Open deltas are generated by installing two transformers and utilizing one unused tap from each to generate a third leg for three phase service.

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Power At Your Site


Site Power – Problem Avoidance
How well do you know the voltage on your project?

One of the challenges many of our clients face during the design and bid specifications stage of a project is what electrical service is required for the equipment. In many cases, the desired voltage available at a particular site is not ideal and it is best to address this issue prior to releasing a bid to vendors. 99% of projects will require a three phase electrical service vs. a single phase service and generally a higher voltage such as 480V is desired over 208V or 240V. In addition, it is very important to determine if the available voltage is offered in a closed delta vs. an open delta configuration.

A closed delta provides a separate transformer for each leg of the three phase service which provides a balanced line voltage to the equipment. Open deltas are generated by installing two transformers and utilizing one unused tap from each to generate a third (fictitious) leg for three phase service.

Open deltas are very problematic and can lead to equipment and/or motor damage due to the voltage imbalance across the phases. Because of this, PRM will not warrant a system installed on an open delta service and this must be stressed to the utility provider.


Another point to keep in mind is most utility providers are currently only offering 208V three phase service to many sites, unless 480V is requested. Some motors are not rated for 208V service and this presents a problem when equipment is selected for a project. Also, 208V service to large motors will result in the highest amperage draw and most expensive electrical components. In most areas, 240V three phase service is no longer available. Duke Energy and Duke Energy Progress have stopped offering 240V three phase services in their territories.

Some information about 480V:

  • High voltage draws half the amperage load for the same equipment. This equates to less load on circuit breakers, contractors, overloads, and wire. 

  • Large equipment (over 7.5 HP) runs more efficiently at higher voltage and is less prone to failure.

  • High voltage is a more common industry standard for large equipment and thus is more readily available in the marketplace when it comes to parts and repairs.


There are some disadvantages to higher voltage including higher probablility of arc/flash situations and some companies cannot service higher voltage systems. Just because a 480V three phase service is desired on a project does not mean it is available at a particular site. It is very important to check with the local utility provider and speak with an engineer before bid specifications are released to contractors. In many cases, this will require a site visit and PRM recommends this practice. By taking the extra steps in the planning stages, the consultant and the utility will both be clear on the available voltage and the service requirements. PRM is always available to assist when making the design decisions on a project, just give us a call.

Open Delta vs. Closed Delta

Open Delta

An Open Delta system uses two primary high voltage legs and two transformers. This configuration creates two secondary phase legs with a third derived, albeit artificial leg. This system is never balanced and will lead to motor failure as Phase A, Phase B, and Phase C of a motor will be out of sync, thus creating motor damage from day one. Looking at and testing site voltage is not a good indicator. Look at your power pole coming in.  Two primary lines and two transformers only is a give-away for an Open Delta system.  Don’t be afraid to ask the Utility Supplier to close the Delta.  It is important to note that a 3 Phase Open Delta Supply is less efficient and wastes power.

Closed Delta

A closed Delta system will typically have three transformers and will always have three primary legs.  This is the preferential supply as the load is typically always balanced and more stable.

Wye System

If it is not economically feasible to close the Delta, then ask for a Wye system. It is typical that a 120/208V Wye be supplied. These systems are balanced systems. The benefit of Closed Delta over Wye connection is that the Delta system is typically at a higher voltage, thus reducing the overall system amperage.

Electrical Services

PRM has multiple licensed electricians on staff who can consult with you for power at your site and power issues. PRM began as an electrical company over 35 years ago so our experience and expertise are unmatched in the industry.

PRM Filtration is a tradename of
Product Recovery Management, Inc. (PRM)

200 20th Street

Butner, North Carolina 27509


Main Phone: 919-957-8890

Toll Free: 1-888-873-2848

Fax: 919-957-7230

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