Ready to make a UPS, just need some clarification

[miscrms]

Originally Posted by qbee42

"The heretofore said mentioned party of the first part..."

No, thatsa no good!

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

Marx Brothers Contract Skit

Rob

[Norm611]

Originally Posted by ChapmanF

Hi Norm,


That is, the source of your W/ft limits. Where did you find that #6 can safely dissipate 3 W/ft, #12 can't handle 11 W/ft, and the wire originally in question (was that #12 also?) can't handle 10/16 W/foot?

It sort of makes sense that there would be references giving wire dissipation limits in that form, but I don't know where to find them.

Thanks,
-Chap

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

[andyprius]

Originally Posted by qbee42

. The neutral wire on house wiring is already grounded to earth ground.

Tom

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.

[okiebutnotfrommuskogee]

Originally Posted by andyprius

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.

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.

[Norm611]

Originally Posted by andyprius

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.

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

[andyprius]

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

[andyprius]

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.

[qbee42]

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

[andyprius]

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.

[V8Cobrakid]

.. 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)