ECN Forum Forums General Topics of the Trade General Discussion Area Underground supply

I live in Arizona, I have just built a detached 2 car garage, since the electric supply is much closer to the garage than my house I propose to drop the overhead supply down onto the garage or rather the electrical company will. Then i intend to feed a breaker box in the garage and run a supply underground up to the house.
I have already dug the trench and got the 2" PVC conduit and cable.

I worked as an electrician in England and am a little unsure about American wiring, I am not sure how many cables I need to pull through the conduit, the existing electrical company overhead line has 2, I have been told that I need to run three - what are the three wires for, I can see live and neutral would equal two, is there two hots and if so why?
thanks

To cover all bases, I recommend a 4 wire feeder. This would consist of two hot wires,the neutral and an equipment ground.

At the panel in the house, the neutral must be insulated from ground.

You will need to drive an additional two ground rods at the house & connect them to the equipment ground bus of the house panel.

The house panel will need a main breaker.

The panel with a main breaker will need to be located nearest the point of entrance of the feeder, just as a service disconnect would be.

There are some other code issues involved, I'll let better typists than me tackle them.

Tom

Thanks for the reply,

I assume by 'an equipment ground' you are referring to the ground that I already have at the house and would not be required in the conduit?

My main area of confusion is the two hot thing, as you probably know England is exclusively 240V except for industrial wiring that is and I have never come across more than one hot or live, what is the second one for, does it come from and go to the same terminals, is it simply to spread the load across two wires?

Just to give you a little more info, the run from the garage to the house is approx 120 foot, there is an existing overhead supply to the house which goes to a meter and attached breaker panel, this will be relocated to the garage hence the need for the underground feed. The supply will be 200A however some of that will be for the garage.

Thanks again

I'll let others tackle the code issues, and just describe a bit of theory as it relates to US wiring.

As you know, in both England and the US, power is distributed at higer voltages, and then stepped down to lower usage voltages using a transformer. Except for rare instances in industrial wiring, the secondary of the transformer is always somehow bonded to ground so as to define all other voltages in the system relative to ground and to provide a (hopefully) defined fault current path in the event of a ground fault.

Once you tie a point on the secondary of the transformer to ground, all other points have a defined potential difference to ground.

In the US, the most common supply for residential wiring uses a 240V secondary on the transformer, and bonds a center tap on that secondary to ground. Three wires from this secondary are brought into the house; the two terminals of the transformer (the 'hot' wires) and the center tap terminal (the 'neutral' wire). The voltage between the two hot wires is 240V (nominal); the voltage between a hot and a neutral is 120V (nominal).

Separate from these 3 supply wires, you have your safety ground or Equipment Grounding Conductor. This fourth wire is not supposed to carry current, with a couple of exceptions. The EGC is supposed to carry fault current, is permitted to carry small transient currents from surge suppressors, and is permitted to carry small currents from various grandfathered loads (meaning under current code the EGC cannot carry current, but in prior versions of the code it was permitted.)

Because the EGC is not supposed to carry current, but is supposed to be at the same potential as the neutral conductor, these two conductors are bonded together at one and only one location; the service entrance box.

When you looked at the wires coming into the house, it is quite likely that you saw two insulated conductors and a third wire which appeared to simply be a carrier or messenger wire...quite likely this carrier was also serving as the neutral conductor. So you had three conductors coming in to the service entrance.

You will also have a grounding electrode system connected to the service panel. So that makes your four wires. Any subpanel from the service panel needs to be fed with these four separate wires

-Jon

Just to add to the description of the 120/240V 3-wire system, in American homes you'll find both 120 and 240V appliances.

All your general-purpose wall receptacles are 120V, connected between one of the hot lines and the neutral. Some circuits will be on one hot leg, some on the other to balance the load.

Many large appliances are designed to run on 240V and are thus connected across the two hot wires (via a double-pole breaker). In many cases (range, dryer etc.) the neutral is also run to the appliance so that 120V is available for lights, timer clocks, and so on.

Contrast this with the British system where everything is run at 240V and thus you have just a single hot and neutral on the service.

The EGC (Equipment Grounding Conductor) in America is equivalent to what you would know from Britain as either the CPC (Circuit Protective Conductor) or ECC (Earth Continuity Conductor), depending upon how long ago you left these shores!

Bonding and grounding/earthing arrangements are where the U.S. differs considerably from British practice. All U.S. services are equivalent to British PME (TN-C-S) for example, but the point of the bond is at the main panel instead of before the meter as in England.

Thank you for the wonderful replies, it makes perfect sense to me now.

I guess I have one last question before I can complete this job, it would appear that along with the three THHN-2 cables I am running in the PVC conduit, I will also require an earth cable. What kind of cable should I use for this, is it an uninsulated cable - basically what should I ask for to satisfy code?

Thank you so much for taking the time to answer my questions.

#6 THWN copper, with green insulation would be fine.

Double check your THHN-2 conductors to make sure they are also listed as THWN. I know it doesn't rain much in Arizona, but by definition, an underground conduit run is a wet location.

There is a very good chance that your existing house panel has all the equipment grounds and neutrals (white) terminated on the same bus. You will need to separate the equipment grounds from the neutrals & make sure that the neutral bus is no longer bonded to the sheet metal of the can or to any equipment ground.

Tom

Obviously I accept everything you guys have told me about needing a ground wire and will do so. I do however have a nagging question that my limited experience is unable to answer:

If I have an earth going into the ground at the main panel on my garage and I have an earth connected to the gas lines at my house which will be the sub-panel why do I need an earth in the conduit joining the two panels?

I know I am probably missing something elementary.

You need an earth in the conduit for the purposes of fault clearing. A rod will not open your breaker under fault, as it is very limited in resistance. If you were to assume 25 Ohms of resistance on your rod and divided 120V or 240V by 25 Ohms, you would not have enough current to open the circuit breaker. Please don't take this as gospel...I'm just a dumb inspector and not a theory guy. Wait a while and you'll get a better response.

Now as far as the "requirement" for an equipment ground, there is none. If you have an NEC, take a look at article 250.32 and you will find that you can tie the grounded conductor to the new rod at the detatched structure, if you do not have a metallic raceway or GFPE on your service, which you probably don't have. If you ran a ground wire between the two structures, you will have to use it and not the nuetral however.

Again, if you are used to the way things are done in some British installations you'll have to start thinking along slightly different lines.

From 2002 NEC:

That clearly rules out the TT systems you may have been used to in Britain where the building is grounded just to a local rod, with no bond to the neutral at any point.



[This message has been edited by pauluk (edited 09-29-2003).]