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From the Q&A

With TJ Byers


Strobe Alley Needs Power

Question:

I'm looking for a 24-volt transformer to power 18 strobe lights drawing 0.85 amps each.

The lights will be wired in two circuits, with nine lights on each leg. The distance to the first light is 500 feet, after which each light will be 330 feet apart. I plan on running #4 AWG copper wire for 1,490 feet then switch to #6 AWG copper wire for the remainder of the run. Any help you can give me to find a supplier of this transformer would be appreciated

Robert Mitten
via Internet


Answer:

First of all, I wouldn't use a 24-volt distribution line for this application. Instead, I'd go with a 120 VAC line and tap the line at each strobe location using a small 24-volt, 1-amp transformer, like the Alltronics (408-943-9773; www.alltronics.com) 97N002 or Mouser (800-346-6873; www.mouser.com) 553-F46X.

This accomplishes three desirable goals. First and foremost, it reduces the size of the wire from 4 gauge to 14 gauge — typical house wire. Observe: The current requirement for the 24-volt system is about 8 amps per leg, and the resistance of 4-gauge wire is 0.267 ohms per 1,000 feet, which gives a voltage drop of 1.1 volts in the 500-foot section. (E = IR = 8*0.134 = 1.1 volts.) That's nearly 5 percent of the power lost in the wiring alone. The current of the 120-volt system, on the other hand, is just 2 amps per leg. With a resistance of 2.73 ohms per 1,000 feet, the voltage drop across 500 feet of 14-gauge wire is just 2.73 volts, or about 2 percent of the total power. Furthermore, you can use 14-gauge wire throughout the system, eliminating the need to mix two heavy-gauge wires. Last, but not least, 4-gauge wire costs 43 cents per foot,  ($4,428.00 total with 1650 feet of #6 @ 28 cents per foot), whereas 14-gauge wire costs 5 cents per foot, ($630.00 total), for a savings of about $3,800.00. If a 120-volt system isn't practical because of building codes, the line voltage can be reduced to 72 or 48 volts, as shown in the wiring diagram above. Here are two charts showing wire resistance versus wire size, and recommended wire sizes for the three above systems.

Gauge (AWG) Diameter (mm) Area (cir mils) Resistance (1000 ft.)
0 8.252 53.49 0.105
2 6.543 33.62 0.168
4 5.189 21.15 0.247
6 4.115 13.30 0.425
8 3.264 8.367 0.687
10 2.588 5.261 1.08
12 2.05 3.310 1.71
14 1.63 2.081 2.73
16 1.29 1.3088 4.35
18 1.02 0.8231 6.92
20 0.81 0.5176 10.9
22 0.64 0.3255 17.5
24 0.51 0.2047 27.7
26 0.40 0.1287 44.4
28 0.32 0.0810 70.7
30 0.25 0.0509 112
32 0.20 0.0320 176
34 0.16 0.0201 280
36 0.13 0.0217 445
38 0.10 0.0080 708
40 0.08 0.0050 1093

Recommended Wire Sizes

System Voltage Total Amps Amps Per Leg Voltage Drop (first 500 ft.) Wire Size
120 3.06 1.53 2.09/2% 14
72 4.09 2.05 1.75/2.4% 12
48 7.15 3.58 1.23/2.6% 8
24 15.3 7.15  0.96/4% 4

If you insist on using the 24-volt system you describe, the transformer you want is called a furnace or power control transformer. They are available from Allied Electronics and Newark Electronics, but are very costly. You can save money by cruising the surplus electronic outlets, like All Electronics (www.allelectronics.com) — subject to availability, of course. (Note: Any 24-volt transformer with a 16-amp secondary or greater will work.) A better bet is to use two 8-amp, 24-volt transformers, one for each line, rather than a 16-amp behemoth. They're easier to find and cheaper.


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