Hi all, I’m trying to build the device linked here http://seattlefoodgeek.com/2010/02/diy-sous-vide-heating-immersion-circulator-for-about-75/. The internal wiring is in stretches of about 8 inches or so. The author recommends 16-18 AWG wire. I am wondering if this is thick enough to carry the mains-level currents required for this application? The principal current draw is the 3 heaters, which if you search “jc” you’ll find a comment that calculates they draw about 7.2A.
After searching for the current tolerances of the different wire gauges, I found a bunch of tables like this American Wire Gauge Chart and AWG Electrical Current Load Limits table with ampacities, wire sizes, skin depth frequencies and wire breaking strength which indicate two values, “Maximum amperes for chassis wiring” and “Maximum amperes for power transmission”. What’s the difference between these two ratings? My intuition is that the more restrictive “power transmission” rating tells the gauge required to have no significant loss, while the “chassis wiring” rating is the maximum current before the wires start to burn. Is this correct or am I way off? Which number should I be going by?
I’d like to stick to 16 gauge wire or higher just because the 12 gauge is a bit hard to work with but don’t want to create an electrical fault when I have some high-current devices sitting a few inches above a water bath!
Chassis wiring might (but probably won’t) be handling full current all the time. It’s things like selector switch wiring, maybe relay wiring, something that sees the maximum current for some non-constant amount of time. Push a button, power goes through the circuit, release the button, power no longer goes through the circuit. Even if it does see the current the whole time it can handle a few more amps because it is used for very short internal runs (a few inches of wire) and doesn’t have much of it’s own resistance to add to the current draw (and the potential heat buildup).
Power transmission wires do just that: transmit the power, constantly, to something relatively far away from the power source. Therefore it must have a higher capacity so as to handle the constant (potential) heat caused by the current draw.
So the wires running power to the heating element are going to see the current draw the entire time the heater is turned on, whereas the wires going to the switch which turned on the heater probably won’t (depending on how it’s wired). Plus the heater element wires will be longer, and possibly more exposed, so you don’t want to undersize them. They can buildup more heat, and it takes current just to energize the wire in addition to the current required to energize the device. Notice how the ratings for “power transmission” in your link are almost a third less than for “chassis wiring”? ** It’s just an added safety factor.**
None of this suggests that you can get away with using an under-rated wire size for a switch just because “you’ll only be pushing it for a second”. However they rate the wire differently depending on the application mainly for these reasons.
Resident licensed electricians may feel free to tear this post down and scribble on it mockingly in permanent marker if they wish. I’m not licensed - I just work on industrial equipment for a living and have been playing with the control wiring for a while.
I believe it’s all about length. Wire resistance is proportional to length (for equal gauge). Longer runs of a given gauge have more resistance, so carrying a given amount of current requires a bigger conductor for longer distances - hence the different “chassis” (i.e. short distance) and “transmission” (long distance) ratings.
Mostly, but surroundings matter, too.
Chassis wiring is mostly inside a metal or similar no-flammable box. Transmission wires run through walls made of flammable materials, and enclosed with little air movement to cool the wires.
Ignore the “power transmission” guidelines in that table. And the “maximum amps for chassis wiring” do not seem conservative enough to me. 41 amps through 12 AWG wire? I would never do that.
Personally, I would start by looking at NEC requirements for running Romex in branch circuits, and then (if anything) go more conservative from there.
As you can see here, 16 AWG is good for 10 amps. So I would have no problem using 16 AWG wiring if your maximum current is around 7 amps.
Chassis wiring is assumed to be running separate conductors, whereas power transmission is expected to be multi-conductor cable. For instance, the romex you run through your house has 3 wires all bundled together, inside of the conduit, so it will get warmer.
Chassis wiring (imagine the inside of your fuse box) is considered to be “widely-spaced” wiring, allowing them to get some cooling.
That’s the theory, anyway - avoid fires. I’ve seen plenty of enclosures that are more densely-wired than power cabling.
