Friday, August 17, 2007

What size and type of output wires should I use?

There are two main considerations for sizing DC wiring from the output of a power supply to its load. They are ampacity (fancy term for the number of Amps) and voltage-drop (remember ohms law: V = I x R). Ampacity refers to a safe current carrying level as specified by safety organizations such as Underwriters Laboratories and the National Fire Prevention Association, which publishes the National Electric Code (NEC).

AWG stands for American Wire Gauge and defines the diameter and cross sectional area of the wire. The smaller the AWG number, the larger the diameter, cross-sectional area, and current carrying capacity of the wire. Always use insulated wires with solid or stranded pure copper conductors (do not use aluminum or copper-clad steel wires). The voltage-drop is simply the amount of voltage lost in a length of wire due to the resistance of the conductor.

DC wires may be sized for either ampacity or voltage drop depending on the wire length and conductor heating. In general, ampacity considerations will drive wire selection for short wire lengths (less than 50 feet) and voltage drop will drive wire selection for longer lengths (greater than 50 feet). Note: If you are using the Remote Sense feature of the power supply, remember to stay within the maximum voltage drop across the cables that the Remote Sense is designed to compensate for, which can range from 0.3V to 1.0V (check the power supply’s user-manual for details).

The National Electric Code table 310.16 provides ampacity values for various sizes, bundles, and insulation temperature rated wires. ALWAYS FOLLOW THE NEC RULES, LOCAL CODES, AND YOUR COMPANY’S PRACTICES WHEN SELECTING DC WIRING.

Table 1 shows the MINIMUM recommended wire sizes for different load currents. The use of larger diameter wires (with a smaller AWG number) would reduce the voltage drop (and heat generated) across the wires. The current ratings in Table 1 are based upon using 90° C rated insulated wire. If using a lower temperature rated insulated wire (e.g., 60° C), the wire diameter would need to be larger. Refer to the following web site for more information about wire gauges: http://en.wikipedia.org/wiki/American_wire_gauge .

For example, per Table 1 below, a load current of 200 Amps would require a minimum of two # 2 AWG wires connected in parallel for each of the output connections (one pair or wires for the positive (+) and one pair for negative (-) output connections to the load). Again, larger diameter wires would decrease the voltage drops across these wires.

2 comments:

Vacation Rentals said...

But what voltage is this at?

Anonymous said...

Hi Vacation Rentals,
It does not matter what the output voltage is. All wires or cables have some resistance. When current flows in these wires or cables that are connected from the power supply's output to its load, the current is affected by this resistance, which causes a voltage drop (Current times Resistance = Voltage Drop, or IR=V).
Hope this helps.
Power Guy

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