Unexplained voltage drop in residential circuit
tlark
14 years ago
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christophersprks
14 years agonormel
14 years agoRelated Discussions
2 circuits sharing neutral
Comments (36)All previous posters on this subject have somehow missed the 500# gorilla in the room. Multiple branch circuits cannot be allowed to âÂÂshareâ a common neutral conductor run without increasing the current carrying capacity of that run (larger size, DO NOT DOUBLE RUN WIRE, gang breakers). In such application power lost to heat more than doubles for a doubling of current, following square law (I-squared-R), dangerously violates code and it will burn down. This is the very reasoning behind licensing of electricians; if you donâÂÂt understand this, do not perform wiring and do not shuffle the load centerâÂÂs runs. However, âÂÂbalanced neutral/phaseâ is valid for load runs and mandatory on the supply-side of the bus; and is taken advantage of, by discriminating technicians. Balanced is green, more efficient, reduces losses, brownout, etc. When a load is balanced between legs (2 just happens to work out to be single phase; and 3, three phase), it is NOT sharing a run, but current is mutually excluded from the common neutral run (i.e., current does not circulate to the neutral runâÂÂs lug) to the extent that each leg mirrors the other(s). Hexus, think exclusive OR (XOR, only analog). Each circuit when used alone has exclusive use of that run. Under no circumstances does current exceed the runâÂÂs rating, ANYWHERE. When the circuitâÂÂs legs have matched loads (in current mirror, reactances in quiescence), think about identical hot plates on each leg, then the neutral run to lug will be cold, out of circuit and may be snipped to no affect. The neutral run BETWEEN hot plates will carry just one circuitâÂÂs current for either/all leg(s) (in series for 2-leg circuit; 3-phase âÂÂmotorsâ have all neutrals removed--there is no need for them with no current to run). Instead of I-squared-R loss over MULTIPLE neutral runs to lug; the loss is over a short distance ONLY between hot plates. Circuit interrupters (breakers) are now required to be âÂÂgangedâ (hence branch circuit fuses are not allowed) in order to arrest all branch neutral current. Sleeping rooms (including family/den/living rooms) require a 'split circuit' AFCI. GFCI's will de-energize with PROTECTED neutrals in common....See MoreHouse circuit over/ under voltage problem
Comments (14)You have described the classical unbalanced load problem on a house transformer. The outside transformer has a center tapped 240 V winding. The center tap is grounded. The voltage from either end of the transformer winding to ground is 120V, but the voltage from one end of the winding to the other is 240 v. Inside the house in the distribution box are two "hot' bars and one neutral bar. One end of the transformer bar is connected to one of the hot bars and the other end of the winding is attached to the other bar. The center tap is connected to the neutral bar. Here's a peculiarity of the transformer. The total voltage across the transformer winding tends to be constant. If a heavy current draw in one half of the winding reduces voltage in that portion, the voltage in the second half of the winding will rise and tend to maintain the total voltage. Therefore, if half of the transformer falls by 10 volts, the other half will rise 10 volts. That is why electricians try to "balance" the circuit loads so that on the average, the load on each half of the transformer is approximately equal. If an out-sized load, say a welder, hits, one leg of the transformer, enough to drop the voltage in its leg, the voltage in the other leg will rise. Whats happening is the RV is connected to a single house circuit and at times draws an out-sized current especially if other loads in the house also appears on the same circuit. The solution is to split the load from the RV and connect to two circuits, one that draws from one bar in the distribution box and the other drawing from the other bar....See MoreBack-Up House Battery/Generator Conundrum
Comments (25)LOL. Getting back to fuel, I do wonder about diesel. Wonderful engines etc and probably the go-to for commercial power backup but I wonder about the fuel life in terms of standing....it does go off after time, and I suspect the standby maintenance is bigger - and maintenance is something many probably don't factor into costs. I wonder if you can get the special diesel that doesn't include the road tax and is cheaper? The gas equivalent here in Canada is called "purple gas" but don't know if there's a diesel equivalent. If it was legal to do so and you were committed to having the tank topped off regularly, or had two tanks, I suppose you could, theoretically, have a diesel vehicle and use the fuel from tank A) when it was getting old, keeping tank B) full, then swap and use up tank B) - I believe you can have such fuel tested too. Reading the Costco fliers I thought the Natural gas gensets sounded appealing - no dangerous fuel storage, no old fuel to get rid of, but then you are reliant on the outside gas supply. I suppose propane beats this, but how long does IT keep?...See More12-3 for 2 discrete 120 circuits?
Comments (11)Thanks for the reply, Brickeyee, and I'm sorry I couldn't get back to this sooner. I've read a bit about the problems with harmonics that seem to happen mostly in commercial settings where a bunch of computers or fluorescent lighting is wired on MWBCs on three-phase Wye systems, and the arguments for so-called super-neutrals. But what I suppose I'm not getting in the context of the OP's scenario is the effect on the neutral when the currents are 180 degrees apart. I would have thought that the neutral would still present a decent enough in-phase 0V reference to the computer's power supply when it's fed by an opposite-pole leg. But if it's inductance that really tends to confound things, that's beginning to make sense to me. terribletom - Just trying to understand the angle you are coming from. Is your argument that the neutrals are going to be connected in the panel anyway? Thanks for asking, billl. Well, no, I wasn't theorizing to that extent. I was merely seeking to isolate the problem beyond mere voltage drop (which can, of course, be "felt" by independent ccts from the same panel--more so if it's a small panel and the two hots come from the same pole). It's specifically the current-carrying characteristics of a shared neutral on a MWBC that I was interested in. So I thought that by distinguishing between circuit types, I'd get a more targeted answer. Let's say this is more about learning than arguing, eh?...See Morejoed
14 years agoaspen31
14 years agotlark
14 years agobillhart
14 years ago
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