Development of a System One-Line

Power system designers communicate their design requirements through a combination of drawings, schedules and specifications. One of the key tools in developing and documenting an electrical power system is the System One-Line (also called a Single Line Diagram). This drawing starts with the incoming power source from the utility service and/or on-site generation and their associated distribution equipment. It then follows the power flow down through the various conductors as well as any voltage transformations to feed distribution equipment buses for the key loads served. Initially, the System One-Line provides a framework for the incorporation of different types of required information such as:

1. Incoming service voltage and utilization voltages required.

2. Electrical distribution equipment ampacity and short-circuit ratings.

3. Overcurrent/short-circuit protection.

4. Conductor types (i.e., cable or busway) and sizes. (Cable lengths may also be estimated to determine voltage drop and any upsizing necessary.)

5. Transformer kVA sizes, ampacity, impedance and voltages.

6. Generator kW sizes and voltages.

7. Motor loads and voltages.

8. Other power quality equipment such as surge protection devices, power factor correction capacitors or uninterruptible power supplies.

A System One-Line may start out in the Design Development Phase of a project as a basic concept. Other information can be added throughout the design cycle. It can then be copied and modified to create a number of alternate drawings showing different system approaches. This permits the power system designer to analyze the impact of each arrangement on cost, redundancy and projected physical space requirements.

The System One-Line takes on increasingly more importance as the project evolves through the Design Development Phase. Input from the other architectural, mechanical, plumbing, electrical and fire protection professionals on the design team helps to better define the various equipment loads and develop the power system one-line to accommodate them.

At some point in this stage, a construction manager may be brought in to assist the owner and architect in assessing the design’s constructability. Various improvements that could increase energy efficiency and/or reduce construction costs are often suggested. Moving toward the end of the Design Development Phase, the One-Line and associated drawings such as equipment room elevations and floorplans are provided to the client for their review and approval. At this point, both the client’s comments and the construction manager’s additional inputs are integrated into the design. This final set of approved design development drawings, which include the Power System One-Line, are used as the basis for the development of the construction drawings. Moving into the Construction Document Phase of a project, alterations are made to the Design Development Electrical Drawing set. At some level of completion (typically 90%), these drawings are sent out to finalize budgetary estimates and narrow the field of contractors to be included in the selection process. During the push from 90% to 100% completion of the construction documents, the construction manager or the general contractor is asked to provide a Guaranteed Maximum Price (GMP). During the Bid or Negotiation Phase of a project, Bid Document Drawing Sets are sent out to a list of potential contractors. Estimators at these contractors review the Bid Package and tabulate the value of the electrical equipment, conduit and cable costs plus manpower necessary to build out the project.