This Arduino is going in a weatherproof box on an outside wall. There will be ten sensors cables that go from the Arduino through a conduit to the air conditioning compressor unit. (Monitor temperature, pressure, voltage, current) Multi wire cable gets real expensive real fast. Each sensor needs three wires, common, power, and signal. So I tentatively figure to use ten strands of 3 wires each. Looked that up at Digikey and each one is 0.182 max outside diameter. Call it 0.20 just to be safe. The relay wire is about 1/8 inch, or 0.125 inch diameter. I don’t want to cram them tightly in the conduit so will use a conduit bigger than absolutely necessary.
My calculations don’t look right.
The radius of each cable is 0.2 / 2 = 0.1.
Area of a circle is pi * r squared so: 3.14 * 0.1 * 0.1 = 0.0314 square inches
I need ten so make it 15 for comfort.
Multiple the area of one cable by 15 to get about 0.471
That is how much area is needed for the 15 strands.
That is total area so the radius needed to contain that much area is: square root ( area / pi )
Calculate that circle as: square root ( 0.471 / 3.14 ) = 0.3872 square inches
that is how much area 15 strands will take. Looks too small to me.
A one inch conduit (inside diameter = one inch) has an area 0.78 square inches.
I must be visualizing the 0.182 wires as bigger than they really are, because it seems like cramming 15 of those cables into a one inch conduit is tight. But the numbers say one inch conduit is much larger than is really needed.
What say you? Am I calculating this correctly?
And just before clicking send I figured out part of my problem. Circles don't pack together well in another circle. How do I determine the wasted space?
I think your cables will be a very tight fit in a 1" conduit, especially if the conduit is more that a few feet long.
I think your approach to try to calculate is not the best.
There are published tables for "fill factor" of a transformer winding. This is similar to your situation as they are putting wires in a defined space. The most you can expect for the fill factor of a randomly wound transformer is 70% and more likely 50%. This means you will only be able to get 50 to 70% of the wire (cable) in a space.
In addition, "pulling" cable through a conduit is stressful on the wire and reduces even further the amount of conduit area that is usable. In many cases electricians use "cable lubricant" when pulling cable though a conduit. If you are near a Home Depot look in the isle where the electrical tools are. You will find cable lubricant. Its like industrial K-Y jelly (i.e. can not petroleum based)
I suggest you take a nickel and draw as many conductors as you will need on a piece of paper. Don't pack them line to line but spread them out like you might see in a bundle of wires. The draw a circle around them. The diameter of that circle factored by the diameter of the nickel divided your wire diameter will be the minimum conduit ID you require. It is important to know that as the conduit gets longer or if there are any bends things get tight quickly.
I would also suggest you include a few spares in case one of the cables fail later on. Forget leaving a string or cord in the mix to be able to pull a new wire through in the future. Unfortunately it doesn't work (i've learned).
Just an idea why not use something to convert from parallel to serial so you can run less wires? And actually the common and power can be split at the location. I would work on reducing the wires and a 1/2 inch or 3/4 would be more than enough. Keep in mind for the signal wires you don’t need heavy gauge.
Amongst other things I install network cabling into offices and the like. When I wanted to know how many cable would fit into a particular sized hole or conduit I just drilled some holes in a piece of chipboard then experimented to see how many cable would fit. I think you can spend ages calculating but you'll never be sure your calculation is correct and a calculation won't take into account the springiness and flexibility of real cable.
In addition, "pulling" cable through a conduit is stressful on the wire and reduces even further the amount of conduit area that is useable.
Also a good point, as soon as you have a bend to negotiate the number of cables you can easily get in goes down.
Just make sure your hole in the bottom of your box is plenty big enough. It will nicely double as ventilation opening, preventing water to build up inside.
Protect your electronics with conformal coating, and as long as it's all powered on all the time it'll live long and happily out there.
I once had to run some alarm and telephone wires from my brothers home to his shed which was a run of about 300 feet. We used 3/4 PVC conduit. To make pulling the cable easy and less stressful on the wire we laid out the conduit in a trench, ran the wires then assembled the conduit Except for being a little dirty it worked great.
To make pulling the cable easy and less stressful on the wire we laid out the conduit in a trench, ran the wires then assembled the conduit.
Another useful trick is to put a vacuum cleaner suction hose on one end and feed the cables in the other. The airflow seems to make a big difference to feeding the cables in.
wvmarle:
Suck a piece of rope through first, then use that to pull your electric wires through.
Cable pulling is much easier than cable pushing.
wvmarle:
Just make sure your hole in the bottom of your box is plenty big enough. It will nicely double as ventilation opening, preventing water to build up inside.
Protect your electronics with conformal coating, and as long as it's all powered on all the time it'll live long and happily out there.
Not only that, but many conduit-to-box adapters have a smaller ID than the conduit. There was also the mention of bends in the conduit. Depending on how the bend is made, the ID may be reduced in the corner as well.
Except for being a little dirty it worked great.