Reply posted by Jason on April 02, 2001

I run a small production set - lights, sound, etc. I have a small single-phase distro. I built the unit myself with the help of an electrician. I do not consider myself ignorant when dealing with power, so forgive me if this sounds dumb.

In your discussions you constantly said each hot leg when measured across the neutral leg should meter 110 volts and across the two hot legs it should measure 220 volts. I have hooked in my distro about 150 times now, after being "trained" on what to look for, and what to stay away from. Typically I run into single-phase panels like found in home services with 100-200 amp service and sometimes I run into 3 phase 208 panels with 100-200 amp services.

That said, I was told that the voltage should measure 120 volts hot-nuetral and 240 volts across to hots. Evey panel I tap into, I measure the voltage to make sure I'm not tapping into something weird. Why is it that you say it should measure 110 volts hot-nuetral and everywhere I have tapped int has measure 120 Volts hot-neutral? Is it a regional thing?

One thing I also have noticed, however unscientific, the power meters on Furmans' read power from 116 volts to 122 volts as green, or safe, and 115 volts and below register red, or unsafe. Just a question and an observation.

Jason


Reply posted by Doug Matthews on April 03, 2001

“Regional thing.” - In the example of the previous post, I used 110/220 because that is what the original poster used.

Where I'm located, it is "120/240". The local tariffs (rules set by utility commission) allow the electric utility a deviation of +/- 5% (6 volts on 120), so the legally allowable voltage is between 114V and 126V. Typically, you'll see higher voltages in the range close to the distribution substation (and downstream voltage support devices - let's not go there) and lower voltages downstream as the utility attempts to keep things within the legal all along the line.

Voltage level will also change as load on the power lines change (i.e. residential areas might see better voltage during the day when people are at work, worse in the evening when home and using lighting, heat, TV, computer, etc.)

Doug


Reply posted by Bryan Wright on March 31, 2001

Doug, I'm totally fascinated by this topic and your well-explained response... thank you. Very well written for people like me.

So I assume the growing prevalence of double-neutral-equipped (usually big) distros is to provide a return path capable of handling a high-current load in case the loading of your hot legs becomes radically unequal? Or is each neutral specific to a particular leg?

One other thing I'm not quite getting... when hots are unequally loaded, what is happening between them that induces current flow in the neutral and how are they connected? Inside the distro? Related question: why is it necessary to have this "return line" as you put it, and where does that returned current go once it gets back to the tie-in point?

I'd also like to say that the term "neutral" is a grossly misnamed... not anybody's fault... just an observation.


Reply posted by Doug Matthews on March 31, 2001

Thanks!

“So I assume the growing prevalence of double-neutral-equipped (usually big) distros is to provide a return path capable of handling a high-current load in case the loading of your hot legs becomes radically unequal? Or is each neutral specific to a particular leg?”

I cannot comment specifically on this - I am unaware of these units (make? model? custom?).

It likely has something to do with handling huge imbalances in the phase loading, or electrical code requiring "de-rating" of cable.

“One other thing I'm not quite getting... when hots are unequally loaded, what is happening between them that induces current flow in the neutral and how are they connected? Inside the distro? Related question: why is it necessary to have this "return line" as you put it, and where does that returned current go once it gets back to the tie-in point”?

There is no "induction" of current in the neutral. When current goes out one wire, it has to come back somewhere (or be shunted to earth).

When you plug something into 110V, you are forming a circuit. Current goes out the hot wire, through your load, and back the neutral wire (and vice-versa on the other side of the waveform). In the case of 220V, you are forming a circuit where current goes out one hot wire and comes back the other hot wire - no neutral involved.

So, lets plug two 110V things into opposite legs of a single-phase distro.

Since the voltage waveforms in each leg are out of polarity with each other, when current is going out one hot wire it comes back the neutral wire to the neutral bus of the distro. On the opposite leg, the hot wire is pulling current back toward the distro - it gets this current from the neutral wire, which gets it from the distro neutral bus.

So, we have one hot leg dumping current into the neutral bus of the distro, and one hot leg pulling current out of the neutral bus of the distro. If the loads are exactly equal, no current flows in the feeder neutral (feeding the distro).

If the loads are unequal, and there is more current getting pulled out of the distro neutral bus than there is coming in from the other leg, current flows toward the distro in the feeder neutral.

If the loads are unequal, and there is more current getting dumped into the distro neutral bus than the other leg is pulling out, current flows upstream from the distro in the feeder neutral.

The sum of the currents flowing into a busbar IS zero. This means that some current goes in, some current goes out, but the in's MUST equal the out's.

“I'd also like to say that the term "neutral" is a grossly misnamed... not anybody's fault... just an observation.”

If you think that's misnamed, try atomic charges. A guy named Benjamin Franklin (if I'm not mistaken) decided that an electron should be called "negatively charged" and a proton should be called "positively charged." The upshot of this is that when we talk about current flowing in a given direction in a wire, what is actually happening is that electrons are flowing in the opposite direction in the wire.

Doug