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Mini Home Generator System Design #190021 11/03/09 01:21 PM
Joined: Nov 2002
Posts: 244
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wewire2 Offline OP
Member
A friend asked me to install a small 4 circuit transfer
switch little combo panel next to his sub-panel in his laundry room. The goal is to provide some lighting, a few outlets and keep the refrigerator going during a power outage. He has a typical 5000 watt portable generator with twist-lock and u-ground receptacles. How much circuit tracing and load calculating is typical in the planning of a small system like this. The homeowner knows he has limited power to only run the refer and a few lights.

Work Gear for Electricians and the Trades
Re: Mini Home Generator System Design [Re: wewire2] #190022 11/03/09 01:38 PM
Joined: Mar 2005
Posts: 1,213
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SteveFehr Offline
Member
5000W will power everything in his house but the heat and hot water heater. I only have a 3500W, and I have to run around turning on lights to load mine down enough to drop it from 126 to 125V so my UPS stops beeping about the "high" line voltage. He could probably even run his stove or a modest (2-3 ton) heat pump on it, but not both at the same time.

If he's using an MTS, there are no load calculations required per NEC, he just has to be aware he has to open a few breakers on his panel before transferring to generator.

He may not even need a physical MTS; something as simple as shuffling breakers in his panel and installing a sliding piece of sheet metal that forces you to open the main breaker before closing the generator breaker is acceptable.

Re: Mini Home Generator System Design [Re: SteveFehr] #190023 11/03/09 02:10 PM
Joined: Oct 2006
Posts: 745
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EV607797 Offline
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WeWire2:

You can get a surprising amount of backup power from a typical 5KW generator. As always, you just have to be careful with load management. Mine covers my stove, water pump, ejector pump, sump pump, frezer, two refrigerators and just about all lighting/small appliance circuits. Granted, we can't fix Thanksgiving dinner and have all of the lights on at once, but it has never let me down. We also can't run the heat, water heater, washer/dryer or A/C at all, but beggars can't be choosers.

During one particularly long and frigid outage, we used a 1,500 watt portable heater in one room and spent most of our time in there. We did just fine.

I'm using a MTS on mine that is rated at 60 amps which feeds a 100 amp sub panel. If the load use gets carried away, I'll just turn off some of the branch breakers to prioritize the available power. In your situation with only four circuits, you should be just fine with even a 3.5KW generator. I'd be inclined to pick up a couple more circuits while I'm at it if I were you.

One thing that I've found to be valuable is to include the lighting circuit that serves the area of the panel(s) so that you can see what you are doing. I've also found that an emergency light set in this area helps to get things going without fumbling for flashlights.


---Ed---

"But the guy at Home Depot said it would work."
Re: Mini Home Generator System Design [Re: EV607797] #190027 11/03/09 07:37 PM
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Posts: 244
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wewire2 Offline OP
Member
If I'm getting this right, no load calculations are necessary for the feeder,xfer and mini-panel. Is this related to 702.5? system capacity and rating to supply all of the equipment "intended" to be operated at one time. The "intended" part gets you off the hook if the owner overloads the system? It just seems strange for the NEC to not require a formally designed system. This residential temp gen. stuff is a first for me and I'm trying to save a little digging time. Thanks for the input!

Re: Mini Home Generator System Design [Re: SteveFehr] #190036 11/03/09 09:40 PM
Joined: Aug 2007
Posts: 853
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leland Offline
Member
something as simple as shuffling breakers in his panel and installing a sliding piece of sheet metal that forces you to open the main breaker before closing the generator breaker is acceptable.

I saw 1 for a square D, but can't find it again.

I suppose for a RESPONSIBLE electrician we could make one.

I'm using a 10 ckt gen tran right now. and 5500 honda-
I'm on a well and gas heat/HW and never had a problem (over a week some times)

This year I WILL have my CHRISTMAS lights on the generator.

Neighbors hate that. but warm up some nice rum spiced cider...... All is good again!!

Re: Mini Home Generator System Design [Re: leland] #190037 11/03/09 10:26 PM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
Member
Why do we make thinga harder than thay have to be? Has no one looked at a manual transfer switch?

I strongly reccomend the purchase of one ... there are enough little details to keep track of to make this a bit more involved than it seems, but here's the gist of it:

Imagine a SPDT switch, such as the 'hand-off-auto' switch on a control panel. This switch lets you choose between either 'in' to supply a single 'out'.

Our 'out' from the switch is the connection to the circuit were're feeding. The 'in' that is supplied by PoCo power comes from the usual breaker in the panel.

The other 'in' comes from a fuse or breaker mounted on out transfer switch, and is supplied by the generator.

Now, there is another complication, in that, IMO, you really want to switch the neutrals over at the same time. I like separately derived systems.

The grounds of both systems also need to be bonded together.

Re: Mini Home Generator System Design [Re: renosteinke] #190040 11/04/09 12:26 AM
Joined: Jul 2004
Posts: 9,569
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gfretwell Online Content
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I came up with an idea for my ex, who doesn't want any kind of "fancy" (read:expensive) transfer equipment. I told her to put a 3 way switch in place of the regular snap switch that functions as the disconnect for her furnace blower and flange mount plug out other side of the switch. Then in a power out situation she can just run an orange cord from the genny over to the furnace, flip the switch and she is good to go. A couple more cords for the fridge, the TV and a couple of lights gets her going.

Other than the 110.3(B) I am not sure where the problem is.


Greg Fretwell
Re: Mini Home Generator System Design [Re: gfretwell] #190043 11/04/09 01:13 AM
Joined: Jul 2007
Posts: 1,335
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sparkyinak Offline
Member
If the genset has an internal bond and the trailer is plugged into the main power source and running the genset can cause havoc with the power. A double bond can create CEMF in the wiring and have seen especially extension cords that were properly sized and used burn up without tripping the breaker. Odds are failure will not be that bad plus being you ex, then it is a sound way to go....JUST KIDDING! smile


"Live Awesome!" - Kevin Carosa
Re: Mini Home Generator System Design [Re: sparkyinak] #190045 11/04/09 01:47 AM
Joined: Jul 2004
Posts: 9,569
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gfretwell Online Content
Member
The generator is not bonded (Honda) but even if it was, the worst thing I can see is you would have some neutral current on the ground. Which wire would burn up?

I have seen plenty of improperly wired SDS transfer switches and no outward problems. As a practical thing it is the same as those 4 wire sub panels with a rebonded neutral.


Greg Fretwell
Re: Mini Home Generator System Design [Re: gfretwell] #190050 11/04/09 10:14 AM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
Member
"No outward problem"

Yea, that's the kicker. I made a very easy-to-make mistake on a UPS system, and everything tested OK .... until there was an actual power failure, and some 120 circuits got 240 instead. There goes another tuition payment ....

Yet another reason to 'buy it' rather than 'build it.'

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