Vimpex Volt Drop Calculator Instructions When a number of electrical/electronic products are connected to a cable (such as fire alarm sounders to a sounder circuit), the electrical current flowing in the cable combined with the resistance of the cable causes a reduction in voltage a volt drop which means the products receive a reduced voltage at their terminals. If the voltage is reduced too much, the products may not operate correctly. The Vimpex Volt Drop Calculator assists in determining the voltage drop encountered for a given current loading and cable length. The voltage drop that can be tolerated is dependent on the specifications of the equipment used. The Volt Drop Calculator can also be used to determine the maximum number of sounders that can be connected to a cable of a given length (resistance), to achieve a specified volt drop. It can also be used to determine the maximum cable resistance (and thus length) permissible for a given number of devices to achieve a specified volt drop. Cable types directly calculated are solid copper conductor cables and denote the crosssectional area in square millimetres (e.g. 2L1.5 is two conductors of 1.5 square millimetres each). For types of cable not shown, you will have to note the resistance readings from the inner wheel's outer scale and calculate the cable length separately. It may at first seem confusing to use the Volt Drop Calculator but once you become familiar with it, it is rather straightforward. To develop familiarity, it is advisable to work through each of the examples below to check you get the same results. This will confirm that you are using the calculator correctly. How it Works The voltage drop is calculated according to Ohm s law, which states that when a current of 1 ampere passes through a resistance of 1 ohm, a voltage of 1 volt will appear across the resistance. This is the well-known formula: V = I x R where V is the volt drop, I is the current and R is the resistance. In the case of the volt drop calculator, the current I (which is the total current) is found by multiplying the number of devices by the individual device current. That is to say, the formula used is modified slightly to give: V = n x i x R where n is the number of devices and i is the individual device current. Note: it is assumed that all devices connected have the same current consumption. Page 1 of 5 LVVDC Instructions Issue 1
The Volt Drop Calculator can be used in a number of ways to determine acceptable conditions, as shown in the examples below. The main use is to work out the volt drop that will be produced by connecting a given number of devices to a specified cable. You may then assess whether this volt drop can be tolerated according to equipment specifications. Worst Case Volt Drop Calculation (all devices connected at end of cable): Step 1: Determine the current consumption for a single device. Set the red cursor line to this current on the OUTER scale marked Current per device or total Current (ma & A). Example: each sounder takes 32 ma. Set the red cursor line to 32 ma. Step 2: Turn the BLUE scale marked Number of Devices so that the start of the BLUE scale (starting at the number 1) lines up with the red cursor line, making sure the red cursor line remains set to the current chosen in Step 1 (32 ma in this example). Step 3: Determine the number of devices to be connected. Example: 25. Turn the red cursor line so that it rests over the number 25 on the BLUE scale marked Worst Case. (You will now see that the total current, shown on the OUTER scale, reads 800 ma, this being the total load. This should not exceed the total rating of the sounder circuit.) Step 4: Keeping the red cursor line fixed in position, turn the INNER wheel (with the cable length and resistance markings) so that the start of the scale (starting at 1 ohm) lines up with the red cursor line. Step 5: Determine the required length of cable and cable type. Example: 2L1.5 cable, 350 metres. Move the red cursor so that it is over the Length of 2L1.5 Cable (m) scale at the 350 m mark. You will see that the red cursor also lies over the ohms scale at 8 ohms, this being the resistance of 350 m of 2L1.5 cable. Step 6: Read off on the Voltage Drop (V) scale (the second outermost scale) under the red cursor line the volt drop. In this example, it is about 6.4 V. You need to determine whether a voltage drop of 6.4 V is acceptable or not. For example, if the sounder s minimum voltage is rated at 20 V and the lowest voltage applied to the circuit is 24 V, you can only withstand a 4 V drop. So this current load and cable resistance is not acceptable. But this is a Worst Case calculation which assumes that all 25 of the devices are connected at the very end of the cable, i.e. that the current drawn by each and every devices passes along the entire length of cable. This is not likely in practice. The Volt Drop Calculator also includes an Even Distribution scale, which assumes that the total number of devices are spread evenly along the length of the cable. To perform an Even Distribution calculation, proceed as follows: Page 2 of 5 LVVDC Instructions Issue 1
Even Distribution Volt Drop Calculation (all devices distributed evenly along cable length): Steps 1 and 2: same as above. Step 3: Determine the number of devices to be connected. Example: 25. Turn the red cursor line so that it rests over the number 25 on the BLUE scale marked Even Distribution. (You cannot determine the total current as done previously. You can only do this when using the Worst Case scale.) Steps 4 and 5: same as above. Step 6: same as above. The volt drop now reads (for the example used) as about 3.3 V rather than 6.4 V. This would be acceptable for devices with the specification described above provided the distribution of devices along the cable length was even. If you have multiple device types with different current consumptions In this case, you can calculate the total current consumption by hand (the Volt Drop Calculator can assist in this) and then proceed as above (but only with a Worst Case calculation). Step 1: determine the number of devices of each type and their current consumptions. Example: 20 devices at 28 ma, 14 devices at 36 ma. Determine the total current. For the example given, this is: (20 x 28 ma ) + (14 x 36 ma) = 560 ma + 504 ma = 1064 ma. You can use the Volt Drop Calculator s OUTER and BLUE Worst Case scales to do the individual multiplications for you, but you will need to add them manually. The Volt Drop Calculator can only multiply, not add. Step 2: set the red cursor line to the total current on the OUTER scale (Current per device or total Current). Example: set to 1.064 A (the scale is not fine enough for this to be seen, set to just over 1 A). Step 3: Proceed from Step 4 through to Step 6 in the Worst Case calculation as above. Finding out how many sounders can be connected for a given volt drop and cable This is really just working part of the calculation backwards. Example: each sounder takes 24 ma each. Volt drop permissible is 6 V, cable required is 420 m of 2L1.0. Steps 1 and 2: same as above for Worst Case calculation. Step 3: Keeping the OUTER and BLUE scales pressed together to avoid them moving, turn the red cursor line to the required voltage drop on the second outermost scale (Voltage Drop). Example: turn to 6 V. Step 4: rotate the inner wheel so that the required cable type and length lines up under the red cursor line. Example: 2L1.0 cable, 420 m. Page 3 of 5 LVVDC Instructions Issue 1
Step 5: move the red cursor line to the beginning of the scale on the inner wheel, at the 1 ohm point. Step 6: observe where the red cursor line crosses the BLUE scales. Example: 16.8 on Worst Case, 32.5 on Even Distribution. These are the number of devices that could be connected. Numbers must be rounded down to the nearest integer (you can t connect a fraction of a product) so you could have 16 devices if they were all mounted at the end of the cable, or 32 if they were evenly distributed. Finding out what Cable Type and Length needs to be used If you know the number of devices that need to be connected and the permissible volt drop, you can determine what length of various cable types will be permissible. This can be done for both a Worst Case and for an Even Distribution scenario. Example: 25 devices taking 32 ma each. Maximum permissible volt drop is 7.5 V. Calculate for a Worst Case condition. Step 1, 2, 3 and 4: same as Steps 1, 2, 3 and 4 in Worst Case calculation above. Example: 32 ma, 25 devices. Step 5: keeping the outer and inner wheels from rotating with respect to each other, move the red cursor line so that it lines up with the permissible volt drop on the second outermost scale (Voltage Drop). Example: 7.5 V. Step 6: Read off on the inner wheel scale the resistance permitted (on the outer scale of the inner wheel example 9.4 ohms) and the maximum length of each type of cable permitted. In this example, they are: 2L1.0: 280 m 2L1.5: 420 m 2L2.0: 560 m 2L2.5: 700 m Remember that these are Worst Case calculations, assuming that all devices are fitted at the very end of the respective lengths of cable. To do this calculation for an Even Distribution scenario, repeat the above steps but using the Even Distribution scale in step 3. Using the same example, this gives up to 18 ohms and the following cable lengths: 2L1.0: 520 m 2L1.5: 800 m 2L2.0: 1080 m 2L2.5: 1350 m Page 4 of 5 LVVDC Instructions Issue 1
Conclusion We hope you find the Vimpex Volt Drop Calculator effective and helpful. Please let us know if you have any queries or comments. Vimpex Ltd Star Lane Great Wakering Essex, UK SS3 0PJ Tel: +44 (0)1702 216999 Fax: +44 (0)1702 216699 Email: sales@vimpex.co.uk Web: www.vimpex.co.uk Page 5 of 5 LVVDC Instructions Issue 1