December 2008
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Cabling for DDC Systems
Specify the proper conductor sizes and cable types for your BAS project

Steven R. Calabrese


Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

The wiring associated with building control systems is typically low voltage wiring. Of course there are exceptions to this, but for the most part, the cabling that originates from the digital controllers and proliferates outward to all of the sensors, transmitters, and end devices making up a Building Automation System (BAS), is in the low-v range (24 volts or less). This column endeavors to lay down some basic guidelines and rules of thumb when selecting the appropriate wiring for digital control systems.

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In order to fully understand the requirements for control wiring, we need to first lay down some terms and definitions that relate to the subject:

Wire Gauge

AWG stands for American Wire Gauge, and there is an actual formula that’s used to calculate AWG, which is simply based on the diameter of the wire. Rather than share the formula, suffice it to say that, as the diameter of the wire increases, the wire gauge number decreases. To demonstrate, 12 and 14 AWG wire is what is typically used as building wire, carrying 120-volt power to your home’s receptacles, for instance. The next smallest sizes are 16 and 18 AWG, which are suitable for 24-volt power wiring and low-v control wiring. Even smaller yet, 20 and 22 AWG wiring is used for communication wiring, and some low-v control wiring as well, if conditions allow. The smallest diameter wire we’ll typically ever see in the BAS business is 24 AWG, as some manufacturers may spec their communication wiring to be this gauge.

Just as pipe size is selected based on how much (water) flow is to be handled, so too is wire gauge selected based on how much electrical current is to be handled. The more flow, the larger the pipe. Likewise, the more current, the larger the wire gauge. It is also important to bear in mind that, with longer wire runs, there can be an appreciable loss in voltage, due to the inherent resistance characteristics of the conductors. Which means that you may want to upsize your gauge selection if you have to go a long distance with your wire run. You can enter “wire gauge vs. current”, or something to that effect, into your search engine, to find assorted charts, graphs, and calculators on this subject.

Solid vs. Stranded Conductors

Solid conductor wiring consists of a single conductor covered with a layer of insulation, whereas stranded conductor wiring consists of multiple conductors, all together and whose bundled diameter equals the wire gauge, covered with a layer of insulation. The differences between the two types of wire, at least in the commercial controls industry, have more to do with preference than anything else. Stranded wire is simply easier to work with, as it is more flexible than solid wire. It is the standard choice for the industry. Which isn’t to say that there aren’t instances when solid wire may be desired, required, or even specified.

Twisted Pair

Twisted pair wiring is that in which a pair of conductors, each having a layer of insulation, are twisted together for the entire length of the wire run. This is done for the purpose of decreasing external noise and interference. The twisted conductors are covered with an external insulating jacket, which needs to be stripped away at the ends in order to make the terminations. The number of twists per feet varies with the manufacturer and the application. It is common to find twisted pair cables with 12 or more twists per foot.

Shielding

Twisted pair wiring may or may not be shielded, depending upon the application. The shield is in essence a metal jacket covering the twisted pair, under the overall insulating jacket. Shielded cable further prevents interference from external sources, however in order for this to be effective, the shield must be grounded at one end of the run.

Power Wiring

Whether 120-volt or 24-volt, the wiring used for control power will be sized according to the current draw that the wires are to carry. 16 gauge wire is a good reference point, as it can carry 2 amps upwards of 100 feet with little appreciable drop in voltage. If you intend on either going a longer distance or on carrying more current, then you will need to go to the next larger gauge (14 AWG). On the flipside, if you’re current draw is on the order of less than one amp, then you may be more inclined to use 18 gauge wire. Just be aware of how long your runs are, and upsize to the next larger gauge if need be, by using a voltage drop chart or graph.

Input Wiring

Reliable Controls An input on a digital controller typically has two terminals, and is set up as a “voltage-send / voltage-return”. The input sends a dc voltage (say 5 vdc) from one terminal out to the input device, be it a temperature sensor, dry contact, etc. On the other terminal is the return signal from the input device.

For digital (two-state) inputs, the voltage-send terminal is wired to one side of a dry contact (be it a switch, relay contact, or whatever). The voltage-return terminal is wired to the other side of the dry contact. With the contact open, the voltage-return terminal sees no voltage, and with the contact closed, the circuit is completed and the voltage-return terminal sees the 5vdc sent out to the device from the voltage-send terminal. As far as wire gauge goes, we’re talking miniscule current draws here on the input, so you’re never going to need anything smaller than 18 gauge wire. 22 gauge is often suitable for digital input wiring, unless the engineer or the manufacturer specifies a larger gauge, or the wire run is extraordinarily long. The wiring should be twisted pair, to cut down on interference, however it doesn’t necessarily need to be shielded.

For analog inputs connecting to passive devices such as thermistor temperature sensors (which are nothing more than temperature-sensitive resistors), the voltage-send terminal is wired to one end of the sensor, and the voltage-return terminal is wired to the other end. As the temperature fluctuates, the resistance of the sensor varies, and this is read at the input. Again, we’re dealing with small current draws, so the same rules apply here as they do for digital input wiring, in terms of wire gauge. The wiring should be twisted pair, with a shield.

Output wiring

A digital output on a controller is typically a dry contact that’s rated for upwards of a couple of amps at 24 vac. The wire gauge should be able to handle what the contact is rated for, although the output load at the other end is generally going to be somewhat less than what the contacts can handle (at least good engineering and design practices would dictate!). 18 gauge unshielded is a good starting point, and although it typically doesn’t need to be twisted pair, it certainly doesn’t hurt to use it. Remember from the section on power wiring, that if you’re carrying more than an amp of current, or if you’re going a long way with it, you should upsize to the next largest size.

An analog output on a controller is generally a varying dc voltage that feeds a proportional end-device such as a valve or damper actuator. The voltage output is to match the input voltage requirements of the end device, and is typically in the range of 0-10 vdc. The amp draw at the load end is minimal; nothing more than what 18 gauge wire could handle. Even over fairly long distances. The cable should be twisted pair. The shield is generally not required, unless of course the manufacturer specifies it, and/or the consulting engineer calls out for it.

Communication Wiring

Digital communications cabling between controllers in a BAS will vary from manufacturer to manufacturer, however we can still lay down some attributes that are common to most manufacturers. First off is wire gauge, which will virtually always be in the range of 24-22 gauge, and will be shielded twisted pair. From there a given manufacturer may have other electrical characteristic requirements for their communication cabling, ranging from specific twist rates to nominal capacitances and resistances. Always refer to the DDC manufacturer’s cabling specifications, and ensure that the selected cable vendor provides a spec sheet for their cable, one whose specs match the DDC manufacturer’s requirements.

Tip of the Month: The following is a quick reference for selecting cables for BAS. Please note that these are minimum requirements, and remember to always check the engineer’s specifications and the manufacturer’s recommendations for additional or more stringent guidelines.

Power Wiring (120v) 16-12 gauge, depending upon the current draw
Power Wiring: (24v) 18-16 gauge, depending upon the current draw
Digital Input Wiring 22-18 gauge, twisted pair
Analog Input Wiring 22-18 gauge, shielded twisted pair
Digital Output Wiring 18-16 gauge
Analog Output Wiring 18 gauge, twisted pair
Communication Wiring 24-22 gauge, shielded twisted pair
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