3-wire and 4-wire question

[iceno9]

So i see there are tubs that are 3 wire systems (Live, Neutral and Earth) and 4-wire systems (Live, another Live, Neutral and earth).

My question....Why? What for?
I can't imagine its for load sharing between the 2 'hot' wires because the return current still all comes back via the neutral wire which must be the same wire guage. Likewise, the earth wire must be substancial enough for the full fault current.....so i don't get the 4-wire systems. Can someone explain?

Am i right in assuming ALL tubs in the UK will be 3-wire because we are at 230V?

Regards
Matt

[Dr. Spa™ Ret.]

UK electric is VERY different than in the USA. I believe in the UK, there are only 3 wire systems (230v hot, neutral and ground/earth[in the US the "Live" wires are 120v, and it requires using two live wires to achieve 240v]).

[Pers Onal]

....so i don't get the 4-wire systems. Can someone explain?
Regards
Matt

Dunno bout the UK.

I can tell you a bit about the US, though.

The fourth wire is a ground and is not insulated.

In the US, when I was growing up, all consumer/residential 230 volt was three wire. Two hot 115 legs and a neutral. Electric dryer and cooking range plugs had three prongs. At the time, most outlets had two prongs and no third ground prong. Adapter plugs to adapt a three prong plug to a two prong outlet were common. It's been so long since I have seen one of those adapters that I couldn't tell you when I saw the last one.

In recent decades on 230 ( now commonly called 240), four wire has become the norm. Two hot legs of 115, a neutral (insulated) and a ground (not insulated)

I know, I know, the neutral and the ground go back to the same ground at the panel. But, if you think about it a standard 115 volt circuit does the same. Has a hot leg of 115, a neutral (insulated) and a ground (not insulated).

All that when all I really had to say is that the 4th wire is a ground and often not insulated.

[Isaac-1]

Ok, let me try to explain things a different way, the U.S. and U.K. electrical systems are very different.  In the U.S. typical home electrical power is supplied as what is commonly called 120/240 single phase, technically though it is really split single phase.  Meaning there are 2 hot wires and a neutral (ignoring the safety ground for now), The 2 hot line wires are referred to as L1, and L2, and the neutral is L0.  U.S. breaker boxes contain a combination of single leg 120V breakers and double leg 240V breakers.  The vast majority of 120V breakers are either 15 or 20 amp sizes and run to light fixtures or common outlets, due to code deratings this means that the maximum draw on on of these outlets is between about 1,500 and 2,000 watts.  So 240V breakers are usually used when one needs to draw more than about 2,000 watts (1,500 watts traditionally), such as on electric ranges, water heaters, and clothes dryers, or in this case hot tubs, as using 240V allows to the use of much smaller wires than would be required to run these higher watt draws at 120V.  Many smaller hot tubs like my 3 person Hot Spring Jetsetter can be connected either at 120V or 240V,  in 120V 20 amp mode it can only run the heater or the jet pump, but not both at once, and the heater operates at 1,500 watts.  In 240V 50 amp mode the pump still runs on 120V, but the heater is wired 240V and draws 5,000 watts, and both can run at the same time.  In realtity this is an unbalanced load where there is still about 20 amps available not being used on one of the 120V legs.

My understanding is in the UK, household electricity is distributed on a line to line 230V (or line to neutral 230V) system and the few 120V appliances mostly power tools are powered through  a transformer device that drops 230V to 115V called a yellow box.

[iceno9]

Great response Isaac...
Yes the UK uses isolation transformers (yellow boxes as you suggest) for the lower voltage appliances.

So if i understand your comment correctly, the 2 hot wire US system will have a center tap from the transformer.....i.e. L1 is 120V away from the center tap (Neutral) and L2 will be -120V away from the center tap. Therefore L1 to L2 is indeed 240V.
So assumingly then the current return is from one phase to the other as opposed to a phase to neutral?

Regards
Matt

[Isaac-1]

Yes that is correct, I should also mention that the L0 center tapped neutral is also tied to ground at the service entrance point, typically where the meter base is coming into the house and a 4th safety ground wire is also ran so a typical 4 prong outlet would be L1,L2, L0 and ground, under non fault conditions there should be no current on the ground wire.

Ike

p.s. One advantage of this system is when there is a balanced (or nearly balaced) load from 2 120V appliances across an L1-L0-L2 connection there is no or little current flowing on the L0 neutral as it only carries the difference of the loads, this often by code allows for the use of under sized L0 nuetral wires. (usually one size smaller than their associated L1,L2 wires)