DDC - VAV Electrical Controllers Can Produce Faulty Electrical Output

It is usually required that each electrical DDC controller be powered by its own 24 VAC power supply (transformer). All wiring connections to the controller should be via copper conductors unless the manufacturer specifically states that aluminum wire is acceptable.

The wiring connections must all be made in accordance with the NationalElectrical Code and/or the local code as specified. Do not run Level 2 bus wiring in the same conduits as line voltage wiring(30 VAC or above) or wiring that switches power to inductive loads, such as contactors, coils, motors, generators, etc.

Only use shielded cable to run bus wiring. If the runs will exceed 500 feet, use only twisted shielded pair type wire Belden #28 gauge, Beldfoil 8760 or the equivalent as specified by the manufacturer of the controllers. If the runs will be greater than 500 feet, they will usually require a properly designed and selected repeater.(For more details on this topic check out my books  listed as NRC Publications #79, 84 and 37)or purchase here.

To further minimize sensor error caused by field wiring, unless otherwise specified by the controller manufacturer, the total resistance of all passive sensor wiring should be less than 3 ohms. Shielded cable will not generally be required if the sensor wiring runs are less than 50 feet.

When you are dealing with runs of between 50 feet to 100 feet, it is recommended that you utilize Belden #22 gauge, Beldfoil 8761 or the equivalent. With runs up to 250 feet, consider utilizing Belden #18 gauge, Beldfoil 8760 or an equivalent. With runs of up to 500 feet, consider utilizing Belden #16 gauge or Beldfoil 8719.

Wherever a manufacturer is mentioned above, it is for example only. I do not recommend one manufacturer over another. All engineers, users, designers etc must do their own research concerning product manufacturers they wish to utilize.