Cable Size When Increasing DC Voltage

Maybe you should check it with your ampprobe...

What increased current??????

Difference Between Current and Voltage | Difference Between ...

I = U / R

So, the current will increase when changing voltage from 24VDC to 110VDC, as the resistance stays same.

This may cause problems with the cabling, or will it?

I'd also be concerned about the cable insulation breakdown voltage.

There will be just some PLC-relays as load and, yes, they will be also changed for new 110V voltage level. But I don't think that they will affect so much.

About cable insulation breakdown voltage, I don't think that it will be a problem because cable data sheet says that voltage is 150/250V, am I right?

So the question is, will the wires last the current. For me, 0.5mm2 wire sounds a bit small for 110VDC, but on the other hand, the load is not that big either. I am confused because the cable data sheet don't tell me anything about the current withstand.

A few things

1) The same piece of electrical equipment running off 24VDC will require less current if running off 110V (ohms law again), so unless you are adding a lot of extra equipment your overall current will go down.

2) Suitable cable size is directly related to both current rating and permissible cable volt drop (which is a function of distance), however if you are increasing the voltage then the voltage drop becomes less of an issue, and the current reduces (when the same equipment is used at a higher voltage).

3) 0.5mm2 cable seems too small for an industrial application (but again it depends on the application). The standards I work to are 1.5mm2 standard for hookup wire, 0.75mm2 as an absolute minimum in special cases.

Based on the information given however the cable seems suitable for 110VDC, but without knowing what the total current and cable length is we can only guess as to the suitability of the cable size.

Interesting observation.

However the voltage level depends on the application.

Unless the PLC is localized within a small, clean area such as an equipment cabinet or small clean room, a 24VDC system exhibits/experiences severe operating flaws/failures.

The BELL Standard for system equipment interrogation is 48VDC for several proven/verified reasons. (The voltage level standard is significantly higher for AC applications. I suggest reading of the standard thoroughly.)

1. 24VDC systems are very succeptable to and consistently experience loss of current carrying capacity due to any low level of oxidation, moisture, and/or inadequate conductor or terminal pressure and/or the voltage drop caused by excessive condutor length. (Simply not enough pressure to overcome the resistance created by either of these conditions.)

2. Beacuse of this; interrogating field equipment with 24 volts (AC or DC) such as conveyor emergency stop (pull cords), local equipment stop/start contacts, and any other device in an unclean environment is a recipe for system unreliability, high maintanance cost, and loss of production due to constant equipment control issues/failures when the field devices cannot communicate to the PLC.

3. Most 24 volt PLC system require and many have additional interposing relays to allow interfacing of and execution of the PLC I/O with the field devices at a hgiher voltage level. (These relays are confined to the immediate vicinity of the PLC cabinet/location.)

4. This additional equipment significantly adds to the cost of installation, adds dificulty and confusion to the troubleshooting process, and increases the chance for equipment/device failure. (More moving parts.)

5. If in this case the application is simply converting field wiring from 24VDC to 110VDC for use as Digital/Discrete input or output feedback, the only concern would be if the conductor insulation and the PLC I/O equipment is rated for the higher voltage. (The voltage rating is usually stamped on the outside of the conductor insulation or the equippment/device and if not, there is usually a part number from IEE or another system number that can be looked up to determine the voltage rating.)

6. If there are any analog devices curently in the system operating at 24VDC, the application of 110VDC will no doubt remove them from being operable as soon as voltage is applied. (The smoke will be let out of the device(s) and you won't be able to put it back in.)

7. Most newer PLC input components are optically isolated therefore allowing a broad range of voltage levels to be applied without experiencing damage to the device.

8. Many of the older PLC input devices are not rated for higher voltage levels and may work for a short time but will fail when you can ill afford it.

9. Common sense should prevail:

If the application is in a clean, very low moisture, low-vibration, protected environment and the distance(s) from the PLC to the field devices is short (< 500 feet), using 24 volts may be a good idea.

If the application is in a dirty, wet, hot, high-vibration, acidic, or unprotected location with conductor distances exceeding 500 feet, use of 110-120 volt PLC I/O would be the wise choice.

A sound rejoinder but flawed in its application here. This system already operates at 24VDC and has been operating long enough for the OP to call it "old". Regardless of the plausible advantages a higher voltage system might have in certain applications, clearly they don't apply here or the system would not have gotten old. Additionally our OP came here asking only if the 0.55mm² conductors can handle the higher current. What current? What load? What environment? What switches? This OP appears to not understand the fundamental concerns of working with electricity. The OP should hire somebody.

I'm weighing in a little late on this but, I liked both of your comments #8 & 14

current rating requirement not given .pl give

Would any reader like a copy of British Standard 7671 as a seasonal gift?

Can you find the current carrying rating of 0.5mmSq cable in the data sheet which is telling the voltage ratings.

If you are not drawing more than the current rating how you fear about the overheating. Instead you care about the insulation breakdown when you are increasing voltage.

As all PLC digital input circuitry is optically isolated, the current is negligible (normally < 40mA) however the length of conductor(s) and the associated impedance per ft/m is critical and can result in significant voltage drop. Hence; If the impedance created voltage drop results in the circuit voltage level applied to the PLC terminal(s) being less than the threshold limit that is required to activate the optic coupler, the input will either not function or experience intermittent, unstable operation.

Certainly the same physics apply to analog input and output signals. However, depending on the current/wattage rating of the analog power supply, the system usually will automatically increase the voltage (pressure) enough to overcome the cable/wiring impedance and allow the signal to function within design tolerance.

Weighted digital output and input circuitry suffer the same cable/wiring length constraints and are very susceptible to "noise" impingement as well.

As far as the periphial equipment being "old" affecting the allowable voltage level:

If the equipment is indeed old enough that it is only designed for 24 volt operation and it is not rated to accept a broad range of applied higher voltage levels, then applying 110 volts whether AC or DC will destroy the periphial equipment as well as the PLC. This is a certainy, not a guess, and replacement of all the lower voltage rated equipment whether in the PLC or elsewhere is required.

I am assuming and hoping that the engineer/technician doing the installation design is knowledgeable and cognizant of these constraints and is merely questionable of wheter using the old existing cable/wiring (significant savings) is allowable.

You do realize that we don't even know if this is a PLC system. I introduced the term PLC as support that 24VDC is still a standard for industrial control, not as a description of the system the OP wants to change to 110VDC. You continue to assume many things that are not evident.

As a matter of fact, the only thing evident is that the OP is uncertain if old wires that were rated for 150 V can now handle 110 VDC. Does that question demonstrate knowledge and recognition of the constraints of any electrical system to you? It doesn't to me.

OOPS! Right on. I have assumed too much. Thanks for bringing me back to reality. Hopefully the conversations have stimulated a thought process in the asker that will yield positive results and a viable solution.

Have a great holiday!

Dear Mr.McFenox,

Under Identical condition of operating system, a mere voltage INCREASE will draw less current. You have not given the power consumption and type of operating equipment.

Pl.provide more details.

DHAYANANDHAN.S

Really?! This is only true with a dynamically adjustable load like some switch-mode power supplies. According to Ohms law (V=IR) an increase in voltage by about 4.58 will increase the current by about 4.58. This will mean an increase in power consumption of about 21 times the original power consumed IF the load stays linear and does not self ignite from over voltage.