Gear Thread

Nathan the Machinist

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Nice!
What size tank will you do?
Will you do auto or manual transfer switch?

It has a little 12 gallon tank on it and an auxiliary pump to run from an external tank. I'll probably get a tank for it, 200 gallons or so. At first I was thinking about something like one of those home heating oil tanks. But I'm seeing some transfer tanks that are mounted on a skid that I could load on a truck and take up the road to get filled, I might do that, I haven't decided.

They make an automatic power transfer kit for these, but I think I'll do a manual transfer. No need to make it complicated.
 

Phill50

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Nice. I want one. My mom and dad met in the military and both specialized as diesel generators mechanics. Although I don’t think either one of them actually ever worked on one.
 

Odog27

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I'd be surprised if the utility would allow an auto transfer switch in a residential installation. Not to mention the safety crap that's needed on them now.
I’ve installed quite a few residential units with automatic transfer switches. Hell that’s safer than some of the rigged manual set ups I’ve seen. The auto transfer switch ensures full disconnect from utility. Hots and neutrals all switched simultaneously. On most rigged set ups there is no grounded conductor switching going on. People usually switch off their main breaker and hook up generator power by back feeding through a 30 or 50 amp outlet in the house, or by hooking generator directly to a branch circuit breaker. Problem is they are still hooked to utility neutral. That’s where the back feed potential becomes a problem.
Manual or Auto, just make sure to use a rated generator transfer switch so neutral is switched along with the hots.
 

TRfromMT

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I’ve installed quite a few residential units with automatic transfer switches. Hell that’s safer than some of the rigged manual set ups I’ve seen. The auto transfer switch ensures full disconnect from utility. Hots and neutrals all switched simultaneously. On most rigged set ups there is no grounded conductor switching going on. People usually switch off their main breaker and hook up generator power by back feeding through a 30 or 50 amp outlet in the house, or by hooking generator directly to a branch circuit breaker. Problem is they are still hooked to utility neutral. That’s where the back feed potential becomes a problem.
Manual or Auto, just make sure to use a rated generator transfer switch so neutral is switched along with the hots.

Interesting. Does a residential automatic transfer switch have to do annual tests (like a hospital emergency backup generator for example)?
 

Odog27

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Interesting. Does a residential automatic transfer switch have to do annual tests (like a hospital emergency backup generator for example)?
No. Not so far as a test that is reported to any authority. Most people get service contracts. Tech comes out manually transfers power, does an oil change, filter change. They are all set up to exercise once a month but there is no power transfer during that.
The commercial units at the facilities I contract with have to have numbers recorded monthly. Full load test annually.
 

Nathan the Machinist

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I’ve installed quite a few residential units with automatic transfer switches. Hell that’s safer than some of the rigged manual set ups I’ve seen. The auto transfer switch ensures full disconnect from utility. Hots and neutrals all switched simultaneously. On most rigged set ups there is no grounded conductor switching going on. People usually switch off their main breaker and hook up generator power by back feeding through a 30 or 50 amp outlet in the house, or by hooking generator directly to a branch circuit breaker. Problem is they are still hooked to utility neutral. That’s where the back feed potential becomes a problem.
Manual or Auto, just make sure to use a rated generator transfer switch so neutral is switched along with the hots.


The need to disconnect line is intuitively obvious. The need to avoid multiple ground loops is pretty self-explanatory particularly if running a modern gem set with GFCI that will get tripped up. And I understand that code dictates a separate neutral and ground even if running to a sub panel one foot away, so a ground-neutral bonded utility generator like one would use at a construction site has to be unbonded if running a house. But I'm having a hard time wrapping my head around the physics behind back feeding neutral up the utility neutral? It is not a part of the generator circuit and the only return path would be through ground and the utility neutral is attached to ground in a number of different places including at my panel and at the transformer and at the pole. How is it physically possible to backfeed the utility neutral? It would take two wires to complete a circuit and the other "wire" is the physical ground which is inherently grounded. Trying to understand this...
 

Odog27

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It’s more about reducing potential. In your above example all the right grounding and bonding is in place. Grounding and bonding serve different purposes but physically seem the same.
So you have to account for sub optimal conditions for a generator system and reducing potential when they will actually be used. During storms it is not uncommon to get line surges or to lose a single phase for a few seconds before the imbalance trips a line fuse or main breaker. Or utility neutral becomes hot due to storm damage. That’s why I said “potential” - it’s not about how things would work if all pieces are installed correctly and are intact, it’s about when there’s a failure on one system. And making sure the failure on one system doesn’t damage both systems.
 
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