Wire gauge for low voltage, high current DC application

Question

For the past few weeks I've been shopping for a remote controlled car for my son, along with some batteries and a charger that will work with them and his other RC toys. To make a very long story short, it would be generous to say that the RC industry has many "standards" for it's electrical connectors. Rather, there appears to be an almost limitless menagerie of arbitrarily shaped plastic and metal pieces to connect batteries to their respective toys and chargers.

Consequently, the consensus advice I've received from friends, online research, and our local hobby shop is to cut off the heterogeneous connectors from our various components and solder on a standard type across our fleet. OK, I can do that.

I'm having difficulty, however, in choosing appropriately-rated connectors and wires, because I don't understand the safety implications in this application. I've done enough home wiring to choose the right wires/breakers/outlets/etc. for 120V AC. Unfortunately, I don't trust my layman's understanding of electrical/electronic principles to translate safe home wiring to safe RC car wiring.

The crux of the issue is that I'm seeing staggering numbers for some of these RC components - like 100A maximum draw for the car's electronic speed control (ESC) unit, or 65A for a high-performance brushless motor. I say "staggering" because the main breaker in my house is 100A, and we're talking about a little toy car here.

Of course, my house uses 120V AC, and this toy uses (nominally) 7.4V - 11.1V DC. So, my gut tells me that I don't need a big slab of metal like the main bus in my home's breaker panel to safely move electrons from the toy car's battery to its motor. I mean, 100A@120V has to be somehow "more" than [email protected], right? This is supported by the fact that the car comes with 12 AWG wires, not the hot dog sized cable that carries 50A to my oven, for example.

Still, in looking at spec sheets for my connector and wire options, most things seem to be rated based on how much current they can handle, without specifying a voltage. Similarly, I've read a number of posts on other forums that say essentially "amps are amps" for safety/heating purposes, regardless of whether they're AC or DC. Again, few mention voltage.

So, with all of that as context, my question is:

• Does either the type of current (AC or DC) or the voltage matter when choosing a safe connector/wire to carry a given number of amps?

To recap, I'm looking to standardize on connectors for, say, up to 15V DC over wires a few inches long. In practice, the current will probably be a few tens-of-amps. But, the battery is rated to discharge at 300A (no kidding!) and the thingy it powers (the ESC) is rated at 100A. So, I assume I should plan to use connectors that can handle 100A@15VDC.

Apologies for my verbosity; I just want to make sure that I neither burn down my son's new toy, nor our house. Thanks in advance for your help.

Answer 1

Forget such things as the motor start current - what matters here is the current required to charge a discharged battery. Charging is low voltage, so insulation is not an issue. In all probability, the existing charging cables are adequately sized (probably only marginally, to save cost) so use wiring which is a bit thicker. For example, if the largest charger cable is 18AWG, use 16AWG for your connections. Start with this in mind, and check the heat developed on charge - a minor rise in temperature is OK, but distinctly warm to the touch means that the cable is undersized. Better to be safe than sorry.

Answer 2

Voltage doesn't burn connectors, the current does. Regardless of the electrical potential difference. Wires are heated (and sometimes smoked) by current as well, and connectors smoke by current. Wire get burnt by power dissipation over wire resistance and flowing current. That's why the RC industry doesn't mention any voltage. [the voltage is however important when you make the initial contact, or use an electromechanical switch].

Why don't you let your son to figure this out on his own, he probably will be better prepared for real life if he fries a toy or two (mine did!).

And yes, you need to size your connectors and wires for proper ampacity of the RC toy. That's why the RC industry has a range of banana-type coupling for each case, just as the electronics industry has thousands of barrel connectors for proper use cases. You can "standardize" across your fleet for the highest denominator, but it won't be optimal.

Answer 3

It's possible the "little toy car" is an entry level model and has more modest requirements than the extreme systems built by enthusiasts - but if your son wants to hot rod his Christmas presents, (that's a victory, right?) you'll eventually move into that territory.

So depending on the size and style of motor, controller etc, 25A may be enough to begin with, 50-100A later. But if you're starting with 100A rated components, rate connectors likewise. And below 50V there's not much reason to worry about voltage compatibility or insulation safety for connectors or wires. Anything like a motor controller, switch, or relay that's only rated for 12V or 24V will say so on datasheet and (usually) case.

Two things are different between your domestic stove and the car, even when both are rated at the same current.

1. The car is much smaller than your house : resistance over a few inches of cable creates some loss, but resistance over fifty feet causes a much more serious waste of power. That said, every little loss eats performance. If you ave two evenly matched cars, upgrade the 12awg wires in one to 6awg and see if it goes faster.
2. Duty cycle is limited to a few minutes by the battery, and as you increase performance, battery life falls further. You can tolerate short term overloads that would melt components in continuous operation. (Visiting a friend in America for Thanksgiving, her house wiring failed just as the turkey was done!) Check for overheated wires or connectors after a fast run and upgrade if necessary.