US 120VAC/240VAC question

[mramos1]

I am not an electrician and this is not 100% electronic, so I figure I would ask here. Any electrical types out there. This is for 240vac in the US.

I have an Intermatic ET103C that I fixed. I want to know if I can run my
hot water heater with it (240vac 30amp 2-30 amp breakers).

The timer has a 120vac pair for the clock and relay coils, and two SPST 30amp 230vac relays.

It clearly has 2 relays and they are not connected in any way.

manual (4 pages)
**broken link removed**

 

[Hero999]

I'm not domestically electrically biased and I'm not that familiar with US power.

That looks like it might be alright but isn't it safer to switch the hot rather than the neutral?

 

[mramos1]

I will break the two hot lines and drop across the relay contacts. I sort of screwed the drawing up. will kill the drawing. Glad you caught that.

I guess the big question is, is one leg 120vac. I think it is, will get a meter and check, if to so the clock side with one leg, and then break the hot wires and put them across the relay contact.

Thanks

 

[CheapSlider]

USA uses a split phase supply to domestic premises. This is:
Line1 (Hot -120V) and Neutral and Line2 (Hot +120V).

The heater should be connected to the Line1 and Line2 wires, which will give 240V (120 + 120) You will be switching 2 hots (thats why there are the 2 sets of contacts in the time switch).
The timer should be connected to the Hot and Neutral (120V supply)

Edit
I found a wiring **broken link removed** for ET103C also the **broken link removed**.

 

[mramos1]

Thanks CheapSlider, that is what I thought but wanted to make sure before I hooked it up.. And thanks Hero999 for your switching hot, I caught that after you mentioned it.

 

[Sceadwian]

I had a problem with that a year or two ago I made an ass out of myself at work saying something was only 120 volt because I checked the hot against ground, at the time I didn't know anything about polyphase power and I didn't check the neutral and thought the third line was a local ground Not thinking anything of the fact that all three lines were going through 10amp fuses. The electrician set me straight and I really felt 'owned' about my lack of knowledge. I can tell you very seriously I try to check my facts before I speak now! Though I'm still well known for making at ass out of myself by speaking too fast.

 

[mramos1]

Sceadwian I hear that.. I do the same. Figure 3.3, 5, 12 volts. Hard to make a big fire with the parts I use. But 240vac, I figured I better ask.

Glad I did, I had it right on how it works, but screwed up the original drawing (that I would have wired from). I did not see it until Hero made a comment on switching the hot wire, looked at if and went oh crap.

 

[CheapSlider]

Polyphase supplies.
The (USA) domestic supply is "3-wire single-phase" it is NOT polyphase (which is used to supply over 100kVA). Polyphase usually means 3 phase.

A 3 phase supply would give 120V from Hot(phase) to Neutral and 208V between phases (-this is why you can find aplliances for 208V and 240V).

A "delta 4-wire system" will give 120V; 240V (single phase) and 240V (3 phase)

 

[Sceadwian]

There is no 'neutral' in any three phase device we have at our plant. Most don't even have ground connections, just the three phases. Any connection has a 120volt refrence in relation to ground and 220 in between phases. For 220 they use any two of the phases. This is where I have trouble understanding modern electrical systems in the U.S. as 208V single phase could be either two phases of a three phase 120V system or a single 220V monophase line with a single ground leg. I have seen enough screwed up electrical systems to know that elecrticians around here don't diffierentiate much between the various voltage types sufficiently enough to know the proper voltage avaiable in relation to ground or any other phase in a local system.

 

[mramos1]

At me office, we have 208, 240 and 480. So I guess we have it all. The 208 and the 240 all seem to have a white (neutral) and a red (208 anyway). And we have large water pumps I guess that are delta, I think it was 4 wires and I saw it had 480volts on the panel.

 

[Hero999]

Sceadwian,
I'd be reasonably sure that your system is grounded for safety reasons, otherwise the phase could float at whatever voltage it liked. Just because there isn't a neutral or earth wire it doesn't mean it isn't mean it isn't earthed, even if it's via a high impedance path and a protection system.

A 120-0-120V system isn't a two phase system, the correct term is a split phase system.

We use 110V in the UK for some portable industiral tools, it's normally supplied by a 55-0-55V transformer with the centre tap earthed so the maximum phase to earth voltage is 55V. Aslo 55V isn't normally used but 24VAC is very common in control panels.

 

[CheapSlider]

Electricity is generated and distributed as 3 phase.
There are 2 ways to connect a load to a 3 phase supply:
"Delta" or "wye"

Delta connections are between the phase line wires (no neutral, no earth)
wye connects between a neutral and each line wire. The neutral can be connected to earth at the transformer*.

The supply transformer's 240V output coil is centre tapped to give a 120(L1)-Neutral-120(L2) supply which is known as "3-wire single-phase".

A polyphase supply brings in a supply from more than 1 transformer coil. This can be 2 or 3 phases. The voltage between the Line wires of the different phases will be be 208V (120 x 1.732#). Such a supply may be labelled 120Y/208. There could be any combination of 120 and 208 and 240 Volts available. Higher power devices are available in 208 and 240 V versions in the USA


For more heavyweight (industrial) users the supply transformer output coils can be providing 240, 480, 600 volts (120, 208 and 277 volt supplies can be obtained from these).

Large motors will be 3 phase and are usually delta connected.

There is no NEC requirement for colour coding*# so it's not easy to know what sort of supply is being dealt with. Black is often used for 120V hot, and red for 208V, but this is not universal. (No wonder even electricians can get confused)

Earth and Neutral
Most devices will be earthed. This is not the same as connecting neutral to earth.


* How and where the neutral is connected to earth varies according to differing local codes and practices.

# Phases in a 3 phase system are seperated by 360/3 = 120 degrees. If you plot graphs of 3 sine waves of ampltitude 1, spaced at 120 degrees, the ampliutude between them is 1.732. (√3)

*# NEC requires: Ground = Green or Green/Yellow; Neutral=White; High Leg "must be durably marked"

 

[Sceadwian]

As best as I can gather 208 is two phases of a three phase supply as that's the RMS voltage I get when I plug in the 120 degree phase shift into ltspice. I get 240 volts RMS when I set it up as you call it 'split phase' where it's two AC sine waves 180 degrees out of phase.
It's still two phase though, the only difference is the degree of phase shift.
On a proper 240 volt line how is the 180 degree phase shift made since it's 120 between phases at the pole, or am I missing something here? This is something I would really like to understand a little better since entire countries run on these kinds of systems.

 

[Hero999]

A 180 degree phase shif is made using a centre tapped transformer.

Phase shifting transformers are also used to convert a 3 phase system to a 6 or 12 phase system to eliminate harmonics in large rectifiers.

CheapSlider,
I the UK we call "wye" "star".

You seen to know a lot about the US electrical system, have you worked in the states before?

I've never been there before, I've picked up all I know from the Internet.

 

[Sceadwian]

I forget how versatile tapping a transformer at the right winding can be.

 

[mramos1]

CheapSlider taught me a lot and I live in the US.

 

[CheapSlider]

Sceadwian
You can determine the type of supply by measuring the AC volts across L1 and L2. If this is 240V then you have a single phase supply. This is the voltage across the output coil of the pole (supply) transformer. The coil is centre tapped and the neutral is taken from this centre tap giving you the 2 supplies of 120V. (-120 and +120 with respect to centre tap neutral)
The "3-wire single-phase" system is unique to North America*

If you you measure 208V between L1 and L2 then you have a polyphase supply. Large apartment buildings may be supplied polyphase.



Hero999
You are quite right about the centre tapped output transformer and the star connection
I used the USA Wye terminology to try not to complicate a subject that is already complicated enough.

The wye or star connection is also known as a 'T' configuration and the Delta as a 'Pi'.
The Y star / Delta terminology is used by power oriented engineers and the 'T' / 'Pi' is used by signals engineers. One uses high powers with narrow frequency ranges while the other uses low powers and wide frequency ranges. The equations and maths are the same.

I first encountered 'bi-phase' when I was helping a mate sort out the electrics in his new home in Missouri. (He was originally from near Croydon, he married a girl from Missouri - she didn't like Croydon).
I gained further knowledge of US practices from my job in Telecomms when we were buying a lot of equipment from the USA which came supplied to work on US power.


*Electricity supplies
The 110v is supposed to have originated from Edison. When AC took over from Edison's DC, the (unique to N America) 3 wire system was introduced so that existing 110V lights could continue to be used without modification.

Mains frequencies - With AC there is the question of what frequency. Lower frequencies are good for traction and large motors, higher frequecies allow smaller transformers. 25Hz is used by NYC subway (thats why the lights flicker), 16.67 Hz is used for traction by some european railways. Niagra originally supplied 25Hz before changing to 60Hz.
50Hz is used in Europe. (Is it because it fits in with the metric/decimal 1,2,5 10,20,50 sequence?). Japan uses both 50Hz and 60Hz at 100V

Nikola Tesla was supposed to have recommended 200V at 60Hz which is where the usa 240V comes from)


Sorry about these long posts (- maybe this needs another thread ?)

 

[mramos1]

I will impress them all tomorrow at work. My project was a timer on the hot water heater at home this weekend.

But the company I work for owns a number of buildings. And it is 120 on the office outlets. 208, 240, 277, 480 and 600 in all the none office places. Most of the little things are 277.

Thanks for the great tutorial.

 

[Sceadwian]

Okay, so 120 is single phase with neutral being ground.
208 is two 120 volt phases with 120 degrees of seperation.
240 is two 120 volt phases with 180 degrees of seperation.
What's 277?
I'm asuming 480 is two 240 volt phases with 180 degrees of seperation.
600, no idea..

Do I have things relativly straight here?

 

[gerty]

277 is one phase of 480 to the neutral, primarily used for lighting in industrial settings. 480/1.73 =277