.24(C) to make sure on sizing, etc).
You now have your grounded conductor and can use it with the other 120v phases and equipment ground to supply the single phase load. Unless you have a considerable single phase load I would not be concerned with balancing. If the load is considerable as compared to your service size you might run it by POCO as they normally like to know of significant load changes.
You now have your grounded conductor and can use it with the other 120v phases and equipment ground to supply the single phase load. Unless you have a considerable single phase load I would not be concerned with balancing. If the load is considerable as compared to your service size you might run it by POCO as they normally like to know of significant load changes.
Last edited:
- Location
- Tennessee NEC:2017
- Occupation
- Semi-Retired Electrician
I'm not well versed in transformers other than following connection diagrams. I have never had to wire one to supply a neutral.
My question is how do you derive a neutral from a transformer if there is not a grounded conductor brought to the transformer?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Since you say that you have only three service wires and are measuring the phase voltages as 120, 120 and 208, you must be measuring the voltages with respect to ground.
This means that the transformer which is supplying the two 120 volt legs has a center-tapped 240 volt winding, and the center tap is already brought out of the transformer and grounded. All that is missing is a wire (the neutral) from that same tap on the transformer to your connection point.
Except for the missing neutral wire, what you describe is a standard "high leg" grounded delta configuration.
The third phase line (208 volts to ground) is fed either by one 240 volt winding from one end of the center-tapped winding (open delta) or from two 240 volt windings connected to opposite ends of the center-tapped winding (closed delta).
- Location
- Tennessee NEC:2017
- Occupation
- Semi-Retired Electrician
I think you are confusing me with the OP. I didn't measure anything, I just have heard several times about deriving a neutral from a transformer and wondered how that was done.
I do know and understand what a Delta service with a high leg is.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
If you, unlike the OP, do not have a transformer with a candidate for a neutral connection brought out already, you can still create a neutral, but it will require adding another transformer, most commonly three phase using a zig-zag winding to create a wye neutral or center tapped autotransformer for creating a split phase 120/240 neutral), to provide an electrical balance point to connect to.
I do not know of any way to do it using only the original transformers if they do not have a "neutral" tap.
And, yes, the reference to the high leg delta was intended for the OP.
You are correct. I was measuring the voltages to ground and there is a neutral tap on the POCO transformer. As I stated earlier, I was able to run an additional wire down the riser and connect to the messenger wire which fortunately was already connected to the neutral tap. As this point, the dilemma was that the customer had run his own conductors approximately 250 ft and had only run 3 3/0s with a ground. He says he cannot fish another conductor and is not willing to re-pull or pay me to do it. I was wondering if I should use a transformer to provide the neutral at the building or just use the 2 120 legs and use the 3rd conductor for the neutral. I was concerned about balancing but I think that concern has been alleviated and I will go without the transformer. I can get a 50KVA 240D to 208Y for about $800 but again the customer is trying to go cheaper.
BAHTAH
Senior Member
- Location
- United States
Delta Service Transformer
Delta Service Transformer
Your single phase load should not exceed 10% of the three-phase KVA of the transformer bank supplying your service assuming that the utility used a closed-delta
configuration with all three pots being of equal KVA value. You need to do a load calculation of your single-phase load and check with the utility to be sure the messenger they
used as a ground is sized as they would like for a neutral. You also need to verify as mentioned in a previous answer that your single-phase load will not require the utility
to increase the KVA on the single-phase pot on their transformer bank.
Delta Service Transformer
Your single phase load should not exceed 10% of the three-phase KVA of the transformer bank supplying your service assuming that the utility used a closed-delta
configuration with all three pots being of equal KVA value. You need to do a load calculation of your single-phase load and check with the utility to be sure the messenger they
used as a ground is sized as they would like for a neutral. You also need to verify as mentioned in a previous answer that your single-phase load will not require the utility
to increase the KVA on the single-phase pot on their transformer bank.
- Status