Effects of twisted pair in 120V AC

Question

In Square D's marketing copy for their plug on neutral load centers, they have a photo for demonstrating "cleaner wiring" in which the hot and neutral wires are twisted inside the panel before reaching the breaker. I agree it looks neater, but it got me thinking and I know just enough about electricity to be confused. I assume the practical answer is that a slow (~1 turn per inch) human-made twist in the last couple feet of a cable that runs 30-40 feet will have no real impact, but I'm curious.

In low-voltage DC data wires, twist is used to reduce emitted EMI and reject received EMI. In a typical home I doubt contractors care about the noise on the voltage line, but as someone with a lot of sensitive electronics it interests me. But I've also seen it suggested that the increased capacitance between the twisted wires could increase current drain, which in turn could cause nuisance trips of AFCI breakers.

And I pretty much only understand electricity at all in DC. I don't know or understand what impacts AC may have on this.

So what kind of impacts, if any, could we expect to see from twisting a previously parallel load/neutral pair, carrying 120V AC?

Answer 1

There is no electrical reason for the twist; it merely serves to keep the two wires physically associated with each other once the Romex jacket is removed. It both "looks neater" and is easier to do subsequent work on.

There is no increased capacitance (the distance between the wires doesn't change), except for the fact that the twisted wires need to be slightly longer than the untwisted wires.

Also, even if there were some increase in reactive current, this would have no effect on an AFCI (which is looking for high-frequency noise) or GFCI (which is looking for a current imbalance between the two wires).

Note that excess capacitance between hot and ground (not neutral) can pass enough current to trip a GFCI. I've seen some kinds of plug-in power line filters do this.

Answer 2

If you locate a sensitive audio system behind the wall of the panel, the twisting may matter.

I'm not sure the heavy gauge 1cm diameter incoming wires can be twisted at 1 twist per inch. However, even at 1 twist per 6", the flux cancellation at moderate distances away (6" or further) will become a bunch of dB.

The steel of the breaker box may be the better reduction method.

Copper boxes do very little at 60 Hz, unless nearly a cm thick (and would be very expensive and heavy); standard copper foil (1.4 mils) is beginning to be effective at 4 MHz, and provides nearly 30 dB attenuation at 40 MHz.

On the other hand, a single wire carrying 100 A (that A/C and oven and range) will induce this (estimated) voltage into twisted pair running from the vinyl record playback cartridge down inside the turntable:

V_induce = 2e-7 * Area/Distance * dI/dt (100 A at 60 Hz)

V_induce = [2e-7 * (1/2 meter * 1 mm)/1/2 meter] * (100 * 377)

and lets assume you have a moving coil vinyl cartridge with 100 uV output, and you want 100 dB signal-to-noise ratio which requires 1 nV maximum interference. The wires from cartridge down and into a coax are 1/2 meter long and 1 mm apart, and we model as UNTWISTED for our initial math.

Will this "breaker box located 0.5 meter away" be a problem? Lets run the math.

V_induce = 2e-7 * (1/2 meter * 1 mm)/0.5 meter * 37,700 A/s

V_induce = e- 7 * 1e-3 * 40 kA/s = 8e(-7 -3 +4)

V_induce = 8e-6 = 8 uV, which is 8,000X too strong.

NOW---- I assumed the twisted pair INSIDE THE TURNTABLE ARM has no benefit. Depending on the quality of the twists, you may get 10 to 100X benefit.

And I assumed the STEEL of the Square D breaker box has no effect. Any attenuation depends upon the MUr of the steel, and the steel thickness. And gaps at the corners of the box.

And I assumed the twisting of the 100 A hot/ground has no effect.

Answer 3

For AC power / Hum in audio systems it goes in stages.

Bad: Romex

Good: Conduit

Better: isolated ground and conduit

Best: Isolated ground and conduit. The hot and neutral twisted and the safety ground in the same conduit but not twisted.

https://www.jensen-transformers.com/wp-content/uploads/2015/02/AES-Ground-Loops-Rest-of-Story-Whitlock-Fox-Generic-Version.pdf