I took the physics for scientists and engineers course sequence in college and know the fundamentals of electricity but don’t really know anything about circuits beyond the basic laws (KCL, KVL, capacitors, inductors, resistors, and analyzing simple circuit diagrams). I was wondering if anyone here could show what the equivalent circuit would be for a typical home’s electricity wiring, for light bulbs and lamps only, (ignoring refrigerators, electric powered appliances, televisions, etc).
Putting the question into a specific context, let’s say that there is one floor, and four lamps that have standard light bulbs, where each light bulb is controlled by 1 switch.
Thanks. (Any method of explaining it is good for me :))
incandescent lights are resistors, switches are switches. treat all plug in lamps just like a wired in ceiling/wall fixture. make a floor plan, draw the lights/lamps and switches in their positions; then connect them with wires. an electrical schematic would likely be similar with the wire connections made shorter and straighter.
Thanks very much. Some follow-up questions. Again, with lightbulbs/lamps only, what reasons would there to place resistors in series or parallel, if any? Also, what about controlling a light bulb with two different switches?
In general, that is, considering all common household electricity appliances, is there a need for resistors in series or in parallel, inductors in series or in parallel, and capacitors in series/parallel? Would you consider household wiring to be a very simple equivalent circuit, even modern American homes built in the last few years, or somewhat simple, or rather complex equivalent circuit?
Did you pick up this knowledge from experience, from your job, or in school? Are you an electrical engineer? (jw, you don’t have to reply to this part)
It’s not clear what you want this for, but there is a format for household wiring that is used by home designers, builders, and electricians. It uses it’s own set of symbols (see here) and has very little in common with electronic or electrical engineering circuit diagrams.
Some examples of home electrical diagrams are here and here, if that’s what you’re after.
electrical wiring feeding 240V or 120V circuits or devices is in parallel electrical circuit sense.
a 3way switch is a SPDT, a 4way switch is a DPDT wired in a certain fashion.
all wiring has inductance and capacitance. for electrical wiring in a house most of the wiring is spread out. wiring making a right angle bend will have inductance, too tight a bend might have enough inductance to be significant that it will create a fire hazard. a nicked wire (when stripping insulation) will create a high resistance point.
there are general wiring principles of good practice or electrical code to minimize or prevent such problems.
to quantify by calculation you would need all the measurements in 3D of every bit of wire in the house. some electrical engineers somewhere have done work that applies to this. in real life if you needed that information you would take instrument measurements at various places. this just for considering the conductors in the circuit.
terminology, diagramming will be will be different between electronics as most would learn it and residential electrical wiring.
it all started when as a kid i disliked reading fiction.
All of the loads in your house are generally connected in parallel. This is so that they all get the same supply voltage. It is fairly rare for you to find anything in series in a household circuit, with the exception of protective devices which are always placed in series so that when they open they break the entire circuit.
This is done using two “hot” wires. The power feed comes in from one side, then the first switch selects either the top or bottom “hot” wire. The second switch also selects between the top and bottom hot wire, and connects that to the load (the light bulb). When both of the switches are connected to the same hot wire then the light turns on. When they are both connected to different hot wires then the light is off.
See here:
Residential wiring is relatively simple from a circuit point of view. Your home is typically fed from a single center tapped transformer (aka a single split phase) with the center tap grounded to become the neutral. This makes the voltage from either line to neutral 120 volts AC, and the voltage from line to line 240 volts AC. Half of the 120 volt loads in your house (lights, wall sockets, etc) are connected in parallel between one hot line and neutral, and the other half of your loads are connected in parallel between the second hot line and neutral. 240 volt appliances like your dryer and oven are connected in parallel to the two hot lines.
In your typical equivalent circuit type of stuff you generally just assume one ground. Residential grounding is a bit more complicated than that due to safety concerns. There are two “grounds”. One is the neutral conductor, which is grounded to keep the system stable (it is possible to run ungrounded or isolated systems, but this is difficult enough to keep isolated in the real world that it isn’t done in residential wiring). The second “ground” is the safety ground or protective ground. This ground is run separately for several reasons. First, it carries no current, so even if you consider the wire to be a resistor (as all wires really are) it will still always be at true ground potential. Second, if you connect the case of an appliance (like an oven or refrigerator) to a separate safety ground, then if any one wire breaks (hot, neutral, or ground) you don’t end up with an unsafe condition. If you only used the neutral as your safety ground, if the neutral connection in the house wiring broke and you switched the appliance on, then the case would become electrically hot and would be unsafe.
I’m an electrical engineer with 24 years of experience. I design hardware and software for industrial control.
What? Where did you hear this? All wires have inductance, not just wires bent at an angle, and inductance does not dissipate heat, aside from the resistance of the wire, and that wouldn’t be an issue unless you tightly coiled up a long length of wire; inductance would still be far too low to have any effect at 60 Hz unless you put it on an iron core (and if both conductors went through, carrying current in opposite directions, as in a cable, they would cancel out); if that were true, then transformers and inductors would be mostly useless.
Your terminology is a little non-standard here, and might be confusing.
Those 2 wires between the switches are normally called “switched” rather than “hot”. Because they are only hot when selected by the switch, and only 1 of them will ever be hot at once. Only wire coming in from the power source is referred to as “hot”, since that is the only one that is always hot. (The other wire (white one, in USA) is the unswitched or neutral connector – it should be a continuous connection to the light, never switched. Plus there is a bare ground wire, also continuous & unswitched.)
i agree that all wiring has inductance because i said.
good practice is to make wide smooth curves and not small curves or sharp right angles. it has been found that sharp tight bends in air can be a fire hazard in some situations.
Ten turns of 12 gauge wire around an air core (something like this) is only going to give you about 730 uH of inductance. At 60 Hz that works out to about three-tenths of an ohm of inductive reactance. You could theoretically get a volt or two across that, but you’d have to pull 20 or 30 amps through the wire to do it. And if you’re regularly pulling that much current through the circuit, I’d think the wire would be 10 gauge or larger (resulting in even less inductance).
A 90 degree bend is going to give a tiny fraction of that, if anything. So unless the bend caused a stress fracture in the wire, I don’t see the hazard. But in general I agree that it’s not good practice to have sharp bends in your household wiring, if for no other reason that to have good pull-ability in the wire.
It also still won’t cause abnormal heating, unless it was from the wires actual resistance (i.e. 30 amps x 1 volt = 30 watts, but only for resistive loads; it would still cause voltage drop though). Also, a nitpick - you’ll only get that much inductance for a particular size of coil (very large in this case), with smaller coils having much less inductance, and if it were a cable with two wires carrying current in opposite directions, the magnetic fields cancel out so you have zero inductance (not quite, but essentially zero).
Anyway, what johnpost probably means is what you said, or more likely, stressed insulation eventually breaking and shorting out at the bend.
Physical stress is the problem. The wire will go through repeated heating and cooling cycles where stresses build up in a sharp bend. This can cause fractures or stretching at this point, causing dangerous heat build up and/or sparking.
One llight with two switched. Using two single pole double throw switches. At one switch the hot lead is connected to the common of the first switch. The two runners (wires connected to the other two conection points) run to the second switch. The common from the second switch is connected to the light fixture.
There is no need to connect any resistors, inductors, or capacitors.
Where did I pick up the knowledge. College and attending the California Maritime Academy. Over 40 years as a Stationary Engineer. Also several inspected remodels of my house.
The power system will work most efficiently when the capacitance and inductance balance out. Since in practice inductive loads are more common than capacitative ones (mostly due to motors of various sorts), capacitors are deliberately added to balance those loads. On a residential scale, this is handled by the power company: They have some estimate of the total expected inductance of all of the houses they’re connecting to, and just have big capacitors at the stations or substations that match that expected inductance. For industrial applications, though, the consumer will generally do the balancing themselves (this is called correcting the power factor), and the electric company will charge extra if it’s too far out of balance.
Note that none of this is relevant for a simple light-bulbs example, since light bulbs have effectively zero capacitance and inductance both, and hence will always be balanced.
A bigger concern nowadays is the load presented by electronics, including CFL and LED lights, which are neither inductive or capacitive but aren’t resistive either; rather, they are complex non-linear loads due to the use of a rectifier and filter capacitor, which draws current in short spikes (many times higher than the average/RMS value) at the peak line voltage, but no current otherwise (this applies to both SMPS and conventional linear power supplies, the latter also does have an inductive component). For this reason, the EU mandates that power supplies of 75 watts or more have power factor correction (which is much more complex than adding a capacitor or inductor, using an electronic circuit instead).
