Electrical theory question

I have been thinking about this for a while. While laying a power cable to my shed today I though a good deal about it.

I ran a wire to my shed today for power out there. The wire was 10/3 w/ ground. So I have 2 hot conductors, 1 neutral, and 1 bare ground in a nice sheath designed for underground burial.

Now, all the wires appear to be 10 gauge in size. And 10 gauge is rated for (I think) 30 amps. The 30 is really immaterial anyway for my question. For now, let's just say 30.

This wire was hooked to my outside breaker panel with the hots going to a double pole breaker and the neutral and ground going the the grounding bus.

In my shed, I just have two circuits. One is from 1 hot (say black) and neutral (white). The other is from the other hot (red) and the SAME neutral (white). Ground is properly used.

Now, when I use circuit 1 and load it to 30 amps, the black and white wires are carrying 30A apiece. At this time, no load is on circuit 2.

Now, the reverse works the same, load 2 to 30A. Red and white are carrying 30A each.

But, when I load 1 and 2 to 30A... I used to think the neutral (common to BOTH) was carrying 60A, while black and red carried 30A each. This didn't make sense in that the white conductor was the same physical size as black/red. If this was true, either white was being pushed way beyond rated specs or black/red were oversized.

Then it hit me... black and red are out of phase by 180 degrees. So..... the neutral conductor's amperage is really the difference of the black and red load amperage.

Anyone that KNOWS for sure care to comment?

I think the current rating also depends on how long the cable is.

you have 2 ground wires one of them is earth ground.
the hot wire changes polarity (sine wave) 60 times per second. so the current flow changes direction 60 times per second too.

the extra ground(earth ground) is usually used to ground
the outer metal case( or other internal components)of what ever you plug into the outlet.
just in case their is a short to the case and/or the return ground path is interrupted or protection against the neutral wire having a potential difference relative to earth ground for some reason.
I am sure I missed something.
oh well I need sleep

Nothing personal... but this wasn't really what I was asking about. Also, in my state, code requires neutral and ground be bonded together for a concrete slab house (trailers are diffent I think).

Turbodog,

You are correct. If you have a 240 VAC service with 120 VAC to neutral that is how you would wire the run. Think of a center tapped transformer.

When you make the connections, make sure that you see 240 VAC across the two hot wires and 120 from hot to neutral (and hot to ground). If, by accident, you connect both hot wires to the same phase, you would then have both return currents flowing through the one neutral--and the probability of an over heated neutral when both circuits are under heavy load.

-Bill

The safety ground...bare copper or sometimes green...should be connected to a premises ground and the white ..neutral...should be connected to the neutral side of the breaker.

Aside from that it's not easy to tell what else you have going on. You could be on two seperate phases depending on how you have the panel wired...or on the same phase. I think most residential provides services provise two phase so you can connect ranges and A/C..etc at 220 V. If you loaded up both circuits to the maximum...not advised for a variety of reasons...the phase relationship, given a two phase setup, would mean less than 60 amps on the neutral.

If you have the neutral and the safety ground tied to the same point they will share whatever current the two circuits draw.

To my knowledge the safety ground should always go to a building or premises ground and not be connected to neutral.

GFI is based on the presumption that a current leak can be sensed to a ground I think. You might want to consider GFI in the shed....

Yeah, that's what I'm talking about.

Several points.
1). Neutral and Ground must be tied together no further away then the building service entrance under the National Electrical code in the USA.

Beware, this is not necessarily the case in other parts of the world. In places where autotransformers are prohibited, it is usually because this is not the case, the neutral can be quite 'hot'... I have personally seen neutral as high as 40Volts above ground.
2). It is rare, but not unheard of for power to be delivered as two 120 volt IN PHASE, legs, in which case the neutral current with both sides at 30Amps will be 60 amps. Things like Electric dryers and stoves won't work if you do that, but many other devices use the hot legs independently, and they will in fact work. 60 amps through 10 guage will make it warm, but well short of starting a fire. It is about 4 watts dissipation per foot of wire. IF there is a problem it will be at the connecting point, not the wire itself.

If your home is serviced by a Unit residential transformer, the phase are likely to be 180 degrees apart, otherwise it is 2 out of 3 phases, in which case the phases will be 120 degrees apart.

The apartment complex I live in was built in the 1980's, it is service by a 6500V/100 amp single phase feed. 240v service can be viewed as the output from a center tapped transformer. hot to the center tap is 120Volts either way. Hot to HOt is 240V. Technically if you have 2 out of 3 phases, you don't have 240, you have 208V service, which is the RMS phase to phase voltage.

Unit Residential transformers are also center tapped, at 120V on each leg.

3). You want to keep safety ground and neutral apart at the load, otherwise safety devices such as Ground Fault interupters will NOT work. These devices work by measuring the current in the hot and neutral. If they differ by more than a few milliampere, the device 'trips', i.e. a path exists for the hot that is not through the neutral. that may be an insulation failure. In any case it means a path exists that should not exist, and is a potential hazard.

I have a personal preference for either metallic conduit or metal armored cable, where the conduit or the armor provides and additional safety ground. The metallic conduit or armor also adds considerably the the heat dissipation capability of the conductions inside, and provide additional fire protection.

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mattheww50 said:
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Technically if you have 2 out of 3 phases, you don't have 240, you have 208V service, which is the RMS phase to phase voltage.
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The 208 is only if the phases are 120 out, not 180. Right?!

I DO have a pretty nice true RMS meter and it is reading 240+/- between hots.

But, I have worked enough with 3phase power to know where the 208 comes from.

You've got a single phase, three wire, 240 volt, system. When one or both 120v circuits are used, current is still limited to 30 amps on the neutral line.

BTW ground fault interrupters will work on ungrounded circuits as they sense current on the hot and neutral lines. Any difference in current on one of the lines and it will trip out the GFI. I in must = I out or that = trip.

Eh? Homes in the US have a three phase supply?!? That I didn't know...

I thought he was talking about 120 volt single phase

I must have mis read his question

The original description was the hot legs came from a double pole breaker. That says nothing about whether or not the supply is single or two phase, 180 or 120 degrees apart.

It is rare for homes in the USA to have 3 phase power. If you have a very large home (with very large Central Air conditioning units), you are likely to see 3 phase. Big electric motors start better, and run better with 3 phase power, and some large electric devices are designed for 3 phase delta connections (277 volts IIRC). Few homes are large enough to justify it.


Power is generated and distributed as 3 phase, so it is quite possible to have 2 phase power, where what is delivered is 2 of the 3 phases, which will be 120 degrees apart. If your true RMS voltmeter says something in the 208 volt range, that's what you have.

With the proliferation of residential transformers, 2 out of 3 is a lot less common, since the transformer is fed with single phase. (as homes moved to total electric living in the late 1970's and early 1980's, the assumptions that made larger pole mounted transformers for communities attractive, disappeared, and with it, most 2 out of 3 phase installations)

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14C said:
The safety ground...bare copper or sometimes green...should be connected to a premises ground and the white ..neutral...should be connected to the neutral side of the breaker.

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In general, a standard circuit breakers must never be connected to a neutral circuit (perhaps I am miss-reading what you typed here). They should only be installed in the hot leg of a circuit--and if you are doing 240 VAC circuits, the two breakers must be connected (ganged) so that an overload in one branch will turn off the other side at the same time.

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If you have the neutral and the safety ground tied to the same point they will share whatever current the two circuits draw.

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You should not normally have any current flowing through the safety ground circuit. Only fault currents should flow through a safety ground. You must not wire a safety ground and a neutral wire in parallel.

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To my knowledge the safety ground should always go to a building or premises ground and not be connected to neutral.

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If you are talking about an individual piece of equipment's safety ground must connect to a ground wire, cold water pipe or some other earth ground and NOT connect to a neutral, you are correct.

But remember that the neutral is connected to the safety/building ground/grounding rod/cold water pipe at one point, typically in the main breaker box. The idea is to keep the neutral ground referenced to help reduce the chances of electrocution and neutral/ground shorts drawing large currents (remember, neutrals are not protected by circuit breakers). In practice, you easily can see neutral floats of 40-50 volts at the end of longer runs (I have seen this in larger buildings and commercial installations. I don't know what the cause is but I suspect that there is a hot to ground short somewhere--perhaps in a motor winding--that sets up a ground gradient but does not draw enough current to trip a breaker).

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GFI is based on the presumption that a current leak can be sensed to a ground I think. You might want to consider GFI in the shed....

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GFI sensing is actually quite simple. Take the two wires that form a circuit (hot/neutral or hot/hot) and run them through a couple of loops of wire (torrid transformer). If the current out one leg does not equal the current returned on the other leg, a current will flow in the torrid sense transformer--and you have a "ground fault".

-Bill

BB, thanks for the clarity. The first statement of mine you quoted was wrong and I can't even begin to say where my head was. I should have pointed out that the neutral connects to the neutral buss bar. As for the rest of it I think you made the point much better than I did. There should be no current flowing in the safety ground.