Most of the time people need to know if they can run two different voltages in the same conduit, well there are multiple variables to this one simple question.
Running different voltages on the same conduit has its precautions and its upsides. Not abiding by the very strict rules and practices can however lead to something you don’t want. If you want to avoid the negative consequences of this, you are going to abide by the necessary precautions that need to be taken.
Can you run two different voltages in the same conduit?
Yes, You can, you just have to take into account the voltages you are trying to run. The actual volts you want to run must come into play. Running high and low voltages have some effects you should know about. Running 0-10V dimming wires in the same conduit as line voltage is not recommended at all.
So, when you look at it, the voltage difference actually matters a lot. When the voltage difference is significant, they need to be run separately or at least be separated by a continuous barrier. So what about 120v and 480? On a well-grounded 120V conductor? Yes, it is possible. You would run a 120v control wire one way, and a 480v power the other. Good Engineering practices recommend that you try separating the wires as much as you can.
Can you run AC and DC wires in the same conduit?
The answer is No since the DC cables could induce a DC voltage onto the AC side. They can run directly side by side. The reason for not allowing them in the same conduit is partly that if your insulation fails in some manner you can have very high voltages passing into low-voltage wiring.
The general practice is not to run AC and low-level DC which is usually defined as less the 50VDC in the same conduit, but like everything in life, there are very few hard and fast rules. The real problem arises when there’s a high current flowing in either circuit and particularly if that current is being switched off or on.
If we were purely concerned about wires touching, it would have been sufficient if both cables were armored in some way. PV cable is pretty thick already, and if you had Surfix running in the same conduit, it should be perfectly safe.
Why should AC and DC wiring be kept separate?
Separation of the wiring is not just because they are just AC or DC, but because they clearly serve a different function, going from different places to different places. More importantly, DC wiring is more likely to be low voltage wiring, you certainly do not want to mix up high voltage cables.
You certainly do not want to mix high voltage cables with low voltage cables. A lot of things can occur when there’s a mix of other low voltage signal cables with high voltage AC cables which could cause a lot of interference and crosstalk.
Imagine this, current flows in both directions alternatively, but a DC wire has only one direction of current flow which may be positive or negative. Now imagine that in a certain moment of time, there is a negative electron flowing inside the AC wire, and the wire accidentally touches the DC wire. In this case, the entire system will get violently short and worst-case scenario? You might have a fire in your system. It’s best practice to make sure that AC and DC wiring and kept separate.
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Can we use the same cable for AC and DC?
Yes, it is possible. However, there is one main thing to consider. When substituting Voltage and Current, It is not advised to take a cable built for a 5V, 1A DC application for an application designed to power 220v, 5A AC device. Due to an excessive amount of high current the cable is most likely to melt causing a fire of some sort.
For AC cables the current flows through one direction and then changes as it flows in the other direction. DC cables, on the other hand, pass through the cable with no alternating characteristics of any sort.
So What Does The NEC Have To Say About This?
According to the NEC, it is perfectly OK to run different voltages in the same conduit, however, you must take into account that once you reach more than 3 current carrying conductors in a conduit a duration factor will come into play (Table 310.15(B)(2)(a) NEC 2005. Also, you must watch your conduit fill.
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