What size cable cross-section should be used for ammeters
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A 5A ammeter is often installed inside the control panel. Can the wire connecting this ammeter to the terminals use a 1.5 square millimeter gauge? Some users have said that according to the standards, a 2.5 square millimeter gauge should be used. However, I believe that the standards are designed to account for voltage errors caused by losses in long-distance transmission lines; therefore, the requirements apply to the cables outside the control panel. As for the wires inside the control panel that are less than 0.1 meters in length, it shouldn’t cause any problems. I would like to ask the experts here at Haichuan whether this understanding is correct.
I think it is mainly for safety reasons; using a 1.5-square millimeter wire for a 5A current is indeed a bit too small – at least a 2.5-square millimeter wire should be used.
Most of the ammeters on the control panels at the site are inaccurate; this is due to the small cross-sectional area of the wires, as mentioned above. Therefore, for safety reasons, ammeters are generally not used, as their impact is minimal. Nowadays, there are many ways to protect important pumps; devices that can provide protection against both rated current and no-load current can be installed in the drawers. What can actually be seen on-site, in addition to speed control via frequency converters, is that it’s necessary to have a frequency meter installed for reference there.
The current meter has a transformation ratio of X/5, with a maximum rating of 5A; in practice, it usually operates at around 3A. Additionally, a cable with a cross-sectional area of 1.5 square millimeters should be capable of carrying a current of over 10A. Why is the actual current lower?
That’s right; according to the manual, a 1.5-square-millimeter cable is indeed suitable, but in practice we choose a larger one for extra reliability, as this makes it safer and more dependable.
This post was last edited by chb1534 on 2015-9-12 at 17:04. The inaccurate readings of the ammeter on the control panel are mainly caused by a large distance and insufficient output power of the current transformer; for example, the output power of a typical current transformer is 10 VA, and if a current of 5 A is considered, then according to the formula: R=1.75×10^-8 (unit: ohms). Meters) X cable length (in meters) / Cable cross-sectional area (in square meters). Line voltage drop U = I R, line loss P = I squared R. If it is 2.5 square millimeters, the cable length is approximately 57/2 = 28.5, which is about 25 meters; in this case the line loss is 10 watts. Thus, the maximum transmission distance while maintaining accuracy is 25 meters. If 1.5 square millimeters is used, the transmission distance is only 34/2 = 17, or about 15 meters ； At an actual current of 2.5A, the maximum transmission distances for 2.5 square millimeter and 1.5 square millimeter cables are approximately 100m and 60m respectively; if the current is 1A, these distances are approximately 625m and 375m respectively. Therefore, the thickness of the cable is crucial for ensuring accuracy in long-distance transmission. However, for cables within our box that are less than 1 meter in length, when a current of 5A flows through them, a 2.5 square millimeter cable results in a voltage drop of around 0.035V, with a power consumption of approximately 0.175W; whereas a 1.5 square millimeter cable results in a voltage drop of around 0.058V, with a power consumption of about 0.292W. In the worst case, the difference in power loss is only 0.12W, which represents roughly 1/100 of the total power loss – in other words, it corresponds to a transmission distance that is 1/100 shorter. I don’t think anyone would design things with such precise specifications.
I think a margin of 50% should be sufficient; with a nominal current of 10A and an actual current of 5A, it should be safe enough.
Using 1.5 or 2.5 square millimeter wires has little impact on a 5A ammeter. In fact, what needs to be considered primarily is the distance between this ammeter and the current transformer. If the distance is within 10 meters, the impact is minimal; either 1.5 or 2.5 doesn’t matter. When the distance is several dozen meters or more, the voltage drop across the wire cannot be ignored. In other words, the current value shown on the ammeter at this time may differ significantly from the actual value. Therefore, it is recommended that when connecting an ammeter in series with a current transformer, use wires that are as thick as possible.
I would like to ask whether an ammeter is needed in the distribution cabinet as well, in addition to one at the site Let me explain that by saying the wires are thicker, I mean there are also ammeters inside the distribution cabinet.
If the wiring of the ammeter is taken from a current transformer, then the thickness of the cable is related to the distance. But don’t you find that troublesome? A current meter is installed directly on the distribution cabinet, and this meter sends a standard signal to the current meter located on the field operation panel. In this way, both the distribution room and the control panel can display the current value, and a 1.5 gauge wire is sufficient
Haha, the main reason is that we are only responsible for manufacturing that small control column; the actual design on site is not done by us. Currently, we use 1.5 square millimeter wires, but some people at the site claim that the standards require 2.5 square millimeter wires, and they also insist that we use 2.5 square millimeter wires for the 4-20mA signals as well.