EMERGENCY LIGHT VOLTAGES AND VOLTAGE DROP CALCULATIONS
The most common voltage for emergency lights is 6 VDC. This means that the power coming from the building’s electrical system must be dropped down from 120 volts and converted to DC current. The main reason for this is that nobody has ever invented an AC battery.
When installing remote heads for emergency lights, we must take into account the distance that the remote lamp head is installed from the main emergency light unit. The farther the unit, the greater amount of voltage drop one will incur, reducing the available voltage to power the lamp. This in turn will reduce the amount of illumination created by the lamp.
UNDERSTANDING VOLTAGE DROP
Steel emergency lights are offered in 6, 12, and 24-volt models.
All wire contains some resistance to the free movement of electrons. This produces a phenomenon called “voltage drop.” If you are running 20 feet of wire, you won’t see enough voltage drop to be concerned about. However, when you start talking about hundreds of feet of wire, voltage drop has to be taken into consideration. In designing for construction, architects and building engineers plan for a maximum of 5% voltage drop. Anything more can adversely affect the operation of equipment in the building.
The higher the current drawn through a wire, the greater the voltage drop. However, the larger the wire diameter, measured as “wire gauge” the lower the voltage drop. So, it is important to size your wires accordingly to minimize or at least control voltage drop.
CALCULATING VOLTAGE DROP
There are several ways of calculating voltage drop. However, to make things easier, we’ve created the following chart:
|
Wire Gauge |
Load Current |
||||
|
.5 amp |
1 amp |
2 amps |
4 amps |
10 amps |
|
|
10 |
0.10% |
0.20% |
0.40% |
0.80% |
2.00% |
|
12 |
0.16% |
0.32% |
0.64% |
1.27% |
3.18% |
|
14 |
0.25% |
0.50% |
1.01v |
2.02% |
5.04% |
|
16 |
0.40% |
0.80% |
1.60% |
3.20% |
8.00% |
|
18 |
0.64% |
1.27% |
2.54% |
5.08% |
12.71% |
|
20 |
1.01% |
2.02% |
4.03% |
8.07% |
20.17% |
|
22 |
1.60% |
3.20% |
6.40% |
12.81% |
32.02% |
Before we try and use the table, we need to understand the relationship between amps and watts. There is a very close interrelationship between volts, watts and amps. Affecting one will always affect the others. You can use the following formulas to find any of them, if you know the other two:
- Watts = Amps x Volts
- Volts = Watts ÷ Amps
- Amps = Watts ÷ Volts
So, based upon this, when we say that a 6 volt emergency lamp has a 9.0 watt bulb, we can find that 9 watts ÷ 6 volts = 1.5 amps
Okay, now let’s go to the table. If we are running a 12 watt remote head 300 feet off of our 6 volt battery, we first figure out that 12 watts ÷ 6 volts = 2 amps. If we are using 18 gauge wire, we’ll have a voltage drop of 2.54% for every 100 feet of wire, or 7.62%. That surpasses the 5% standard in the electric code. However, if we went to 16 gauge wire, we’d only have a voltage drop of 4.8%, which is an acceptable voltage drop.
Now, let’s look at the same installation with a 24 volt emergency light. We’re still going to use a 12 watt remote head and place it 300 feet from our main unit. Since we have a higher voltage, our calculation for the amperage changes considerably; 12 watts ÷ 24 volts = 0.5 amps. If we use that same 18 gauge wire, we’ll have a voltage drop of 0.64% for every 100 feet of wire, or a total of 1.96%. In fact, we could drop all the way down to 22 gauge wire, and still be below the 5% limit at 4.80%.
In this second example, if we stayed with the 18 gauge wire, we could put in a 25 watt remote head and only have a voltage loss of 3.125%. We could actually go 475 feet away with that higher wattage remote head and still be under 5% voltage drop.
Call us at 404-224-9365 for more information on emergency lights, voltage drop, and configuring an emergency lighting system. Customer service representatives are standing by so please call us today.