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ProGrade, Certified Electrical Contractor
Category: Electrical
Satisfied Customers: 542
Experience:  28+ years exp. - installation, troubleshooting, & repair of commercial, & residential electrical systems.
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I have a commercial property that has three phase power. It

Customer Question

I have a commercial property that has three phase power. It has three sets of breaker panels. Two of the panels are full and the 3rd has open spaces. I want to install two new circuits for receptacles for a computer server room. I ran two sets of 12ga wire for two separate 20 amp circuits. I'm using 20amp rated receptacles and two separate 20 amp breakers. I'm wiring the breakers in the panel just as I have done in the past for a single phase panel. When I plug in a three-prong receptacle tester, it indicates that it is wired correctly. However, when I plug in a UPS to the receptacle, the UPS is indicating a site wiring fault. When I turned on the UPS, it fried it. Need help to understand what I am doing wrong. I'm happy to send pictures.
Submitted: 11 months ago.
Category: Electrical
Expert:  ProGrade replied 11 months ago.

Hi J, welcome to

Sorry to hear about your dilemna.

The receptacle tester did what it was designed to do unfortunately it could not tell you that you were dealing with a high leg on that phase. A multimeter would have been a big help.

What it sounds like you are up against is a 3-phase delta high leg. One phase of the 3-phases will read 190 to 210 volts to ground. It is the high leg and 120 volt equipment cannot be supplied from this phase of the panel.

With the UPS damaged, it will have to be replaced. There are some options available for the replacement. You could purchase a ups that runs on 230 volt single phase. Another option, if you have enough electrical capacity would be to add a sub feed panel off of one of your existing panels to give you more spaces for 120 volt equipment.

Please let me know if you have any additional questions or need more direction.

Thank you

Customer: replied 11 months ago.
Hi, thanks for your quick response. I understand about 80% of what you are saying. It's my own knowledge limitations. I'm good at following directions, but I don't have electrical expertise. So your comments about the phase leg, etc is a bit over my head. Here's my knowledge of three phase... I understand that the breakers are not all on the same line bus, there is some pattern where each breaker is actually connected to either the A, B, or C line in. I know there is some benefit for motors and high power consuming equipment to draw power from multiple lines, but that is over my head and does not affect my current need to install a few simple receptacles.My assumption (which appears to be wrong) was that if I install a single 20 amp breaker in any single slot in the panel, that it was no different from a single phase installation. I thought because the three line buses are separate, that any one breaker installation was essentially the same as single phase. Again, I concede that this appears to be an incorrect assumption.That said, it's sounds like I need to measure the voltage and get back to you. I think my preference would be to understand how to connect a single phase subpanel to make this easier in the future. However, I have no problem purchasing a more advanced UPS. Also, in case it factors in to the decisions, the second 20 amp circuit was to be for a portable A/C unit.Thanks,
Expert:  ProGrade replied 11 months ago.

You are welcome Jason. Please check the voltage and let me know.

There are 3-phase systems that do have 120 volts to ground no matter which phase, that is a 120/208Y. Your delta high leg system is not one of these.

If the new breakers you installed were side by side, they are on different phases, one of them on the high leg, one of them not.

I can send separate insgructions to install a sub panel if you want. I will send a quote for the details separately.

If you did not want to pursue this option you could still install a 240 volt UPS. It would be powered from a double pole breaker off of any two phases.

Let me know what you find when you check voltage and what else I can do to help, thanks

Customer: replied 11 months ago.
I found my meter and I checked the voltage today. The two 20 amp breakers that I installed are both on the left side of the panel and are in slots 5th and 6th from the top. So if I assume that all breakers on the left side are the odd numbers and the right side are even numbers, these would be breakers #7 and #9. Breaker #7 measured 234 volts and breaker #9 measured 124.5 volts. Does this mean breaker #9 at 124.5 volts should work for a standard 120v receptacle or is even that extra voltage too much? I'm pricing the higher volt UPS in case that is the best course of action. If that should be the best choice can I just install a 30amp breaker and receptacle for position #7 or must there be a dual pole breaker between the high and low voltage slots? Also the wiring to the receptacle is 12ga copper, am I going to need to swap it with 10ga? The run is about 20 feet.
Expert:  ProGrade replied 10 months ago.

You verified that you have a high leg. Yes the number nine breaker is good for any circuit that needs 120 volts. The number 7 breaker cannot be used 4 230 volts. Equipment requiring 240 volts has to be fed from a double pole breaker.

The 12 gauge copper is good for a 16 amp load. If the load exceeds 16 amp you would need number 10 wire on a 30 amp breaker which would be good for up to 24 amps.

Let me know if you need anything else thank you

Expert:  ProGrade replied 10 months ago.

Hi Jason, I did not hear from you again so I was just checking in to see if you need any additional assistance thank you