need explanation of transfer switching

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  #1  
Old 03-05-14, 11:38 AM
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need explanation of transfer switching

We have a Siemans W0816MB1200CT (200 amp) power entry panel.

To hook up a portable gen set to our place, we'll need a transfer switch.

I don't understand all the functions of a transfer switch.

I know that 200 amp wires are 2/0 gauge (sort of like battery cables).

To prevent backfeed, how does a transfer switch handle that function? I've been told that it somehow does it without going to 2/0 gauge wiring. How? If I can get around backfeeding the system without going to 2/0 gauge wiring, that will be a big plus to me......
 
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Old 03-05-14, 12:09 PM
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A transfer switch has one output and two inputs. The output is the feeders to your panel. One of the inputs is the utility feed and the other is your emergency power feed - your generator feed.

To prevent backfeed, how does a transfer switch handle that function?
The better transfer switches do it by switching the neutral along with the hots. For single-phase residential service. a transfer switch that has three terminals at each position is needed to accomplish that.
 
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Old 03-05-14, 02:19 PM
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There's different types of transfer switches. There are ones that can switch the entire load (house) and there are also ones that switch individual (usually 6-10) circuits. This is the type I think you're thinking of - such as those made by Reliance.

Basically in this type there is a 3 position switch. In one position the circuit is fed by the existing breaker. Center is off. In the other position the circuit is fed by the generator.

If you have a GFCI equipped generator you'll have to switch the neutral.

I did check the Siemens site but don't see an interlock made for your panel.
 
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Old 03-05-14, 02:58 PM
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there are also ones that switch individual (usually 6-10) circuits. This is the type I think you're thinking of - such as those made by Reliance.
Those are transfer panels, not transfer switches. Is that what you're thinking of installing, or do you want a transfer switch?
 
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Old 03-05-14, 04:03 PM
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A whole-house transfer switch is installed in the line between the electric meter and the primary breaker panel.

It lets you disconnects the panel (panel feed) from the utility lines from the meter and connects the panel to the lines from the generator. Vice versa when utility power is restored.

It must have the capacity (150 amps? 200 amps?) to handle your entire electrical service.

You have the fat 2/0 wires from the meter run to the transfer switch (A side).

You still need additional (probably short) 2/0 wires from the transfer switch to the panel.

The wires from the transfer switch (B side) to the generator connection will probably be somewhat thinner since your generator will probably not have the full 150 or 200 amperes to run many items in your house at the same time. If your generator provided a maximum of 40 amps then you would need just 8 gauge wiring from the generator to the transfer switch.

You can alternatively install a smaller transfer switch, say 40 amps or 60 amps, between the primary panel and a subpanel. The switch would have the same character as a whole house transfer switch. Only the circuits in that subpanel would be eligible to receive generator power.
 
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Old 03-05-14, 07:32 PM
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Xfr SW

Here is a basic optional standby system layout.You can see how a transfer switch is an, either, or, situation. The switch either gets its power from the utility via your service panel, or it gets its power from your generator. This just shows feeder conductors, there would also be grounding by conduit or a GEC within the feeder cable.

Name:  Optional Standby system.jpg
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Old 03-05-14, 10:32 PM
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Portable Gen Connection

Sorry I did not notice your installation will be for a portable generator. Here is layout for portable:
Name:  Optional Standby Gen with Receptacle to Trans Switch.jpg
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Old 03-06-14, 07:24 AM
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Those are transfer panels, not transfer switches.
While I agree, the manufacturers refer to them as a transfer switch.
 
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Old 03-06-14, 08:29 AM
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Individual switches in a transfer panel work the same was as a whole house transfer switch except they control single branch circuits.

The A side is connected to the breaker terminal for the selected branch circuit up in the main panel which remains connected to the utility lines. The common terminal is connected to the branch circuit hot conductor that used to be attached to the breaker. The B side needs no attention by the user or installer. It is hidden inside the transfer panel and connected to the generator feed that is part of the transfer panel.

A transfer panel typically comes with a flexible conduit holding all of the A side and common wires (one pair for each switch in the transfer panel) to be fed into the main panel where the connections are made.

When the generator is in use, the main panel breaker is not in the current path and therefore does not protect the branch circuit. A transfer panel has (or should have) overcurrent protection for each individual circuit it serves so it is safe to supply it with a generator whose capacity is greater than any one branch circuit's rating.

Referring to a transfer panel with more than one switch as a "transfer switch" is grammatically incorrect.
 
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