Technical Info: Proper Wire Selection

Proper Wire Selection.
By: Kevin O’Connell, B&M Siren

Here’s a scenario that is all too often repeated:

I get a phone call from a perplexed person; “I just put a beacon on my ambulance, and it hardly turns!”

“The bulbs are dim, aren’t they?” I ask.

“Yeah, the bulbs are really dim! What’s wrong?” Or, “So and so put alternating flashing lights on my ladder truck and they stay on one side after a few minutes”.

I follow up with the next question:

“He mounted the flasher in the back of the rig, didn’t he?”, as if I already didn’t know.

“Well, yeah. But what’s that have to do with it?”

Plenty!

These, and many other ills from sluggish sirens to burned wiring can usually be blamed on the installer who failed to consider a few simple, but important factors.

Essentially, electricity is the movement of electrons from one atom to the next within a conductor (wire). This is often compared to the movement of water through a hose. Just as pressure drop is a factor in hydraulics, so is voltage drop a factor in electrical conductor size. The guy with the ladder truck ran an 18 gauge wire from the front of the rig to the very back, a termination, splice, and connector-riddled run so long that the voltage was reduced to 10 volts at the flasher. As soon as the flasher warmed up, 10 volts wouldn’t operate it.

The guy with the ambulance was even worse. After spending $80.00 for a beacon, he was too cheap to buy the proper wiring components! Several pieces of scrap wire, their ends bared, twisted together and taped, were run to an undersized switch. He’s lucky he didn’t burn up his beacon motor from under-voltage-related heat!

Although current draw can be rated in amps or watts, wire is only rated by amperage. Hence, loads rated in watts must be converted to amps. The watt is the most basic unit of electricity, being one amp of current at one volt of electromotive force. So the conversion formula is simple:

Watts divided by volts equals amps.

A good example to use would be the Federal Twin Sonic light bar, a venerable favorite. The Twin Sonic Model 12 uses four #4464 bulbs. The 4464 is a twelve volt bulb rated at 60 watts. Thus, 60 watts divided by 12 volts equals 5 amps. The four bulbs draw 5 amps each for a total lighting load of 20 amps. Add 3 amps for the motor and the grand total for the light bar is 23 amps. At this point, 14 gauge wire looks adequate. But not for long.

OPERATING TEMPERATURE

Secondly, consider the operating temperature of the device and wiring. The current carrying capacity of wire decreases significantly with temperature increases, and current draw of motors can nearly double. Let’s refer to the capacity chart:

MAXIMUM CURRENT CAPACITY @ 12 VOLTS

WIRE SIZE       120 DEGREES      125 DEGREES      150 DEGREES

20 gauge            15 AMPS                  13 AMPS                   9 AMPS
18 gauge             18 AMPS                  15 AMPS                 11 AMPS
16 gauge             22 AMPS                  19 AMPS               14 AMPS
14 gauge             27 AMPS                 23 AMPS               17 AMPS
12 gauge             40 AMPS                 32 AMPS               24 AMPS
10 gauge             50 AMPS                42 AMPS                31 AMPS

Next, determine the total length of your circuit. Example:

Battery to fuse or circuit breaker             2 feet
Fuse to switch                                            7 feet
Switch to lightbar                                        8 feet

Total                                                              17 feet

After considering the first two factors, 10 gauge wire seems appropriate. So let’s calculate voltage drop accordingly:

.0011 voltage drop X 24.5 amps X 17 feet =.45815 voltage drop through the circuit.

Less than 1/2 volt loss. Not great, but very acceptable. Personally, I use 8 gauge for these applications just to be sure, discarding the factory wiring on the lightbar.

TERMINALS AND SWITCHING HARDWARE

Fourth, good terminals and proper switches are essential. Just as pipe fittings can cause friction loss in plumbing, so can poor terminations cause excessive heat with attendant voltage drop.

Terminals should be installed on CLEAN wire using proper crimping tools. My Thomas & Betts TBM-6 crimping tool cost over $1,000 with dies, but unless you do what I do, most people can do quite well with two or three crimpers costing less than $200 total.

Terminations should be kept to a minimum, and splices should be avoided if possible. If you have a choice between screw terminals and blade terminals, opt for the former and keep them clean and tight. Blade terminals have limited contact, are more susceptible to contamination and can work loose, all which makes for dangerous heat under load.

Switches should be carefully selected as well. I have a friend who continually uses 25 amp momentary pushbuttons for early B&M siren brakes, which draw over 100 amps. He runs them through 10 gauge wire, generating a great deal of heat. An appropriate installation would use the pushbutton to activate a solenoid with 4 gauge to 6 gauge cable. Whenever practical, your switching device should be rated at least 50% higher than your load.
Finally, always, ALWAYS be sure that the electrical device you install is WELL GROUNDED!!

(Kevin O’Connell is an emergency equipment specialist and President and CEO of B&M Siren Company. His articles have appeared in many prestigious national fire engineering publications, and we think him for offering this article for publication..)