One Wire Size Up Means Big Savings

Installing wire only one size larger than has been required by the National Electrical Code increases energy efficiency with dramatic paybacks. This simple technique can yield quick paybacks while increasing the flexibility of the installation. By increasing the wire size, reduced power losses offset the cost of the wire and produce savings on energy costs.

Why is this important?

By upsizing wire in a new installation, the engineer or contractor can demonstrate the real savings to the customer as well as the advantages of lower generated heat and increased flexibility of the installation. In addition, when less heat is generated the result is reduced energy requirements for fans and air conditioning systems.

Of course, there are many factors that must be considered in any installation. But for most new applications, where the cost of labor and conduit for the installation outweigh the cost of wire, the increased size of the wire can pay for itself in less than two years. At the same time, increased wire size is insurance against changing future needs and assures lower voltage drops. Some companies, as a matter of course, specify wire two or three sizes larger than minimum requirements in neutrals, which are often overloaded due to harmonics.

Key elements that affect the payback, and thus the economic incentive, to install larger wire gage, are the duty cycle, load factor and electricity price. When using the same size conduit, the increased cost of wire is minimal. As the examples below demonstrate, the payback for upsizing can be quite short, even in single phase lighting circuits, or one to two shift commercial settings.

How important is this to the owner?

Jim Clarkson, former Corporate Energy Manager of Southwire Company, which has nearly 50 acres of industrial facilities under roof in nine states, requires that all loads under 100 A shall use wire one size larger than that required by code. "Quick payback is assured under these circumstances," says Clarkson. "Larger size applications are evaluated on a case by case basis."

A simple way to understand the dramatic impact of wire size on energy efficiency and costs is to examine the number in these examples, in which one wire size above code minimum is installed. All three examples include a separate full-size grounding conductor, following current recommended practice, and use THHN copper conductors.

Example 1. A three-phase circuit feeding a 125 H.P. 460 V motor, operating at 75% load, 250 ft. from the load center, running 8,000 hours per year. Draw is assumed to be 75% of 156 full-load amps (FLA).
3/0 wire4/0 wire
Conduit Size 2 in. 2 in.
Estimated Loss (at 75% load and 44°C and 40°C,respective conductor temps.) 708 W 554 W
Wire Cost $991 $1232
Conduit Cost $365 $365
Incremental Cost $241
Energy Savings: at 75% load 1,237 kWh/year
Dollar Savings: at $0.07 per kWh Payback $86.59/year 2 years, 9 months
Dollar Savings: at $0.10 per kWh Payback $123.70/year 1 year, 11 months
In this example, the payback is under 3 years, and the savings continue indefinitely into the future.
Example 2.The same I2R savings and short paybacks apply to single-phase systems also. Take the case of a single-phase, 15 amp lighting load operating continuously. To simplify, assume the load is concentrated 100 ft. from the panel.
#12 AWG#10 AWG
Conduit Size 1/2 in. 1/2 in.
Estimated Loss (at 15 amp load and 40°C, and 37°C,respective conductor temps.) 77 W 48 W
Wire Cost $11.82 $18.57
Conduit Cost $42.00 $42.00
Incremental Cost $6.75
Energy Savings 254 kWh/year
Dollar Savings: at $0.07 per kWh Payback $17.78/year 5 months
Dollar Savings: at $0.10 per kWh Payback $25.40/year 3 months
Dramatic, short term paybacks in a single-phase run, with flexibility for future load changes.
Example 3. Even when larger conduit is required, there may be a quick payback with wire upsizing. Consider the case of a wye-connected, three-phase 40 amp lighting load operating only 4000 hours per year. To simplify, assume the load is concentrated 200 ft. from the load center. In this example, a total of 5 conductors are used in a rigid metal conduit: three phase conductors, a neutral and a full size ground conductor.
#8 AWG#6 AWG
Conduit Size 3/4 in. 1 in.
Estimated Loss (at 100% load and 60°C and 45°C, respective conductor temps.) 711 W 426 W
Wire Cost $117 $166
Conduit Cost $128 $192
Incremental Cost $113
Energy Savings 1,140 kWh/year
Dollar Savings: at $0.07 per kWh Payback $79.80/year
1 year 5 months
Dollar Savings: at $0.10 per kWh Payback $114.00/year 1 year
Notice how much cooler the #6 wire operates. And less than one and a half year payback at only 4000 hours operation.