2. tech-talk
  3. 155104-976

Lets talk dedicated lines

About a year ago I installed a dedicated 20 amp line for my system...and went as far as modding the whole power structure from the street transformer to my panel to my house...at some fairly major expense. The result was certainly an improvement for my system.
Recently i got to thinking that a lot of us have a system that utilizes the standard 15amp line that more than likely only supplies one other component in our rooms...and that is the lights. While lights can be noisy on the system, particularly if they are halogens, then this is a source of considerable hash and noise. However, since the only other thing shared by this line are the lights...and not the more power hungry grabbing stuff..like the refrigerator, or the Ac etc, then simply playing the system in the dark should essentially allow for a dedicated line effect to materialize on the gear that is attached to this line as well. The standard room convenience outlet( which is what we are talking about here) is 15 amp breaker protected and is only supplying the lights and the convenience outlet in a room....so ( assuming that this is the case--and the lights are the only thing supplied besides the outlet) what is the real advantage to the 'true' dedicated line ? Twenty amps is more than a lot of auto gear needs, so I doubt that is the issue...thoughts?
128x128daveyf
stevecutler
20 posts
 

AC power is a funny bugger and it helps to understand how “noise” gets onto the line. Basically noise are tiny “spikes” imprinted onto the sinusoidal AC wave from various sources. Use of a scope can show this. I have a Flintstone era Tektronix tube scope which I use and shows this clearly. Some of the worst offenders in residential are motors (with brushes such as vacuum cleaners), compact fluorescent lamps (the self-contained pigtail type which can cause over 100% harmonic distortion on the neutral) and any electronic device which requires a large inrush current. Also remember that most “noise” is running on the utility lines themselves due to harmonics imparted from various sources mostly large motors.

A dedicated line may help and the use of hospital grade aluminum clad MC cable may help due to the sheath acting as a shield against RF. IG hospital grade MC cable uses a second ground (green with yellow stripe) and with the use of an IG receptacle may eliminate some noise off of the line. This is used for sensitive hospital equipment located in ER’s, ICU’s, CCU’s etc. The extra ground wire is connected to a separate isolated bus bar inside of the electrical panel which is then connected to the ground bus.

A residential whole house TVSS (transient voltage surge suppressor) will eliminate some noise, protect sensitive electronic equipment, not costly, easy to install and only requires two spare circuit breaker spaces and a dedicated twenty ampere two pole CB in an electrical panel. An isolation transformer (1:1 ratio) connected to your equipment may also help on a dedicated line by isolating both the hot and neutral lines from the utility but the ground will not necessarily be isolated.  An isolation transformer is rated in Volt Amps (not watts) and multiplying the voltage times the required amperage will give a good estimate of the size needed (120v x20A=2.4kVA) but will not take into account the small losses due to coil efficiency (power factor).

Unless your equipment requires it and you have an extreme case, I cannot see why anything larger that a 20 ampere 120 -volt dedicated line would be necessary using the above methods. Of course some experimentation will be needed to get it to satisfaction. Remember that most equipment and utility grids are 60 to 80 years old and modern upgrades to generating, transmission and distribution have not taken place mostly due to exorbitant cost. Utilities are required to provide power but not necessarily clean power. (My background is an EE degree from PSU some 40 years ago, am a licensed electrician and worked with many local hospitals and utilities in design build). Hopefully this will shed some light on power line carrier noise.


john_g
81 posts
 
My listening room is in a house completed in 2005, with a main disconnect panel outside serving one subpanel upstairs (where my listening room is) and another serving the downstairs. The listening room has one 15-amp circuit for lights and a separate 20-amp for the convenience outlets; I was able to move its breaker to the top of the subpanel by switching wires with another 20-amp circuit. Various finagling ensured that the audio equipment is the only thing on that circuit, although there are still a few unused convenience outlets on it.

The wife's out so I just tried millercarbon's experiment of shutting off all the other breakers in the subpanel, and am pleased and disappointed at the result.

I'm pleased because it indicates that shutting down the other circuits improves the sound, even though there was nothing but a few wall warts drawing current on them at the moment.

I'm disappointed because it means I'll have to figure out some way to run new cable from the subpanel on one side of the building to the outlets on the other, without benefit of attic access. Conduit on the outside of the building would not gain spousal aesthetic approval.

One question:  Do unused outlets on the circuit detract from sound quality somehow?


Ag insider logo xs@2x
cleeds
5,960 posts
 
danvignau
I use a dedicated 30 amp line to power my system, and it works great.
In many jurisdictions, that would require the use of 30A receptacles, which would in turn mean that you'd need to modify your equipment to have 30A plugs, or otherwise make some adapter for them. None of that is justified, imo. I think it's better to simply install multiple 20A lines, which can be improved by derating them.
... a 15 amp circuit uses 14 gauge wire, 20 amps 12 gauge, and 30 amp 10 gauge.
Not necessarily. There are variables such as the length of the run and whether the lines are in conduit, and there can be advantages to derating the line.  If you're in the US, always check local code as well as NEC.



More to discover