[rscholl]

What is the benefit of running a dedicated 240V line into a theater if it gets stepped down to 110 anyway? Is dedicated 240 better than dedicated 110? I'm planning a new home and want to do what's considered best in this regard. Also, what is the advantage of 20A over 15A circuit?

Thanks for any tips.

 

[Larry Fine]

It doesn't get "stepped down". Let me give you a quick lesson. Imagine a standard 2-cell flashlight, where the batteries are in series. You have 3 volts across the bulb, because the two batteries' voltages add. There's 3 volts from the top battery's + terminal to the bottom battery's - terminal.


Note that there is also 1.5 volts between the middle, where the two batteries touch, and either end terminal. However, since there's no connection to this mid-point, there's only a single voltage available; 3 volts. This is what is called a "single-ended" power supply, because only one voltage is output.


Now, if you inserted a terminal between the two batteries and connected a wire to it, you could power both 1.5-volt and 3-volt loads at the same time. This is what is known as a "double-ended", or center-tapped, power supply. This is used in most modern trasistor audio amplifiers and many other circuits.


I almost all cases where a double-ended supply is used, the center point is grounded, rather than one end, so, referenced to ground, both + and - polarities of electricity is available. This is important to many integrated-circuit equipped electronics, which sometimes need both polarities for the circuit it's in.


Okay, now let's move on to the power in your home. If you can, go outside and look up at the utility transformer up on the power pole that feeds your house. (If it's an underground service, go for a drive and find one up on a pole.) On the side of the big gray can, there are 3 terminals.


Remember the flashlight? Except for the fact that this is alternating current instead of direct current, it works the same: there is 240 volts between the two end terminals, and 120 volts between the middle terminal and either end. These 3 wires come directly into your house (through the meter).


Note that the center terminal is connected to the can itself, as well as to the bare wire that runs with the other two wires to your house. This is the "neutral", which corresponds to the middle of the two batteries. The neutral is grounded, which limits the voltage-to-ground entering your house for safety reasons.


Once inside the breaker panel, the two hot wires are fed through the main breaker and on to the 2 "busses", or rails, that the breakers stab onto. Heavy appliances and other equipment use 240 volts, and regular branch circuits use 120 volts. The big question is "why?" Okay, read on:


Wattage, which loosely corresponds to work done, is the product of volts and amps. In order to do a certain amount of work, a certain amount of power is consumed. Watts' Law tells us that, when you double the voltage, half as much current is needed to produce the same wattage, or power.


If you have half the voltage available, twice as much current is required to produce the same amount of work. Because conductor is more expensive than insulation, it's more economical to supply a higher voltage and less current. Plus, the lower the current, the lower the voltage drop (loss) in the wires.


So, your large electrical loads use both hot wires (and the neutral if any part of the load uses 120 volts, like the timer and drum motors in your clothes dryer; only the heater element uses 240 volts), while your lights and small appliances use 120 volts, which means that the neutral is one of the circuit conductors.


If you look in your breaker panel, the double-handle breakers (aka double-pole) feed 240 volts (or 240/120 volts), while the single-handle breakers (aka single-pole) feed 120 volts. This is also the distinction between the 120-volt and 240-volt run you're asking about. Which to run depends on your electrical needs.


If you need only one circuit, it'll be a 120-volt run. If you need two circuits, you can run two 120-volt circuits or one 240/120-volt circuit. The difference is that (as long as the two hot wires come from the opposite hot wires) a single neutral can be shared by the two hot wires, just like your incoming electricity has.


If you need several circuits, a sub-panel run is an option to individual circuits. This means that a single cable brings 240/120 volts to a smaller panel located close to the load, which may provide several advantages: 1) local circuit control, 2) lower voltage drop over one large cable, and 3) possibly less labor.


As for 20-amp vs 15-amp circuits, this depends upon what you need to power with each circuit. Obviously, a 20-amp circuit has a third higher capacity, but more smaller circuits can be more versatile than fewer larger ones. For example, in my own HT closet, I use eight 15-amp circuits; none need be larger.


So, let's see what your system components are, and what their power requirements are. Also, tell us your forseable upgrades, like a projector, larger amplifiers, etc. , as well as present and future lighting needs. This is what is required in designing any electrical system to feed specific loads.


In addition, how far is it between your breaker panel and where the theater will be located? This is another factor when choosing between a sub-panel and individual circuit runs. Note that, beyond the sub-panel, the individual circuits will be the same: 120 volts; this is all about getting the power to the area.

 

[GRBoomer]

Whew! Are you on the publish or perish plan Larry? Great write up.

 

[GPowers]

Thanks Larry for the write up. It made things a lot more understandable.


What is the ratios of circuits to a the total service rating? Example if your house has a 100amp service, what is the branch circuit amp total allowed?


At my house we have a 100amp service but there two sub panels in addition to all the branch circuits. The pool sub panel is 40amps and the Home theater building has a 60amp sub panel.


looks like more then a 2 to 1 ratio.

 

[jnolan]

your house is not bound by the panel, but rather by the meter (there are 3 residential split phase meters used, your meter is most likely 240 amps). But of course, your main panel does have a main breaker which does limit the amount of power that you can draw, and in your case you said that was 100amps.


You local building codes may have some requirements for wiring up multiple sub panels, that's where you should check first. What it really comes down to is not how much supply you have, but what the demand is at any given moment in time. Think about it this way, how much power is being consumed when your pool is on, appliances, washer/dryer (if it's electric), lights, home theatre, computers, etc.


People get tripped up all the time by focusing on the circuits and how much that adds up to when they should be focusing on the consumption side instead. It's easy to figure out how many amps something is drawing, the spec plate on an appliance will tell you, and if it's in watts then the calculation is easy:


amps = watts / voltage


a 100 watt lightbulb draws .83 amps and so on.

 

[BarryO]

Quote:

Originally posted by GPowers What is the ratios of circuits to a the total service rating? Example if your house has a 100amp service, what is the branch circuit amp total allowed?]

That's one of the most involved parts of the residential electrical code. Various demand factors are assumed, depending on the number and type of load.


See Article 220 of the NEC for all the details.

 

[GRBoomer]

Isn't that why once you surpass the first 10kW, all other power loads are considered at 40% of their rated value? The calculations assume you are not going to turn everything on at once. At least this is how the calculations work in my home home wiring how-to book. Sorry I have not read the NEC yet.


JNolan your statements seem to make sense to me.


______________________________________

Just for fun lets try a calculation on the 100 Amp panel
(please beat me back with a limp noodle if I am over stepping my limited knowledge):

1. 10,000 watts / 240 VAC = 42 Amps (this is factored as always being consumed)
   
   
   So now we have 58 Amps left in the panel (100-42), but that is considered at 40% usage because it is all above the initial 10kW. Which equals 58/0.40 = 145 Amps
   
   
   Total design = the initial 42Amps + 145Amps = 187 Amps.
   
   
   This is more than the 100 amp "rating" of the panel, so this shows that you can have more than 100 amps of rated circuits in a 100 amp panel, but once your actual consumption - not just the rated consumption - surpasses 100 Amps you'll trip the main breaker.
   
2. My 100 amp panel has 30 slots in it. For ease of calculation, let just say that each slot has a single 15 amp 120VAC circuit attached. That makes for 54kW (30*120VAC*15Amps) which is 225 amps at 240VAC (54kW/240AC). Again the ratings of the indvidual components/circuits exceeds the rating of the 100 amp panel.

* Disclaimer: I am probably forgetting some safety factor calculation too, but I think they are on the order of derating by 80%. 80% of the above numbers is still more than the rating of the 100 amp panel.



Getting back to the point, the appliance power ratings or circuit ratings are not all taken at face value, because it assumed you will not be using every load simultaneously. Lets see AC and heat (maybe for an HT), electric water heater, electric dryers, oven, home theater, all the tools in the work shop all going at once.


In a home theater, my lighting circuits are not getting used at capacity when I am watching a movie, because I want it dark. And if I were to have all the lights on for cleaning, then I would most likely not be trying to watch a movie. So a sub-panel for a home theater is most likely not going to be using all of the circuits at the rated capacity.


Let the noodle whipping begin:...ahhh...ahhh...ahhh

 

[Larry Fine]

Oh, if it weren't so late, I'd whip out the whipping noodle right now!


Actually, you guys, most of that ain't half bad. I have the NEC on my computer at home, where I'll be Sunday evening. I'll throw some stuff at you then.


Just one note, GR: When calculating the demand for a premisis, you don't have to factor in both heat and AC, only the larger of the two. Can you guess why?

 

[greywolf]

Re: the shop tools part, I believe the NEC specifically refers to a one man shop v. a commercial type shop where a number of people are working the tools.

 

[sirbleckywelcky]

Just to nitpick:

Wattage, which loosely corresponds to work done, is the product of volts and amps. In order to do a certain amount of work, a certain amount of power is consumed. Watts' Law tells us that, when you double the voltage, half as much current is needed to produce the same wattage, or power.


If you have half the voltage available, twice as much current is required to produce the same amount of work. Because conductor is more expensive than insulation, it's more economical to supply a higher voltage and less current. Plus, the lower the current, the lower the voltage drop (loss) in the wires.


The two basic power equations are:


P = I * V


and


P = I^2 * R


They say the same thing, the second equation just has V = I * R substituted. When transfering power over a wire, the power loss at I and V will be x. If you want to get more power to the end, and you double the I, you will quadruple the power loss (4x). While, if you want to get more power to the end, and you double the V, you will only double the power loss (2x).


So, this is why you see cross-country power lines (those 200 foot tall towers running next to the highway or through some farmer's land) running at 100,000 volts or something similar (not sure what the exact voltages are that the power companies use). They lose a lot less power running at a high voltage versus running at a high current.

 

[GRBoomer]

Quote:

Originally posted by Larry Fine Just one note, GR: When calculating the demand for a premisis, you don't have to factor in both heat and AC, only the larger of the two. Can you guess why?

Yeah, I know. I had more written in another paragraph and censored it from my final post.


But to state the obvious, "because you would not simultaneously be using both heat and AC." But often I see that people here are demanding AC in their theater while the rest of the house demands heat. Being that dedicated home theaters, are the exception and push the normalacies of construction, I was just throwing it out that there there may be a simulataneous heat and AC demand. Regardless of what code says, home theaters (and their builders) exceed normal expectations. How many people out there describe their idea of fun as sealing themselves in a dark, soundproof room, with a dedicated HVAC system?

Or at least since the 1950s and 1960s .

 

[Larry Fine]

Quote:

Originally posted by GRBoomer ...I see that people here are demanding AC in their theater while the rest of the house demands heat. ...I was just throwing it out that there there may be a simulataneous heat and AC demand.[/i]

Yes, but for each system (i.e., the air-handling unit and its corresponding compressor/condensor), only one will be operating at a time.

 

[fatmanstan]

Just to nitpick a little further, if you really want to figure things out, you also have to toss real vs apparent power into the mix.


Real power (watts) is the product of voltage times current, times the cosine of the angle between them. Apparent power is straight volts times amps.


Apparent power is the combination of real power and reactive power, which are at 90 degrees to each other. Reactive power is the result of current through inductive or capacitive loads (or portions of a load), and real power is the result of current through a resistive load.


A heater is an example of a resistive load. Power supplies, computers, alot of electronics are full of different components that are capacitive and inductive, but generally are R (resistive) - C (capacitive) loads. Large motors are good examples of inductive loads.


This, of course, doesn't have much use in basic house wiring, but since I saw equations being thrown around, I couldn't resist. I'm an Electrical Technician for a power utility by the way. Feel free to toss any technical questions about electricity my way, if any were to ever come up.

 

[rscholl]

It's very interesting reading all of these posts. I still have some questions though. Why is 240V better than 120V for a HT? Is it because the 240V line is balanced? I guess I need more basic electrical education here, but if you run a 240V line in how do you power 120V components? You run a dedicated 240V line in to a subpanel which then sends off 120V lines to the theater? Where does 15A vs 20A come into the picture? Is that the amperage of the actual circuit hooked to the outlet box in the wall? How does that relate to 120V vs 240V lines? I apologize for these basic questions, but I'm obviously not an electrician and I'd like to understand these concepts and the terminology before I talk to my builder and his electrical subcontractor. Thanks.

 

[fatmanstan]

15A vs 20A only comes into play when you have heavily loaded circuits. Your normal receptacles throughout your house will most likely be 15A receptacles, fed by 14 awg wire from a 15A breaker. I can't off the top of my head recall seeing any 20A receptacles, well, ever in service except in industrial settings, and most of those were twist lock.


A 20A receptacle has one of the slots on its side, so instead of I I, you would have I --, omitting the ground slot. You can also get 15/20 A receptacle which are a mix of the two, kinda like I I--. (hope my representations are easy enough to follow). The wiring to a 20A receptacle would have to be heavier guage, 12 awg. (by the way, this is Manitoba Electrical Code, I think some places the minimum guage to run is 12 awg, even for 15A circuits).


I guess there are no reasons why you couldn't wire up 15A receptacles to a 20 A circuit, you just couldn't do it the other way around. By our code, though, you are still bound to a certain limit of receptacles per circuit, which is 12, I believe. Running a home theater, I could not think of any equipment that would require a 20A circuit, unless you wanted to have, say, a toaster oven, popcorn maker, microwave, coffee pot all at the same time. Electronics typically don't draw much current. I don't even think large amps draw that much power. They draw enough I guess, but unless you had quite a few of them, plus some other big loads on the same circuit, you are not going to run into problems. However, it's always easier to overbuild in the first place then have to pull new cables later.


As far as 120 vs 240, I can't think of anything you are going to need 240v for in a home theater, apart from maybe a large popcorn maker or for baseboard heat. 240v receptacles and appliances have a different configuration again, both slots on their sides, -- --. So the only reason I can think of for running 240v would be to run 2 120v circuits in a 3 conductor cable, split circuit as Larry mentioned (2 circuits sharing the same neutral). It is slightly cheaper to run a 3conductor/14 to get two 15A circuits than to run two 2c/14 to get the same thing.


So in the end, unless you have some really exotic equipment or want to install some heating or cooking equipment, in the end you are going to be using 120v anyway. If anyone out there knows of or has any 240v HT equipment, then I guess I stand corrected, but the vast majority will be 120v. Cost of installation would be the only difference I could see. If anyone else knows why 240v would be better than 120v, let us know, but this is how I see things. As far as balance, things should tend to be pretty even if your electrician know what he is doing in the first place.


Hope this helps

 

[Larry Fine]

Bob, let me answer your questions without having to separate them and juggle the quote/unquote formatting. Keep in mind the stuff I've already posted.


Running 240 volts is a way of transferring twice the power without having to double the amperage. 240 volts IS two 120-volt lines that share the neutral. A 120-volt line is (ignoring the grounding) comprised of a hot wire and a neutral, the latter which is also a circuit conductor that happens to be grounded.


A 240-volt line is comprised of two hot wires with 240 volts between them, and 120 volts between each of them and the neutral. If you connect a circuit from either hot wire and the neutral, you'll have a 120-volt circuit. That's what is done in your main panel. Can you pull off the panel cover and look inside?


"You run a dedicated 240V line in to a subpanel which then sends off 120V lines to the theater?" Yes, exactly. That's exactly how the power enters your home: two hot wires that share a single (grounded) neutral. A 240-volt feeder to a sub-panel is simply an extension of the same system.


You branch out circuits from a sub-panel exactly the same way you do from the main panel. The reasons to use a sub-panel instead of running everything back to the main panel are (1) distance, (2) localizing control, and (3) space for additional circuits in the main panel.


15-amp vs 20-amp has nothing to do with voltage; it's the capacity of an individual circuit. High-current devices (kitchen appliances, hair dryers, etc.) generally demand a 20-amp circuit because of the current required for proper operation of the device. Lighting generally is happy on 15-amp circuits.


If you need to supply, let's say, 10,000 watts of lighting to an area (warning, technical stuff: which translates to 83.3 amps at 120 volts, which because of 80% loading, would require a minimum of 104.1 amps worth of circuit capacity), you can use either seven 15-amp circuits or six 20-amp circuits.


It's all a matter of location (commercial installations don't allow 15-amp circuits) and how you want to divide up the load into several circuits for control. Sure, you could supply everything by one circuit, but if one fixture goes bad and shorts out, there goes all the lighting. Instant darkness!


Let's suppose you choose six 20-amp circuits for the above example. If you run them from a 120-amp sub-panel, it has to be fed from a 120-amp feeder. However, if you run a 240-volt feeder to the sub-panel, and put three breakers on each hot wire, you only need a 60-amp feeder. Why does this matter?


Because a cable with three 6-gauge conductors costs less than a cable with two 1-gauge conductors. Remember Watt's Law? A higher voltage means lower current for a given amount of power. By running two hots, the load is distributed between them, and the neutral only has to carry the difference.


If your main panle has, let's say, 80 amps being drawn on one hot wire, and 60 amps on the other, the neutral only has to carry the 20-amp difference. This is better than a single hot and neutral that each have to carry the entire 140 amps. Plus, with 240 volts, major appliances can also benefit.


For example, a typical water heater has 4500-watt elements (only one of which is ever on at one time). At 240 volts, this is a load of 18.75 amps, which would require a 10-gauge wire. If you only had 120 volts, the current would be 37.5 amps, which would require an 8-gauge wire.


If your house had only 120 volts coming in, you'd have to have a service of twice the capacity. A 200-amp service would have to be a 400-amp service, etc. The reason the neutral is grounded is that it limits the voltage to ground from any hot wire, and makes sure that a short to ground will operate the breaker.

 

[Larry Fine]

Actually, Stan, a 240/120-volt run is technically more efficient than two 120-volt circuits. Let's say you have two 20-amp loads on a 100-foot run. If you run two 120-volt circuits, each load has to run through 200 feet of wire.


If you run a three-wire circuit, each load only has to run through 100 feet of wire (plus the interconnecting neutral), and the neutral would have to carry no current. This halves the overall voltage drop and power waste.

 

[twojciac]

My father is an electrician and I can't get the man to run a 15 amp circuit. Even if you don't have a specific device which requires 20 amps, which by the way a 120V 20A outlet looks like | |- instead of | |, you can still run all of your normal 120V devices on the circuit. So you're less likely to pop the breaker if you run the shopvac in your home office with your laser printer and a couple of tower computers.

 

[fatmanstan]

Ah, touche Larry.


That's a good enough reason to run split-circuit 240v for me.

 

[edfowler]

I installed a new 200 amp service in the garage that I built. I ran a 70' supply line to my old 100 amp circuit breaker box.


My theater is going above the garage right above the new 200 amp service.


I intended to install a seperate 100 amp box for the theater, not for localized control, but with the impression that an isolated box would clean up the ac going to my components. Is there any benefit to having a seperate breaker box as far as isolating noise in the ac lines, or am I just wasting my money?


If it can in any way improve the performance of my theater, I'm all for it.


Thanks.