A question about electricity?

[Adwarrior]

I'm from the UK where everything is 240 and comes down one wire plus a common and a ground.

Here in North America 240 is supplied by 2 wires. I understand this is so, but I don't understand how that works. As if I take 2 wires carrying 120 volts and connect them together they are still carrying 120 volts, correct?.

So, do items that run on 240v in North America have 2 separate connections one for each 120v feed internally or some such?

Can someone clarify?

Thanks in advance

 

[Phillthy]

yes and yes. the two together give you 240.

 

[rives]

The two wires carrying 120v each are "out of phase" with each other. Alternating current forms a sine wave, and it will make a full cycle from positive thru negative 60 times per second. So, when one wire measuring 120v is positive, the other 120v line is negative, and the difference between them gives you 240v. Since the neutral is at the mid-point of the sine wave, you get 120v when measuring from either line to the neutral.

This is a very simplified version of what is taking place, it can get indecipherable to the lay person pretty quickly.

 

[Adwarrior]

Rives, thanks for the reply.

I think I understand the fundamental: that one is 180 degrees out of phase with the other, hence "alternating".

This question arose, as the relay for my flip arrived and as per my other post (that you kindly contributed to) I only ordered the one DPDT 30 amp relay, as I'm only drawing around 18 amps.
So I'm looking at it and I understand how to wire it and what the circuit should look like. However, I'm not entirely sure I can/should combine both 120v supplies. I thought about joining the 2 120v supplies in a junction box and then feed them into the relay. Or alternatively I would fit a 20amp receptacle on the ballast side of the relay and leave the tabs intact and plug each ballast into said receptacle and feed both in that way. Though I would have hot male plugs from the ballasts, which isn't ideal. A receptacle with recessed male plugs like a PC would be better and safer.

Can I do what I'm proposing?

Thanks again

 

[rives]

I'm afraid that you have lost me. You cannot "combine" the two legs of 120v - this will create a direct short. They need to be switched independently and simultaneously (thus the DPDT contacts), and then fed to the ballast line-side wiring. They could go through an intermediate termination at a 240v receptacle, and a double receptacle could be used if the jumper tabs were removed. If the tabs remain in place, both ballasts will fire up when either set of wires is energized, negating the flip-flop's actions.

You should never use a male plug on the load side of any device- a female plug is used there to keep from coming in contact with the energized plug. Standard receptacle/plug combinations aren't suitable for the load side of a ballast - there is a very high voltage spike during the ignition cycle that is far beyond the design limits of the connection.

 

[Adwarrior]

OK, Let me put it this way then.

I want to connect 2 ballasts to my relay. My relay has a pair of contacts for the timer, 2 pair of contacts for the output and only one pair of contacts for the input. How do I connect 2 ballasts to the one pair of input contacts?

 

[rives]

I think that you have talked about switching both ballasts and lamps in the other thread, so I'm not sure where we are at this point.

If you want to select different ballasts with the relay, you will feed the line side of each ballast with it. Connect one hot leg of your 240v circuit to each "common" of the DPDT contacts - this will allow power to flow to the Normally Closed (NC) contact terminals when the relay is unpowered. One ballast will be connected to both of these NC contacts. The other ballast will connect to both of the Normally Open (NO) contacts, and that ballast will be energized when the relay is powered.

If you want to flip between two lamps downstream from each ballast (4 lamps, 2 ballasts total), then you will need to have another relay. Both wires coming from the ballast, going to each lamp, need to be switched. This will require (1) DPDT relay per ballast, which will in turn select which lamp is driven at any given time by that ballast.

 

[Adwarrior]

OK thanks, that's cleared that up, I definitely will need another relay then. I explained to the tech support when ordering what I wanted to do (obviously not in too much detail) and he assured me the one would work. Now I have to sit around and wait for another one to turn up. Thanks anyway Rives.

Yes it was discussed in my other thread, but it kind of got side tracked by the hot switching/cold switching benefits and got a bit off track. Though, Growshopfrank did say it could be done, but that there may be harmonic distortion (whatever that is).

Something I'm still not completely clear on is where is Phase determined? Is it by the device requiring the 240v? As I can't see it determined at the generating source as all the breakers are connected to the same power bar inside the panel. It's really bugging me.

 

[rives]

The phasing is determined at the generation stage. You should have 3 wires coming into your panel - (2) 120v hot legs and (1) neutral. Inside your panel, there should be two busses side-by-side, with attachments for the breakers to stab into. The stabs will alternate from one phase to the other so that a double-pole breaker will be fed by each phase when it is installed.

 

[vukman]

umm, you are using a double pole breaker, correct??? I know this might be a stupid question but a lot of people when wiring up a 220/240 device do not understand that you have to have a double pole breaker or in essence, 2 breakers...one for each leg of the 120....

Please forgive me if I have intruded and also I am in no way insinuating that you do not know what you are doing but believe me when I tell you, I've seen all kinds of things done inside of an electrical panel as I'm sure rives has as well...

Good Luck

 

[Adwarrior]

The phasing is determined at the generation stage. You should have 3 wires coming into your panel - (2) 120v hot legs and (1) neutral. Inside your panel, there should be two busses side-by-side, with attachments for the breakers to stab into. The stabs will alternate from one phase to the other so that a double-pole breaker will be fed by each phase when it is installed.

That makes perfect sense. Thanks for clearing that up.

Adam

 

[Adwarrior]

umm, you are using a double pole breaker, correct??? I know this might be a stupid question but a lot of people when wiring up a 220/240 device do not understand that you have to have a double pole breaker or in essence, 2 breakers...one for each leg of the 120....

Please forgive me if I have intruded and also I am in no way insinuating that you do not know what you are doing but believe me when I tell you, I've seen all kinds of things done inside of an electrical panel as I'm sure rives has as well...

Good Luck

I'm not actually running 240, I'm running two separate 120 circuits down a piece of 12/3. The supply was old was old and only 100amp and the panel was very untidy. So, I rewired, had a new panel and 300 amp supply 2 years ago and have plenty of spare capacity. I'll be using a double breaker regardless, as it's a cheaper option.

Cheers