This from Short Circuit Via HotLine1

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Would you be able to post the attached 1-line diagram in the General Discussion Forum so others can see and discuss the installation please?





Topic Subject: 3 Generators at Dwelling





This 1-line diagram shows the electrical layout to back-feed a dwelling with three 20KW single phase generators. I’m looking for comments on the layout.



thanks, shortcircuit

Looks expensive.

My bias would be for a single Diesel gen-set with dual fuel capacity -- that was always kept warm -- and which was automatically tested once a month long enough to warm it up.

I'd have only one transfer switch -- on automatic -- and omit all of the sub panels and pin & sleeve gear.

I'd have a sub-panel with a cut-out switch that would shed load for non-essential loads. The presumption being that the bulk of the total load is never to be shed. Otherwise, I'd flip the priorities -- putting the all of the priority loads on one sub-panel -- itself subject to the automatic transfer switch.

I'd have an unswitched daughter panel(s) so as to have enough breaker spaces.

Super critical loads would warrant a dedicated UPS.

The dual fuels would be Diesel #2 and piped in natural gas.

I'm presuming that everything would be rigged to survive a tsunami and hurricane. (None of that Fukushima, 36" above sea-level installation nonsense.)

Tesla...that is a great idea.

In the above simple layout...generator #1 is for the most important loads and #2 and #3 are for areas of the property that don't need power all the time. This approach allows for shut down of generator #2 and #3 when not needed, thereby saving alot of fuel.

The idea behind the pin & sleeve is to allow connection to an alternate generator if another were to fail in the group of 3.

As for cost...lets say all equipment cost about $18K-$20K...this isn't that much more than one large generator and would save alot of fuel.

Thanks for the input.

Some comments for both options.

Can't you save the fuel by shutting the loads down for 2&3 with the single unit?

With multiple units and pin sleeve option you have the option the rewire unit#2 into #1 panel in the event of #1 failure?

Depending on a load calc for the items that are considered 'required' you may want to re-think this from square one. A 200 amp ATS with load shed technology and a 20 KW natural gas, or propane, genset should solve this.

Many McMansions here use the above 200/20 Load shed & I have not heard any complaints.

As to your layout, I see no issues of NEC compliance; the only possible issue may be Article 400.7/400.8 cord uses, depending on details of the pin/sleeve setup.

This does bring up an interesting question. Does a 10kva generator running at close to capacity use less fuel than a 20kva generator running at half capacity?

I suppose that might be a question for my niece, she is a Generac rep.

jdevlin...yes the pin and sleeve is for flexablity to connect 1 panel to another generator feeder if a gen-set were to fail.

Hotline...I too see some conflict with article 400 and cord uses. Maybe move the pin and sleeve outside for connection at the generators. They would only be live when the generators are feeding the dwelling.

Would a generator be considered a piece of "utilization equipment"...a bit of a stretch of the definition?

Greg...

Diesel engines are famed for having a pretty linear performance curve from idle up to full power.

Otto engines ( automobile engines, Mr. Otto's design scheme )are much, much, worse than Diesels across all ranges -- and particularly near idle.

An idling Diesel can run very, very, lean -- without losing it's 'flame'... below a certain threshold, Otto cycle engines don't fire off at all -- and just eject the vapors into the exhaust.

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But as true as that is -- it's irrelevant.

The o n l y time that the back-up system is in play is during emergencies. Fuel cost is not a serious factor.

Reliability is critical. Diesels are much, much, more reliable than Ottos. They have no spark circuit -- and are inherently more robust. A pre-warmed Diesel doesn't even need glo-plugs -- particularly in Florida's weather.

A Diesel that starts up on distillate #2 can also accept methane as an auxiliary fuel. ( 20% liquid + 80% gas ) You can go to Youtube to see videos of such rigs running on the freeways of America.

Because of their lower first cost, Otto cycle engines running on Poco natural gas are t h e single most common power back-up that I've commercially installed.

Lastly, one 60 kW set-up is sure to be cheaper than three 20 kW set-ups. That law of economics is why the Poco uses monster machines. There is no other reason.

Tesla:
Yes, the diesel is more reliable then gas/propane/natural. But, for resi, the favorite is gas. Initial costs and the convenience of not having a fuel truck (except for propane)

Hence, our data center has 8 diesel, 2 MW, 13.2KV, with 3800 gal. belly tanks for each unit. Shortcircuit and I are both in climates ranging from 100+ summers, to 0 winters, so a warm diesel is a six month luxury here. Bless the block heaters.

Greg: BTW, the diesels mentioned gulp 180 gal/hr at 85% load, and the load bank testing was very, very close to that figure.

Since the cord is going to be fixed wiring I don't think the cord
would fall under 590 Temporary installations even though it is for
temporary power so
400.8 seems like it would apply.
(1)"flexible cords and cables shall not be used for the fixed
wiring of a structure."
(2)"where run through holes in walls"
(4)"where attached to building surfaces."

Also, as I understand it, the ampacities listed in table 400.4 and 400.5
are the cord ampacity ratings. You can use these figures when derating
but for the termination requirements of 110.14(C)(1)you will ultimately need to comply with
table 310.15(B)(16) Even with 90 degree terminations #4
is still not sufficient for a 100 amp load.
Feedback on this appreciated.

Maybe I am living in the 20th century but aren't these constant speed alternators? Who cares what the engine does at idle, how is it at half load and 3600 RPM (or whatever) and full working load?

Unlike motive power, motor-alternator sets swing up and down the power band without changing RPMs.

(Typically)

As I alluded to above, these units run so rarely that fuel economy is -- in every sense -- not a consideration.

Be that as it may, all Otto cycle engines will use about twice as much fuel per net kW as a Diesel cycle would.

As they 'unload' -- ie drop back to near idle -- their specific fuel consumption is about five times as much as a lean runnning Diesel.

Which means nothing. That relative performance advantage is not going to be seen in any real world application.

I'd entirely ignore fuel economy and focus on first cost... In which case, all smaller installations will end up being Otto cycle/ regular gasoline engines -- usually getting natural gas as the main fuel.

For one facing hurricanes -- gasoline fuel stability is an issue -- as is natural gas continuity.

I'm 3,000 miles too far west to be of any help on that.

I'd peek in on the local grocers. They'll almost always have a back-up gen-set on the roof -- just to hold their freezers steady. I'd follow whatever they deem to be best local practice.

wewire2...If we were to move the pin and sleeve outside for interconnection between the generators then 400.8 (1), (2), and (4) would not apply, IMO

I don't see anything in 400.7 and 400.8 that would specifically prohibit the pin and sleeve outside by the generators, IMO

The rubber cord we would use would be type "W" (PPC) which rates at 96amps...next size up rule allows 100amp OPD

Tesla, diesel as a fuel source is never considered for residential applications in my area.

As for fuel degradation, I was told that today's gasoline would start to go bad in 30-60 days and diesel in 6-12 months.

I agree that continuity of street gas would be an issue in a major disaster.

Propane would be less susceptible to a disaster. For that reason and because there is no street gas for this project, over 5000 gallons of propane will be underground for fuel supply for cooking,heating and the generators.

There will also be an 8KW solar PV system with storage batteries that will allow 24-48hrs of backup power independant of any generator power if need be.

If the pin/sleeve was setup on the exterior then an appropriate length of cord would not raise my eyebrow.

'Cord' (400.7/400.8) useage is one item that raises questions, whenever it appears on a plan/sketch, or is buried within the 'Electrical Notes' of a design professionals plans/specs.

BTW, with 5k gal of propane, & the solar storage, that sounds like someone preparing to be a survivor!

Thanks for your input Hotline!

Survivor or Doomsdayer...

From my experience, and research with comparing generators from the same manufacter and line of gensets have told me that a 10 kw load on a 10kw generator uses less fuel per kilowatt than a 20 kw generator carrying a 10kw load. Totall opposite for a gas motor. Diesel is expensive to setup but can be cheaper than gas to operate. Too many variable to determine that. Scope scope and scope will give you the answers

Shortcircuit...

If this project is to be in the Northeast, then Heating Fuel can substitute for Diesel fuel.

Both are middle distillate #2. The only difference being that the home heating fuel has a dye in it and carries no road taxation.

In every other sense, they are the same.

In which case, any stationary Diesel is going to use 'home heating oil.'

Out here 'home heating oil' does not exist.

The only significant deterioration for middle distillate is water vapor absorbtion. So, middle distillate stores extremely well.

In contrast, gasoline loses its volitile components. No end of lawn mower difficulties are due to this one factor.

If I were in New England, I'd probably favor middle distillate over propane -- no pressure vessel -- and the tank can be buried.

They bury propane tanks here. I have one in the ground as we speak. I got the propane kit for my 5.5KW briggs. It is still in the box.

We bury propane tanks here in NJ also. 'Home heating oil' tanks are not very popular, major concern is leaking, and the nasty liability issue for polution.

Most of the diesel gens I know of have 'belly tanks', surface/above ground that the gen sits on. Gens that are inside tractor trailer boxes have a tank in the trailer, these are mostly rental gens, or former rentals.

Depending on the tank size, and GRHr, a contract diesel dealer is a prudent move.