Ready to make a UPS, just need some clarification

No, thatsa no good!

Hope that reference was intentional, otherwise I'm sounding a bit silly right now...

Marx Brothers Contract Skit

Rob

 

Chap,

I came up with those numbers the same way as I did with my actual dissipation numbers.

If you look at the "powerstream.com" website and look at their wire size table, they provide resistance per 1000 ft, and maximum current. From the Ohms / 1000, you can easily calculate Ohms / foot. Looking at the current limits in the table for #12 wire, you have 41A for "chassis wiring", and 9.3A for "power transmission". (Also, #12 is rated at 20A for household wiring).

I strongly suspect that these ratings are based on the ability of the wire to dissipate heat under various conditions. I suspect that "chassis wiring" is individual wires with air circulation around them, hence the wire can dissipate quite a bit of heat without getting too hot. "Power transmission" is probably either in a bundle, or in conduit without much air circulation to cool the wire. Normal insulation type for each application could affect the maximum operating temperature.

Does this answer your question?

Norm

 

Hopefully NO newbie or other wise will misinterperet the above statement. The neutral wire is generally black, it floats until a load is connected to complete circuit. The white wire is always HOT unless a CB is pulled. The Green wire is your safety ground wire that connects to each and every outlet thruout the house which eventually should lead to a recommended Earth connection. This is the reason for the standard 3 connector power plug. There is no electrical difference between the hot wire and the neutral when a load is connected. Never consider the neutral as a ground wire. This is a common mixup. Use a multimeter to identify which lead is HOT and do not depend on the colors.:cheer2:

 

Sorry AndyPrius, but the neutral wire is white and the hot wire is usually black or sometimes red. In older systems the white neutral wires and ground wires were usually all tied together on one buss in the breaker or fuse box. Several years ago that was changed and now the neutral wires are all tied to one buss and the ground wires are tied to a separate buss.

You are right when you advise to check with a multimeter for the hot lead and not rely on the colors.

 

Andy,

For house wiring on the east coast (and I believe in the National Electrical Code), the white wire is the neutral wire. The neutral wires for all branch circuits are tied to a buss which, as qbee stated, it is grounded (in the CB box). The green (or bare) ground wire is also grounded in the CB box. The black (and red for 220V circuits) wires are the hot wire(s). From the CB box they are always hot (when the breaker is on) up to a switch. The black wire between switch & load is hot only when the switch is on.

The neutral (white) side of a 110V circuit should always be at (or very near) ground potential. The green (ground) wire should not normally carry current, but is used to ground the chassis of a tool or appliance. If, due to an insulation failure or damage, a hot wire within an appliance makes contact with the chassis, the ground wire prevents the chassis from going "hot". Generally it should either trip a ground fault interrupter or circuit breaker.

I agree that you should always check the wires with a tester (and preferably open the breaker) before working on a circuit. Though the white wire should be at ground potential, it could be hot due to a wiring error, or a broken or loose wire.

Norm

 

My error, I always get my black and white mixed up.

 

As a additional precaution, always check your chassis to ground, with a meter after project is complete and powered up. Years ago, I coodinated the EV program here at the local AFB, when the mechanic was wiring up the power cords I cautioned him to use a meter and check afterwards. Walking around one rainy day I noticed a vehicle at the charging station and decided to put my multimeter leads on it, Yup about 60VAC on the chassis ( under slight rain and poor grounding conditions) I don't think that the auto mechanic knew what a multimeter was! Three of the five trucks were wired wrong.

 

Norm has it right. Where it gets interesting is when you derive the 110 VAC from an inverter. Picture running your laptop computer from a 12V inverter while you are driving down the road. Obviously the white neutral wire and the green protective ground cannot be tied to earth ground. It would be a little difficult to do going 70 mph down the highway. In this case the neutral and protective ground are usually tied together at the inverter, and both are connected to the 12V negative.

Now look at the case where an inverter is used to power a load in your house, like you might do in the case of a power failure. In this case it is possible to provide an earth ground, and it makes sense since a floating neutral could lead to a nasty shock. Here you attach the neutral of the inverter to the neutral in the house, and the ground of the inverter to earth ground.

In the case of our boat inverter, this is done automatically. When 110 VAC is derived by inversion at sea, the inverter closes a relay and connects the the 12V ground to the 110 VAC neutral and protective ground. When we plug into shore power, the inverter senses this and breaks the connection between the 12V negative and the AC system, while connecting neutral to the shore neutral and ground to the shore ground.

It's nice to have it all done automatically.

Tom

 

Three interesting situations. Begs the question, what is neutral, ground. So with a boat is water now earth? I remember installing a couple of small inverters on a sailboat and testing my black lead in the salt water, and the red on 12VDC. If I remember correctly, the meter read 12VDC. So the Oceans are really Earth.

 

.. i think grounding a land based line is really only for safety reasons. It's all one huge "just in case something shorts" setup. (that's a lot of power running through those lines)

 

That's correct. There is no other reason on a power circuit. Signal lines will often use ground as a shield to ward off unwanted noise, but that's a different issue.

Here is the thinking behind the separate ground line: Any electrical circuit is a path from a voltage source out to a load and back. This means we need two wires: one to go out, and one to come back. Power companies save money by using the earth as one of the wires. They do some neat load balancing tricks to minimize the return current, but good old mother earth is used as a return wire for power distribution systems. It seems improbable, but it actually works quiet well. By convention, the white neutral line in house wiring is connected to the power distribution earth ground.

With the neutral line grounded, we can complete a short circuit and zap ourselves by touching the hot wire (black or red) while some other body part is grounded, such as standing outside in a mud puddle. It gets more exciting since household plumbing is also grounded. This is why you don't use the toaster in the bathtub. Electricity would short circuit from the hot wire in the toaster through you, into the plumbing, and out to the earth where it completes the circuit.

Without specifying that the neutral line be grounded, it would be anyone's guess which line was hot. This was recognized as a safety hazard long before the days of the three pronged outlet. The first step towards safer grounding came with requirement for polarized outlets. When you look at a standard U.S. power outlet you will notice two slits, one of which is slightly longer than the other. The longer slit is neutral, while the shorter slit is hot. This is why some two pronged plugs will only work one way.

Back when I was a kid, before dirt and the cooling of the earth, it was common practice to use a polarized two prong plug for appliances and tools. The neutral wire would be connected directly to the metal chassis of the tool or appliance. This provided a measure of safety: if a short occurred inside of the appliance, the grounded neutral wire attached to the case absorbed the current and kept it away from the user. While effective and better than no ground, there is a potential problem with this system. The neutral wire carries current under ordinary use. This means that a wiring problem downstream from the appliance may cause the neutral wire to go hot. Said more simply, if the neutral wire fails in the power cord, house wiring, breaker box, or out to the transformer, the electricity has nowhere to go and the neutral wire becomes hot. This turns the metal case of our old fashioned electric drill into a primitive execution device.

To solve this problem, wiring code introduced the protective ground wire. Just like the neutral wire, the protective ground wire is connected to the earth. Unlike the neutral wire, the ground wire doesn't do anything on a normal basis. It's just there in case the other wiring fails.

Using our electric drill example, you will notice that some electric drills have two prong plugs, while others include a ground. You might wonder why. The two prong models will be labeled as "double insulated", which means the drill is built with two layers of insulating material protecting the user. Current code says that this is adequate protection, so no ground is required. The alternative is the third ground wire, which is usually found on drills with metal cases.

Yes, the water is at earth potential. Grounding on a boat is a complex thing. I will spare you the details, but it involves the 12VDC system, the 110VAC system, lightning protection, radio ground, and electrolysis.

As an example of just how weird boats can be, wooden boats are grounded differently than plastic boats, and not for the reason that first comes to mind. On plastic boats, it's common to ground all of the through-hull fittings to keep them at the same potential. If you do that on a wooden boat, the small amount of stray current flowing into and out of these fittings will make the water acidic, which in turn will attack the wood and cause it to decompose.

Gotta love the Marx Brothers!

Tom

 

Hello TOM, Great clarification on wiring. I thank you.

 

Question...

The Prius has an inverter to drive the electrical motors, and it's not grounded to earth while it's running. Why is it so important in the UPS application to have a ground to earth but not when using the much more powerful inverter in the Prius for the electrical motors?

 

After reading all the posts you'll left I got to wondering if my weekend job is done right. I have a 2007 and installed an Aims 1250w inverter with 3 foot #4 wire and cut the hot wire and installed an 80 amp fuse I bought at Circiut City. I hooked the ground wire to the same location as the 12v battery ground. I plugged in a full size frig and a couple of 100w bulbs and ran a 1/2" electric drill. Everything seemed ok but I maybe should have left them run longer that about 15 minutes to see if I was getting hot wires, etc. I never thought about an outside ground. Do you'll think thats necessary? And does anyone think my DIY project may need any tweeking?

 

That sounds about right. Your house wiring already has a protective earth ground, so that takes care of it for you. Obviously you have to pay attention to the load you put on the inverter. There is no way any of us can tell you about that, since each fridge is different. Keep in mind that motor loads like a refrigerator require more current during startup. If you load it up too much, the voltage may drop and cause starting damage to your fridge.

The Prius inverter is completely insulated from the occupants of the Prius. Household appliances, on the other hand, are designed for a grounded wiring system. If you use non-grounded appliances, then the earth ground for the inverter makes no difference. If they are grounded, and you break the ground, a wiring failure can become fatal. Note that as stated above, your household wiring is already grounded, so you should be okay.

Tom

 

Out of a very nice description, here is the only part I would want to clarify. The power co. isn't eliminating a wire or using the earth as a normal current path: the return conductor is a wire that runs unbroken back to the source of supply just like the others (except that, if you have an overhead drop to your house, it's usually bare while the others are insulated). It is connected to earth, but it actually wouldn't work quite well if the earth were the only return path; it works because of the wire. The resistance of the buried electrode to ground is not as low as you would want; at 25 ohms or above the code calls for adding another electrode, but that's the only standard it has to meet. That's far too lenient a resistance spec for any path seriously intended to carry normal operating current.

The wire is called the Grounded Service Conductor in the electrical code (and its counterparts, the white wires in branch circuits, are called Grounded Circuit Conductors). "Neutral" is what the rest of us call them. The wire (usually green) connecting the third prong to ground, and never intended to carry current, is called the Equipment Grounding Conductor.

-Chap

 

Hello Mike, Sounds good to me too. Where did you get the inverter, type, cost? The reason I'm asking is I just bought one to use for my house in an emergency to run a minimum of household appliances, got already to cut my main CB and decided to check the output voltage first: was 91VAC. So I eliminated the very thick extension cord and measure directly: Still 91VAC Output. Since it is supposedly rated at 125VAC, I'll bring it back to the store. ty:cheer2:The reason why we got into ground wires is we were really thinking of house wiring, then we got off on strange electrical phenomina ( that spelling strange too)

 

This is getting more interesting all the time. How does the average homeowner know the grounding is proper. How would they check it ( a megger), would the utility company come out and check it? Would they care? It seems any added resistance would be to the companys monetary favor. I seem to remember reading somewhere that the ground spike should be a minimum of 6 feet. Does that sound right?

 

There is no set specification for a good earth ground. A six foot spike will work in damp soil, but might be completely ineffective in rock or dry soil. Some ground systems require a large field of buried wire, while others can get away with almost nothing. The utility company can check your ground by comparing it to their ground. I have known of houses that had a better ground than the power company, and managed to draw a strong stray current because of it. That will get the power companies attention.

To ChapmanF's clarification, let me add a bit more. Power is distributed over higher voltage primary lines. Typically these are three phase systems, but they don't have to be. Each phase of the primary completes a loop or circuit back to the substation or other source. These lines are used to feed a transformer, which drops the voltage down to it's final value. In the case of a normal house, the transformer is single phase 220V and center tapped. The center tap gives two legs of 110V. It is to this center tap that the neutral is connected. The point I was getting at about the power company using the earth has to do with load balancing. To see what I mean about load balancing, let's look at your house wiring:

Inside your breaker box you will find two banks of circuit breakers, each connected to a single 110V leg. Some circuit breakers will be twice as wide. These are 220V breakers, and they get the 220V by connecting accross both legs. The breaker boxes are built so that the breakers attach to alternate legs. This is so that you end up with a more or less balanced load. Here is the cool thing: if both legs were truly balanced, you wouldn't need that neutral wire running out to the transformer. All of the current would go in on one leg and out the other. The neutral wire only carries the difference in current, not the total current used in the house.

The same thing is true for groups of transformers. A good power distribution system attemps to balance the loads, so that there is little return current on a neutral line. Return current is essentially dead weight, or like having a truck deadhead home empty. Small imbalances are absorbed by the ground.

My first house had a bad neutral connection. Normally everything worked okay, but whenever the water pump kicked on, half the lights in the house got brighter and half dimmed. We had a 110V pump, so its heavy starting load was applied to only one leg. Since the neutral line was going bad, it had a higher than allowed resistance. The heavy current dragged the neutral line away from ground, making one leg higher and the other lower. If you see that sort of thing it's time to call the power company.

In the old days, earth ground returns were fairly common. You still see them in third world installations. As ChapmanF points out, a wire is a better and more reliable conductor.

Tom

 

You guys keep this up and we will all know everything about housewireing, good job!