PDA

View Full Version : 240 volt AC question



RPM
08-21-2010, 09:21 PM
This is for somebody with more electrical experience than I've had. I hail originally from the UK, where 240 volts AC single phase is standard for dwellings at least.

I've done some 110vac electrical installation here in the US, and as far as I understand it, the 'normal' supply to dwellings is three wires - two positive legs at 120vac each, and a neutral eventually going to earth.

For 240vac, the theory I've heard, ( which makes sense) is that if you need 240 vac, you can get this across the two positive legs - so you wire both legs to the 240vac socket , and leave out the neutral, unless on say an oven, you have a 110 vac fan or something similar.

All this makes perfect sense, until I noticed recently that all the 240vac breakers I've seen take both their 110vac connections from the same leg. I've never seen any breaker system that has one connection on one leg and a different connection on the second leg, so how is one able to get 240 vac from just one positive leg?

Thanks in advance

Richard in Los Angeles

gmatov
08-21-2010, 09:24 PM
RPM,

If you look more closely, you will see that the buss bars have alternating lugs. A double breaker takes 120 from each buss bar. Go have a look.

Cheers,

George

alanganes
08-21-2010, 09:27 PM
You have the theory correct. The 240 two pole breakers do not take power from the same leg, it just looks that way. If you look carefully at the construction of the breaker panel, You'll see that the blades that the breakers connect alternate legs. Two adjacent blades are connected one to each pole.

Don Young
08-21-2010, 09:29 PM
Your observation about the breakers is incorrect. A double pole breaker as used for 240VAC connects to two tabs in the panel that are the opposite 120VAC legs. You may have been looking at a two pole half width breaker. They do connect to only one leg and cannot be used for 240V. The panel tabs are arranged so that each leg connects to alternate tabs.

doctor demo
08-21-2010, 09:32 PM
Richard, in the panel every other lug is the other leg A B A B A B etc. In a three phase panel it is : A B C A B C etc.
As a side note the legs are not ''positive'' they are AC so they are positive and negative (60 cycle) usually just called a hot leg.

Steve

doctor demo
08-21-2010, 09:34 PM
I guess I'm a slow type-r :D

Steve

RPM
08-23-2010, 04:05 PM
Thanks guys,
I took a look, and of course it was just as you described -I should have been more observant :-(

Richard in Los Angeles

claudev
08-23-2010, 04:36 PM
RPM

Be sure to run a ground even if you are only going to use the 240 VAC lines. This way you won't find yourself acting as the ground if a short develops in your appliance or whatever. Note that on some older systems the neutral was also used as a combination ground and neutral. You will probably run into some of these systems from time to time. Be careful.

I have been bitten by a few of your UK systems long ago - about 1954/56.

winchman
08-23-2010, 04:38 PM
Also, what you're calling "positive" is usually referred to as "hot".

gary350
08-23-2010, 05:03 PM
A 3 phase generator has 3 output wires, L1, L2, L3. The center wire is called neutral. The only time you use the neutral wire is when you one of the other wires to get singla phase at a lower voltage.

In industry if you have 480 volts between L1/L2 or L2/L3 or L3/L1 then you will have 240 volts between neutral and any of the L terminals.

In home use if you have 240 volts between L1/L2 or L2/L3 or L3/L1 then you will have 120 volts between neutral and any of the L terminals.

The L terminals are all 120 degrees out of phase with each other.

MaxHeadRoom
08-23-2010, 05:05 PM
This is for somebody with more electrical experience than I've had. I hail originally from the UK, where 240 volts AC single phase is standard for dwellings at least.



The difference is, or at least was that the UK uses one 240v phase to a grounded star point of a 3 phase transformer, every so many residences were on the same phase, used to be every three.
So your neighbour may be on a separate phase.
Max.

J. R. Williams
08-23-2010, 05:19 PM
Gary
In most industrial power areas where 480 volts are used the power panel does not use a neutral unless it is designed for a Y connection and the you have 277 volts between a hot leg and the neutral (center of the Y connection). The 277 power is used mainly for lighting circuits. Where 120 volt single phase service is required a transformer is used to obtain the 120/240 volt service.

JRW

Paul Alciatore
08-23-2010, 07:23 PM
Your observation about the breakers is incorrect. A double pole breaker as used for 240VAC connects to two tabs in the panel that are the opposite 120VAC legs. You may have been looking at a two pole half width breaker. They do connect to only one leg and cannot be used for 240V. The panel tabs are arranged so that each leg connects to alternate tabs.

There are also some half width breakers for 240. They take up two standard width slots and connect to two adjacent fingers on the bussbars. They have four separate, half width breakers and the inner two provide one 240 circuit while the outer two provide another. The inner two are strapped together and the outer two are strapped together.

Lew Hartswick
08-23-2010, 08:23 PM
In home use if you have 240 volts between L1/L2 or L2/L3 or L3/L1 then you will have 120 volts between neutral and any of the L terminals.
Except it's 208 phase to phase for that type service. (when it's 120
phase to neutral).
...lew...

gmatov
08-24-2010, 12:34 AM
Gary,

Are you sure of that? A 3 ph line is 120 volts per leg 120 degrees out of phase, all three legs are 120 volts. I don't think there IS a Neutral.

220 AC has 3 wires, 2 of which are 110, and the third neutral, plus a possible green for ground. Neutral is not required to get 220, but highly recommended, and possibly required by NEMA .

"In home use if you have 240 volts between L1/L2 or L2/L3 or L3/L1 then you will have 120 volts between neutral and any of the L terminals.

The L terminals are all 120 degrees out of phase with each other."

I have to question this, too. You have too many "L" terminals for 240 V home power. You have L1, L2, N, and Ground. L1 and L2 are 180 out of phase. That is why you have 240 V. If they were IN phase, you would have 120.. You are describing 3 phase.
Sorry.

George

whitis
08-24-2010, 06:30 AM
There are effectively 1,2,3,6, 9 or 12 phases in the US. Picture each of the three phase windings center tapped for 120/240V service with all the center taps connected to neutral (this might not all be done in one transformer, but the effect is the same):



120 60
L2+ L3-
\ /
\ /
\ /
\ /
180 L1- -----N----- L1+ 0
/ \
/ \
/ \
/ \
L3+ L2-
240 300



There is no real + and - here other than that the legs with the same name but opposite signs have the opposite voltage at any given instant.
L1+ to N gives 120V and L1- to N gives 120V of the opposite polarity (or effectively 180 degrees out of phase). Connect across L1+ to L1- and you see 240V. Same goes for L2 and L3.

Connect between L1+ and L2+ or L2+ and L3+ or L3+ and L1+ and you see 208V. Weirder still, the difference between those isn't in phase with any of the six phases shown above but is shifted 30 degrees (or half way between two of those phases). If you were to do the same thing with the negative phases of those (for balance), you get three more new phases for a total of 12. Basically, the delta (triangle) connected stuff ends up being shifted 30 degrees from the Y connected stuff. Frequently, for heavy duty three phase stuff you would have another transformer, which doesn't need the center tap or balancing the + and -, but if you are mixing usages you might. So, 3 of the 12 phases might be infrequently used. For residential neighborhoods, we run a whole neighborhood off a single phase and its opposite phase. Which means it is hard to get three phase power for your shop if you are in a neighborhood which was wired in a residential frame of mind even it if has gone commercial. Even in some office areas it might be hard to get 3 phase.

Rather than one transfomer generating 6 phases as shown above, you may have 1 transformer producing 3 phases and each phase run to a neighborhood where another center tapped transformer splits each phase into two opposite phases.

Although there are 12 phases most equipment/circuits/loads only see 1, 2, or 3 of them. There are some exceptions. A lathe might have 208V motor for the spindle and 120V for coolant pumps or control circuitry so it might see 4 phases. A big dimmer bank for stage lights might see 6, since the lights run on 120V - and it actually cares about the phases though since the dimming repeats every half cycle it can use the same timing for the opposite phases.

Resistive loads draw current in phase with the voltage they are supplied but inductive and capcitive loads draw current out of phase.

In a typical house, the 120V loads are split between the two opposite phases which helps balance the load. This imbalance only goes back as far as the pole transformer, though.

For fun, run the following command in gnuplot:
plot sin(x),sin(x+2*pi/3), sin(x+2*pi/3*2), -sin(x), -sin(x+2*pi/3), -sin(x+2*pi/3*2), 2*sin(x),2*sin(x+2*pi/3), 2*sin(x+2*pi/3*2), sin(x)-sin(x+2*pi/3), sin(x+2*pi/3)-sin(x+2*pi/3*2), sin(x+2*pi/3*2)-sin(x)

J Tiers
08-24-2010, 09:47 AM
There are lots of different services in the US. Most domestic services are one of three. Light industrial get the same choices, usually.

1) Standard 240/120.

This is a single phase service, with a pole transformer that is supplied one phase of something on the order of 2400V or 4160V by the power company. This is stepped down so that supplied power is 120V either side of the grounded neutral

2) 3phase with 120V (240V 3 phase)

Same as above, except that a third phase wire is added, usually with a single transformer (open delta). This gives 3 phase 240 power AND 120V

3) 208V 3 phase.

This is 3 phase "star", with 208V between phase wires, and 120V to neutral.


Usually, a detached stand-alone house will get service type "1". Farms may get service type 2.

Type 3 is usually limited to apartment buildings, (and light industrial) where the three phases are split up with 2 going to each apartment, typically, and 208V replaces 240V as the stove and heater voltage. Otherwise it is pretty much the same as type 1 for the user. At my work we have 208V (and wish we had 480).

Heavier industrial may get 480V 3 phase, which is 277V to neutral.

Big factories and so forth can get anything,.,, they may have "medium voltage" motors etc (anything over 600VAC), and voltage is supplied based on what makes sense for the power levels.

Richard-TX
08-24-2010, 10:31 AM
... effectively 180 degrees out of phase

It isn't effectively 180 degrees out of phase, it is 180 degrees out of phase.

When explaining the 230 power system used in the USA to someone from another country like England or Australia, it is usually helpful to describe it as a two phase, 180 degree, 230 volt system. That will usually remove all ambiguity.

Barrington
08-24-2010, 11:01 AM
When explaining the 230 power system used in the USA to someone from another country like England or Australia, it is usually helpful to describe it as a two phase, 180 degree, 230 volt system. That will usually remove all ambiguity.As someone from England I feel I must object :p

I think most here would describe it as a 'split phase' system.

A 'two phase' system would involve angles other than simple inversion.

Cheers :)

.

J Tiers
08-25-2010, 12:15 AM
With "systeme deux" per my description it is "two phase"......literally..... it uses two of the three phase wires.........

I would, however, myself describe it as "240V single phase with center-tapped neutral", which is probably the stateside equivalent to "split phase.

JRouche
08-25-2010, 12:47 AM
As someone from England I feel I must object :p

I think most here would describe it as a 'split phase' system.

A 'two phase' system would involve angles other than simple inversion.

Cheers :)

.

But YES!! JR

JRouche
08-25-2010, 12:49 AM
With "systeme deux" per my description it is "two phase"......literally..... it uses two of the three phase wires.........

I would, however, myself describe it as "240V single phase with center-tapped neutral", which is probably the stateside equivalent to "split phase.


And YES again... Amazing how many folks, interpret US power systems. JR

Barrington
08-25-2010, 05:55 AM
There's plenty of scope for international misunderstanding..

In another thread about light switching I became aware that the US terminology was different...

In the UK a light controlled by a single switch is '1-way switching', by two switches is '2-way', by three switches '3-way' etc.

Why is a US light controlled by two switches '3-way' :confused:

Cheers :)
.

J Tiers
08-25-2010, 09:54 AM
Why is a US light controlled by two switches '3-way' :confused:

Cheers :)
.

Why are there "hogsheads", "piggins", "drams", "minims" "firkins", and "pottles" in liquid measure in old UK usage?

Same reason

gnm109
08-25-2010, 11:07 AM
Why are there "hogsheads", "piggins", "drams", "minims" "firkins", and "pottles" in liquid measure in old UK usage?

Same reason


I like the British terms. My old BSA Gold Star Owner's Manual had some: paraffin (kerosene), gudgeon pin (wrist pin(), spanner (wrench), purchase (leverage) and dope (racing fuel).

MaxHeadRoom
08-25-2010, 02:07 PM
In the UK a light controlled by a single switch is '1-way switching', by two switches is '2-way', by three switches '3-way' etc.

Why is a US light controlled by two switches '3-way' :confused:

Cheers :)
.

When I grew up in the UK any more than 2 were 'intermediate'?
The only older English word I know still used in N.A. is 'Fall', UK switched to the newer term 'Autumn'
Max.

gnm109
08-25-2010, 03:17 PM
When I grew up in the UK any more than 2 were 'intermediate'?
The only older English word I know still used in N.A. is 'Fall', UK switched to the newer term 'Autumn'
Max.


I just remember another one from the BSA manual...."Oily Waste"....(rags).

Barrington
08-25-2010, 03:34 PM
Not saying anything's wrong here, just trying to learn a foreign language ! :D

I think my confusion is whether a US lighting circuit controlled by two switches is referred to as '3-way switching' or does it just use '3-way switches' ?

Having now looked at a few US sites it seems to me that a '3-way' switch gets it's name from having three connections, and a '4-way' switch from it having four connections - yes ?

However unless I'm much mistaken, to any electronics or electrical engineer (whether US or UK), one is a '1-pole 2-way' switch and the other a '2-pole 2-way' switch, because switch 'ways' always refer to the number of mechanical positions which provide distinct electrical connections...

In the UK, the US '4-way' switch would be called an 'intermediate' by a electrician when used as such, or properly just a '2-pole 2-way' when used for another purpose.


Cheers

.

Lew Hartswick
08-25-2010, 07:16 PM
Some of this terminalogy is specific to the "power wiring" trade. In
the electronics world switches are refered to as SPST single pole single
throw. (On - Off) then SPDT double throw (like the socalled 3 way)
then the DP double pole in either ST or DT then for more poles just
use 3 for three poles etc. The double throw ones can also have an
OFF in the middle so that is another "option" .
Is that adequately confusing? :-)
...lew...

gmatov
08-25-2010, 10:53 PM
1 way switches are for controlling a light from one location.

3 way switches are for controlling that light from 2 locations, say entering and traversing a room and turning out the light as you exit. You can reenter the room and turn it on, or go a circuitous route and turn it on again from the switch you threw as you entered before.

The next is 4 way switches. Those are intermediate switches, any number you wish, but they must be between the 3 ways. I only have 2 of such circuits in my home. Foyer is entry, stairway, living room, kitchen. 2 3 way, 2 4 way between them

Kitchen is switched from the entry from foyer, from garage, from dining room.

4 ways are relatively expensive, both from the standpoint of the switch cost, though you probably will not use many, and the additional wiring, and the Electrician's charge. I say expensive because I built my own home and wired it on a shoestring. Money was tight, and paying about 8 bucks for those switches, 35 years ago, took money from other things I had to do, when 1 ways were about 39 cents and 3 ways maybe 79 cents each.

But I would not do without them, today.

Correction in my reply to Gary350:

"A 3 phase generator has 3 output wires, L1, L2, L3. The center wire is called neutral. The only time you use the neutral wire is when you one of the other wires to get singla phase at a lower voltage."

I took the "center wire" to be L2, when you are clearly stating that it is a fourth, neutral wire, plus a green ground. Or so I take it.

Cheers,

George

Jim Hubbell
08-26-2010, 01:14 AM
I have a hard time understanding how there may be more than one phase obtained from the secondary of a stepdown transformer that is supplied with single phase at its primary.
I hope someone will explain this.

J Tiers
08-26-2010, 01:29 AM
I have a hard time understanding how there may be more than one phase obtained from the secondary of a stepdown transformer that is supplied with single phase at its primary.
I hope someone will explain this.

They can't, in the usual way... because it doesn't happen without special shenanigans.

It's poor terminology, really, needlessly academic. Even if definitions allow a 180 degree opposite "phase" to be termed part of a "multiphase" system, it is misleading and un-useful to do so except for certain purposes....

For "all intents and purposes", a "180 out phase" is merely a continuation of the first phase, and is best described as such.

Arcane
08-26-2010, 02:28 AM
Time for a vid! :D

http://www.youtube.com/watch?v=9S64r-MjBN4

Turn your speakers way up!

Jim Hubbell
08-27-2010, 02:08 AM
Thank you J Tiers, I see it the same as you described it. Maybe the use of the word ' phase ' has morphed into simply a ' hot pair ' or some other meaning.
I have watched terminology in many trades go flat.

J Tiers
08-27-2010, 09:32 AM
Thank you J Tiers, I see it the same as you described it. Maybe the use of the word ' phase ' has morphed into simply a ' hot pair ' or some other meaning.
I have watched terminology in many trades go flat.

"Phase" in electric usage can mean a lot of things when used loosely. The top ones usually meant are

One wire of a set of "multi-phase" power wires , typically when considering the set as "wye".

One hot pair, when considering 3 phase as "delta".

The relative "phase angle" of one wire or pair vs others or a reference.

RPM
08-27-2010, 02:12 PM
Wow, I'm glad you explained about the US version of 2-3-4 way switching. As somebody said earlier, it's really based on the number of terminals on the switches, not the number of switches in the circuit.
What surprises me is that I've managed to wire-up correctly a 2 switch system and a three switch system ( there's another change of definitions!) without knowing the correct US terms - I must have been lucky - or perhaps I checked out the back of the switches beforehand :-)
Thanks again for all the professional advice

Richard in Los Angeles

MaxHeadRoom
08-27-2010, 02:59 PM
The principle is exactly the same, only the names have been changed to confuse the innocent. ;)
Max.

gnm109
08-27-2010, 05:14 PM
I wired up two three-way circuits for two strairways that we have. I got the schematic out of the yearly electrical code book and did it myself. Still working many years later.

gmatov
08-28-2010, 02:06 AM
RPM,
Not to denigrate, but go to different houses to look at switches. Suppliers, I mean, or look at older vs newer switches. They are not and were not the same as today. They are usually "cut and try".

I have wired new switches into old circuits, only one of the 2 switches is bad. Only 3 wires to connect. The internal bridging is different. Red/white/black do not wire to the same terminals on an old and new switch.

You didn't check out all the switches before hand, and you weren't lucky. You simply bought 2 or more of the same brand and same age switches and did as the back of the box told you to. A genius, you ain't. You got a problem with that, come back and prove how smart you are.

Smart asses, I don't like, and I think you are one. You wired a SWITCH? BFD,.

Cheers,

George

gnm109
08-28-2010, 11:25 AM
In the words of the imortal Rodney King, the professional victim and convicted felon, after he had recovered and discovered that he might have a good cause of action against the City and County of Los Angeles on May 1, 1992:

"People, I just want to say, you know, can we all get along? Can we get along? Can we stop making it, making it horrible for the older people and the kids?...It’s just not right. It’s not right. It’s not, it’s not going to change anything. We’ll, we’ll get our justice....Please, we can get along here. We all can get along. I mean, we’re all stuck here for a while. Let’s try to work it out. Let’s try to beat it. Let’s try to beat it. Let’s try to work it out." :)

http://en.wikipedia.org/wiki/Rodney_King