Which Brand Electrical Panel?

[j j]

to cause a voltage drop no

wires have resistance. drawing a large current will mae the voltage drop slightly

[Jay Levitt]

of wire"" . the lights will blink, The wire becomes a

the service panel and the load of each piece of

Ah hah! Let me repeat that back to you and make sure I am understanding: even if the service itself has proper wire gauge for

400A, if the *line to the compressor* isn't big enough for 150A, the wire essentially becomes part of the load, and that whole circuit draws far more than 150A, thus propagating a voltage drop back to the main service and making everything dim. Yes?

[John Hines]

Motors are basically a dead short, drawing unlimited power, until they start rotating, which generates back EMF, which offsets some of that current draw, leading to normal current draw at operating speeds.

Ohm's law isn't enough to describe inductive circuits which an ac motor is.

The power grid has to add high voltage capacitors to circuits with lots of motors, or other weird power factors, to keep voltage up. This isn't something a consumer can do. Look up inductance-capacitance circuits in your electronics book.

One thing that could be done, would be to put the lighting circuits on

200 amp feed, and the motor circuits on the other.

[Chris Lewis]

According to Jay Levitt :

I don't think a motor-only circuit can exceed LRA. LRA is worst case, even from undervoltage due to undersized wire. A motor can certainly (greatly) exceed rated current due to undervoltage however.

The fact of the matter is that lights dim whenever _anything_ turns on. Simple V=IR. The question is to degree. You can reduce it, hopefully below noticability by proper design (ie: wire sizing and circuit distribution) and workmanship. But sometimes that may be at a prohibitive cost.

[I-zheet M'drurz]

Thank you. Maybe *that* will sink in.

Turning on another 100W bulb will draw < 1A, and the effect on the line voltage will be milli- or microvolts drop. Will you *see* that in another 100W bulb lit up in the next room? Of course not, but the line voltage will indeed *drop*.

[Mike O.]

[..stuff snipped...]

I don't have any information on the other brands, but there is a difference between the SquareD Q0 and Homeline series

It may not be more metal, but the Q0 series uses a tin plated solid copper buss bars, the Homeline series is an aluminum buss (higher resistance = more heat). Also, the Q0 covers most of the buss with plastic, only the areas that the breakers need to contact is uncovered. On the Homeline, the entire buss is exposed.

Last year I did a replacement of our old Federal Pacific panel. My own research as well as everybody I talked to, locally and on the usenet (not just sales people) recommended the Q0 series over the Homeline. When I had the inspection done, the inspector confirmed that the Q0 was the best choice.

There's a bunch of technical info on SquareD's web site; I imagine the other brands have their info available as well.

(BTW, I have no connection with SquareD, other than as a customer)

Mike O.

[Jay Levitt]

And "degree" is the point I couldn't seem to get across to the guy with the bulging veins in his neck - that perhaps, although I may have experienced tiny voltage drops at my last house (with a bigger, less efficient A/C and the same 400A service), I did not experience

*noticeable* dimming.

I think a (dimmed) light bulb is coming on. The voltage drop from the A/C is proportional to, among other things, the resistance of the wire. And the voltage available back at the main panel is a zero-sum game, so if the A/C circuit has higher resistance in this house, it makes a bigger dent in the voltage available for lighting circuits.

The point that I was missing is that voltage is a zero-sum game. For some reason, I pictured some sort of magic regulator at the power company that gave me roughly 120V all the way up to my rated maximum current draw, because that was the only way I could imagine the service staying anywhere *near* 120V as loads got switched on and off. In actuality, I suppose the resistance of the wire is simply low enough that a 400A draw doesn't make a *huge* voltage difference.

[Chris Lewis]

According to Jay Levitt :

Sort of. It's probably not so much the AC circuit itself (providing all the connections are good and solid), but the resistance of circuitry _before_ the circuits "divide" into branches.

Think of it this way: the distribution transformer is supplying a specific voltage. All of the wiring from that point to the AC is providing a resistive drop. If all of the resistive drop was between the panel and the AC, the other circuits wouldn't notice a dip.

What this suggests (again assuming there aren't faulty connections) that AC-induced dimming in other circuits is the drop in the feed before the breaker panel, not the AC circuit.

As a corollary, if two homes have different dimming characteristics with the same device, the difference is in the main feed resistance. The expensive wire...

I should mention, however, that when we're talking about 120V circuits, the situation can be different. If you pull hard on one leg, the voltage on that side can go down, and the other side go up.

That being said, if you get noticable brightening of lights when a big 120V load comes on, you have a big problem (loose neutral).

[Jay Levitt]

OK.. that makes significantly more sense to me than the zero-sum idea - I didn't *think* voltage drops back-propagated, but I knew how little I knew - but now we've come full circle.

What I was trying to say at the very beginning, though obviously I didn't use the right terms, was that I had thought a 400A service meant, by definition, that I had low enough resistance to continue providing something very close to 120V even when I'm drawing 400A. It sounds like that's not necessarily the case, and that there are varying grades of wire that might be used in a 400A service, so that one house might have a very tiny voltage drop at 400A and another might have a significant drop. Which also means that it'd be fairly expensive, if not impossible, to improve, since that's all buried.

I remember why I went into software now.

[Chris Lewis]

According to Jay Levitt :

Heh.

Voltage drops do propagate. V=IR losses. Power really is a zero-sum game (at least insofar as short duration events go). If you pull a watt more, the rest of the system (the rest of North America ;-) gets a watt less - until the generation system compensates.

The electrical standard basically says that the power company has to provide you 240V +- 3 (or 5%) at the panel under any load up to the maximum. (plus a 3-4% drop to the end of the circuit is permissible, but this doesn't directly contribute to dimming of other circuits).

If the wire is sized for a 5% drop in voltage during that AC's startup surge, it will represent something close to a 9-10% drop in wattage at the bulbs. That's a lot.

Not just varying wire _sizes_, but different lengths. Double the length, double the voltage drop.

Not necessarily. It could be a connection/wire size problem _before_ the transformer.

If it really is objectionable, contact the power company and tell them that you have a properly designed/installed AC circuit with plenty of ampacity, and you think you're having sufficient voltage swings during AC startup to cause a problem.

If you're lucky, they'll come out and put some recording voltmeters on your system to identify _exactly_ where the problem lies. For free.