By:  Mike Chancey, PE, LEED AP, MEP Dept. Manager

I get questions about 1-phase (1PH) and 3-phase (3PH) power from architects and other clients.  This will be the abbreviated version to give you a little better understanding of 1PH and 3PH power.

First, what voltage are you getting if you have a 1PH or 3PH service from the power company? With a 1PH service, you will have a voltage of 120/240.  This is a standard service for most single family dwellings.  Most, if not all, residential appliances, light fixtures, HVAC equipment and water heaters will operate from either 120 or 240 volts.


When you have a multi-level building, commercial or industrial facilities, a 3PH service is likely required due to the size of the service and the equipment involved.  With a 3PH service, you can get either 120/208 volts or 277/480 volts. Determining which voltage you need is the topic for another blog.  Elevators, large HVAC equipment, large process equipment, commercial kitchen equipment, etc. require 3PH power.  The building size and the equipment being used will dictate the need for a 1PH or 3PH service.


If 3PH power is available at the site, 3PH has some advantages over 1PH.  Let’s say the building load is 100,000VA (100KVA).  Using a 120/240, 1PH service, the current would be 417 amps. Using a 120/208, 3PH service, the current would be 278 amps due to an additional factor of 1.732. Lower amperage means smaller service entrance conductors and conduit and lower rated distribution equipment, which means lower cost.  There is an extra wire involved for 3PH, but the overall cost is still lower.

How can you tell if you have 3PH available at the building site?  I’ll say 95% of the time when I look at site utility plans, only the power poles or pad-mounted transformers are shown with no indication of 1PH or 3PH.  If the power is overhead, there is a quick way to determine if 3PH is available.  If there are (3) conductors attached on top of the power pole with insulators, then 3PH is available. If there is only (1) conductor on top of the pole, then only 1PH is available.  That should be easy to remember and recognize.

If the utilities are underground, you may need to contact the power company to determine if 1PH or 3PH is available. If you can find the dip pole where the overhead transitions into underground, then the same logic for overhead conductors works for underground.  If (1) conductor runs down the pole, then you have 1PH. If (3) conductors run down the pole, then you have 3PH.

1 Phase:                                           3 Phase:


In closing, the local power company should be contacted to determine the type of power available at your building site.  Your electrical engineer can help you to determine the service type and voltage that best suits the project needs.

To see MEP projects Allegheny Design Services (ADS) has worked on, please click here.

Thanks for reading,



Written by Mike Chancey, PE, LEED AP, MEP Department Manager

mike-chanceyMike Chancey is a native of West Virginia. He grew up in Hurricane and is presently residing in Bridgeport. Mike graduated from the West Virginia Institute of Technology with a bachelor’s degree in electrical engineering. After a brief two year stint with Duke Power in Greensboro, NC, Mike spent the rest of his career designing electrical systems for various building types and facilities. Mike is a member of the Clarksburg Kiwanis Club and active in community service work in Harrison County as a board member of the Harrison Co. YMCA and current president of the board for Empowerment through Employment.