Can someone please explain the US 240v system...1 or 2 phases.

elkangorito

As the topic heading suggests, I'd like to know about the US single and/or two phase 240v system.

__________________
Switchboard design engineer & Licensed Electrician (Australia).

J. V.

In the US we do not call anything two phase unless we are only using two conductors derived from a 3 phase system. It's either single or three phase.

120 volts is measured from the single phase conductor to the neutral. The neutral usually being white in color. Neutrals and grounds are bonded in the MAIN service panel. Never in a sub panel (another panel being fed from the main service panel).
Single phase also has a ground wire (ECG) to be NEC approved. (Older buildings and houses that are not under major restoration or construction) are exempt from the ground reguirement. *

240 volt systems can be three wire or four wire. Medium voltage feeds our utility transformers and they usually output 240 volts single phase to dwellings. 3 wire.

3 wire consist's of two hot conductors with a ground/bond. ( earth bond)
4 Wire consist's of two hot conductors, one neutral and one ground. (EGC)*
Each hot leg is measured across each other = 240 volts.
Each hot leg measured to neutral = 120 volts
Each hot leg measured to ground should not be considered a true reading but will be around 120 volts.

*
(EGC) equipment grounding conductor.
*
Dwellings that only have two wire branch circuit systems are exempt unless major work is to be performed on the premises. Where a permit might be required.
The NEC does not force homeowners to tear up walls ect... to install a grounding conductor.

I hope this helps. It is very generic and others here, I promise will chime in.

Stubbie

Hi Kangaroo

For residential dwellings

First we do not use a 2 phase system, that proved ineffective years ago or maybe it was politics.... What we use in residential is a transformer with a secondary of 240 volts single phase centertapped step down type. Primary voltage is generally around 2.4 KV to 7.2 KV from a 3 phase source. This allows us to serve both 120 volt loads and 240 volt loads. So our systems for single family dwellings are actually 120/240 volt single phase 3 wire sometimes called the edison system.

This is a common pole mount with diagram graphic yours vs. ours





What actually happens is we split off 2 single phase voltage legs at the top and bottom of the secondary winding for +120 volts and -120 volts with a center tap so total potential between the top and bottom with respect to each other is 240 volts. V1 and V2 with respect to the neutral is + or - 120 volts the resulting wave form (both legs 180 degrees offset from one another) allows cancellation of current in the neutral given perfectly balanced loads otherwise it carries the difference between legs. If you look at the image there are 3 wires.. left and right are the hots V1 and V2 with respect to the graphic. The center is the midpoint tap (neutral) also notice the casing bond then the center tap is earthed with a grounding wire running down the side of the pole to a ground rod.

In a balanced load situation driving two 120 volt loads between V1 and V2 would be like this...using a double pole breaker or two single poles connected to opposite legs at the panel. Notice also that in this configuration we can drive both 120 and 240 volts loads on the same circuit. You can substitute smaller values for amperages if you like.


and 240 volts not using the neutral


Notice the 180 offset but it is not 2 phase it is a split single phase source. So essentially each leg returns on the other to the transformer as the waveform cycles between polarities.


Stubbie

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" One nice thing about the NEC articles ... you have lots of choices"

Stubbie

Stubbie

Here is some good reading showing both 120/240 single phase systems wye (4 wire) and delta (3 wire) plus a lot about the panels we use and hardware. Might want to snuggle up to a tall beer before reading this one.....

This is one international manufacturer we use several different domestic companies in the USA also.

http://www.sea.siemens.com/step/pdfs/load_3.pdf

Another graphic

http://www.mikeholt.com/onlinetraini...99596162_2.jpg

Stubbie

__________________
" One nice thing about the NEC articles ... you have lots of choices"

Stubbie

goose134

Good work stubbie!

joed

Just to clarify the situtation. While it is only single phase service most people still improperly call the too hot legs "phases". At least most home owners and non electricians do.

Stubbie

Yep... actually even many electricians refer to it as 180 out of phase or lost both phases etc...I liken it to calling the white wire a neutral in a two wire 120 volt circuit. We all know what it really means... lol

This kinda shows what is happening with this shift idea......the 180 degree shift is the waveform we have on our 240 volt circuits....If the dotted line is the grounded conductor you can see why we get cancellation of balanced loads in the neutral wire given above the neutral is positive value and below is negative value & of course you have to visualize this whole thing rotating. I think Joed is the only one that can visualize it ...it gives me a freakin headache....!!

__________________
" One nice thing about the NEC articles ... you have lots of choices"

Stubbie

frenchelectrican

i can visual it very clear i do have the " scope " meter to read the power wavelengths as well the phase shift 180° is very common with single phase but when you run the single phase off from three phase network all bets is off you will follow the wavefourm from the top two like 90° shifted but basically in three phase system it actally useally shifted 30° or 60 °typically

i know it will confuse some peoples in here but bear with us this kind of stuff normally found in commercal / industrial area where we have 3 Ø supply is common.

Merci, Marc

elkangorito

Thanks guys for your effort & the great explanations .

I am aware of the split-phase tranny but wasn't sure if you actually used 2 phases or not. It appears as though many 'would be' Australian electricians still insist that you folks use 2 phases when quite clearly, you do not.

Cheers.

__________________
Switchboard design engineer & Licensed Electrician (Australia).