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bobkatz

Well-known member
Joined
Jun 25, 2006
Messages
138
Location
Orlando
Hi, guys. Being in Florida, I've been putting off buying a generator, and it's already hurricane season. Generators are 6-8 weeks backordered, so I'm way behind on the curve for this year!

I'd like to see if any "power experts" here see any flaws in my calculations.

I just measured the following with both air conditioners running and all the audio and computers connected:


Phase 1 40A

Phase 2 32A

Neutral 12A

I figure this means 72A draw at 110 Volts


72 X 110 = 7200 watts

considering the surge of the startup, that sounds pretty safe for a 16KW generator, don't you think? I think there are still some 13 KW generators in stock and available. I could use one it seems to me, unless you see there's some flaw in my measurements.

16KW generators now come with 100A (per leg) circuit breakers. Isn't that too high a breaker? (110 X 2 X 100 = 22000 wats). There's a 200 A master breaker at my service entrance. It's a combined master breaker, would that also be 100 amps per leg?
 
Don't confuse KW with KVA. Generators produce power (KW) but may be feeding genuinely inductive loads such as motors and pseudo-inductive loads such as power supplies that rectify to d.c. In these cases, the line current can easily exceed the rated power divided by the line volts, and the fuses and breakers have to be set appropriately.

In addition, the short-term power output of a generator is often a lot higher than its rated continuous power.

13KW sounds fine for your needs, maybe even over the top.
 
> There's a 200 A master breaker at my service entrance. It's a combined master breaker, would that also be 100 amps per leg?

No, 200A each leg. And in civilized areas, 120V per leg. You are provisioned for 240V*200A= 48KW. This has been standard residental practice since at least 1970.

I know that if I actually used 48KW inside my house, I could bake bread in every room.

It's a ploy by Reddy KiloWatt, who does not want your use of electric toys limited by old-type 230V-50A service entrances. The difference in fusebox cost between 50A and 200A is quite insignificant, compared to total cost of wiring, or amortized over decades of use.

A 200A service allows all the kitchen fires (stove, ovens, toasters) to be electric, electric water heater, electric dryer, all-electric heat in many homes, and A/C in nearly any home.

My father lives on a 30A total feed. He's had to re-wire his dryer to run half voltage for four times as long, and he can't dry clothes while cooking. I think a 200A fusebox is wise.

But you would NEVER want anything like 48KW in your house for any length of time. I use about 3KW of spot heat in mild weather, and it can take the chill off fast. If I put the overn on self-clean, the only time it eats its full 5KW for more than 10 minutes straight, I have to get out of the kitchen.

A/C is funny because it throws the heat OUT of the house, and when you need it most it is least efficient. Still I'd think 5KW of A/C ought to get you through.

What I noticed, when I looked at generator economics: those things GULP fuel. The smaller ones appear to be very inefficient gallons/KWH, and have small tanks. So you put in a couple gallons 2 or 3 times a day, and then what? Drive to the gas station for more gas? If the outage is widespread, the electric pumps in the gas station will be out. You can tap a few gallons from a car. You could plan ahead and store gasoline, but there are fire codes and insurance hassles with storing significant quantity of volatile fuel around your home.

There are of course street-gas generators. These will run through some kinds of disaster (wind-downed wires), not through others (massive flooding and drowning of gas appliance pilot-lights). And I have the impression that much of FLA has grown faster than its gas lines.

Do you need "all the audio and computers" to run through an outage? Normally you look to essentials first. Keep the sump pump running to mitigate flooding. Ability to boil contaminated water, and make dry pasta or beans edible. Power for the blower on the furnace so the water pipes don't freeze and burst. Minimal lighting so you can keep an eye on the storm. TV set for news. In temperate climates, I would not call A/C essential, though in FLA some spot-cooling (a few rooms) just might be worthwhile.

Note that a Whole House generator system MUST be wired through a Transfer Switch. You better not get caught with any way to put power out onto downed lines that repairmen may be working on. I'm sure you know better, but reminding everybody.

The alternative is to have a network of extension cords, from each essential load back to the generator. Obviously you won't be able to buy these after the power goes out on the checkout lines at the store, so you have to be prepared. It is hard to imagine doing this for more than 5KW or 10KW of load. (I have run my whole upstairs on a 10/3 120V/30A cable bypassing some ugly wiring; it's no fun.)
 
A diesel genset is more efficient and more reliable.

For any major outage, you'll have to store a lot of fuel on site though, no matter what.

What kind of A/C are you running? Window units or a full split central job? Those can require a lot of start-up current to get motors in the compressor and ductwork fired up.

http://www.backwoodshome.com/articles/thomsen43.html

Chris
 
I've researched this a few times, originally for a local FM radio station's studio. We discovered that 2 kW would easily power the vital elements in the control room (including minimal lighting). However, keeping the transmitter on-line (which was remotely located) was another story...as in 20 kW!

Definitely look at natural gas powered "generators" if gas is an option..no need to stash gasoline or diesel on-site. However, in the case of a major disaster, even natural gas service may be cut off.

Bri
 
Is 2-phase standard (tranny to house)in America? :?
120V phase voltage in a 2-phase system.....that should yield some thick wiring/cables. :shock:
 
Here in the USA we have what is called "single phase 120/240".

Three wires from the electric company: two legs that are 180 degrees and 240 VAC apart with a "neutral" that is grounded. From one of the "hot" leads to neutral is 120 VAC.

Bri
 
Bob,

You really want to pay attention to the start-up surge of your AC central units. I do not know the details of what is used in AC systems, but up here in Idaho we need to size our generators to start water well pumps. Inductive motors (rule of thumb here) need appox 5x their running current to start. If you have two AC units to deal with, figure they will want/need to start at the same time.

Also, I recommend a low RPM diesel generator over a gas or LP generator. They are more quite, more reliable and seems to have fewer problems. Good quality diesel genterators are available through RV/custom coach building companies. Get a good one.

Cheers,
 
[quote author="PRR"]> There's a 200 A master breaker at my service entrance. It's a combined master breaker, would that also be 100 amps per leg?

No, 200A each leg. And in civilized areas, 120V per leg. You are provisioned for 240V*200A= 48KW. This has been standard residental practice since at least 1970.

[/quote]

Thanks for the clarification, PRR!

I run my mastering business from my house, have four employees, office space and all, so all the computers are necessary :).

What I noticed, when I looked at generator economics: those things GULP fuel.

The cost is enormous, but in 2004 we were out one time 10 days straight with no power. Not to mention the inconvenience of being without hot water for showers (which I can live without, of course), it wreaked havoc with our business and that's the key. This is a business decision. The cost of a tank of LP gas for about 3-4 days continuous running is about $500. So it's definitely not cheap!

And yes, we're doing the ATS switching and all power details properly. The last thing I want to see is a lineman burned up as happens a lot in Florida from backlash in the line. People hook up their generators and the only thing separating them from the power line is the main breaker, which they forget to throw.
 
[quote author="FredForssell"]Bob,

You really want to pay attention to the start-up surge of your AC central units. I do not know the details of what is used in AC systems, but up here in Idaho we need to size our generators to start water well pumps. Inductive motors (rule of thumb here) need appox 5x their running current to start. If you have two AC units to deal with, figure they will want/need to start at the same time.

Also, I recommend a low RPM diesel generator over a gas or LP generator. They are more quite, more reliable and seems to have fewer problems. Good quality diesel genterators are available through RV/custom coach building companies. Get a good one.

Cheers,[/quote]

Around here in Florida everyone recommends LP gas. This is the first I've heard of a diesel alternative. As for the starting current, the model I'm considering can handle it.

BK
 
Hi Bob,

I installed a 15 KW LP generator at a friend's ranch in Santa Barbara. It was a back-up power source and a battery charger for his wind and photovoltiac system. It SUCKED the LP out of that tank. I could not believe how quickly it used up a 1000 gallon tank. It also ran at the same RPM as a gasoline powered generator, which is somewhere around 3k rpm. This make for a loud generator. One of the advantages of diesel is you can get low RPM generators (something like 700-900 RPM) and that is much more quite and easy to build a sound baffle for.

Bottom line for me is LP is cheap, but inefficient in terms of power per gallon. Gas is better in the power department, but not very safe to store and expensive. Diesel is not very dangerous to store, has a very good power per gallon ratio, and can operate a generator with a good power to rpm curve (low rpm).

But the locals know more about operating such systems in your area than I do, so you may want to listen to what they are saying. I can only speak from my own experience.

Be sure to double/triple check that surge power rating. It's very important.

Cheers
 
There is also the hideous diesel smoke smell, and depending sensitively on combustion details, the carcinogenic aspects. So it's not all that simple a decision.

Of course in an emergency these may be relatively small concerns.
 
[quote author="FredForssell"]Hi Bob,

I installed a 15 KW LP generator at a friend's ranch in Santa Barbara. It was a back-up power source and a battery charger for his wind and photovoltiac system. It SUCKED the LP out of that tank.

[/quote]

I know, I know. Well, the LP gas generator is probably noisier than the diesel. But I don't know anything about diesel and no one else here does. The 16 KW ultra quiet LP gas generator with an electronic governor cost me $3700 with an automatic transfer switch including free delivery.

Just pray for very few or no hurricanes!



I could not believe how quickly it used up a 1000 gallon tank. It also ran at the same RPM as a gasoline powered generator, which is somewhere around 3k rpm. This make for a loud generator. One of the advantages of diesel is you can get low RPM generators (something like 700-900 RPM) and that is much more quite and easy to build a sound baffle for.

Bottom line for me is LP is cheap, but inefficient in terms of power per gallon. Gas is better in the power department, but not very safe to store and expensive. Diesel is not very dangerous to store, has a very good power per gallon ratio, and can operate a generator with a good power to rpm curve (low rpm).

But the locals know more about operating such systems in your area than I do, so you may want to listen to what they are saying. I can only speak from my own experience.

Be sure to double/triple check that surge power rating. It's very important.

Cheers[/quote]
 
Gasoline and natural/bottled-gas engines are "the same thing". You replace the liquid carb with a gaseous carb. There are differences in power: good natural gas will make more power in a given size than gasoline, if the engine is designed to run on good natural gas. But you can't get the good stuff outside the gas fields (street gas is diluted to a low common spec, bottle gas is just a little richer) and most engines under 200HP (130KW) are gasoline engines with hasty adaptations.

> A diesel genset is more efficient and more reliable.

A diesel will always cost more and weigh more than a spark-engine of the same power. Spark-ignition goes "fwoosh", Diesels go "BANG!". A Diesel always runs harder than a worst-case sparker, and has to be built about 3 times stronger to live at all. 3 times stronger also means it can't run as fast. However the resulting heavy expensive engine will run a looooooong time before wear is a problem. Diesels are slightly more efficient than sparkers at full load, but much more efficient at part load.

If you only run a few hours a year, the up-front cost of a Diesel will never pay-back in fuel or replacement costs.

If you run many hours a year, the Diesel is miles ahead on fuel and ring-job costs.

As a very rough rule: less than 1,000 hours in the next few years, get a cheap sparker. Cars run 2,000 hours in 10 years and, at US fuel prices, are mostly sparkers. Simple economics. Heavy business trucks run 2,000 hours in one year, and are mostly Diesels. Same economics.

In Bob's situation, up to 250 hours every year or two, the mass-market sparker is the "obvious" way to go. Especially since he can lose the ~$3,000 cost of a 13KW sparker genset in just a few days of lost business.

For Roger's friends who live off-grid, runtime/year is much higher and the balance shifts toward Diesel. If you run much of the day almost every day, Diesel will usually pay-back.

13KW of Diesel seems to run $10K, $15, and up and up.

http://NorthernTool.com has pictures and prices for a wide range of gensets, from a 1KW job with a handle (looks like a beercooler with an AC outlet) to a 150KW V10 that self-tests weekly for $22K. Northern isn't real good about having what you want in-stock, but they do catalog a wide range and give you low-ball prices.

Northern does list the RV motor-home modules. These seem to span 4KW to 7.5KW, which will run "two 13,500BTU A/Cs with 1,500 to 4,000 watts reserve". They are boxed so you can slide them under your RV floor and sleep.

Oh: you could build a heavy sparker to run as slow, and quieter, than a Diesel. Nobody does. If you are going to invest that much money and metal, Diesels ARE more reliable. The spark system on sparkers is usually cheap and gives much trouble. A Diesel has potential trouble in the injection system, but can't be cheap so is usually quite reliable. Sparkers had carbs which were often junk; this point fades because sparkers now use injection systems, and Diesels are adding (unreliable) computers to their injection (for smog and knock control).

You want slow: I know a spark engine that is red-lined at 750 RPM. I understand it is not quiet. It is as big as a house, and pumps a lot of natural gas through the very long skinny pipe from Tulsa to NYC.

The difference in cost between a big slow engine and a small fast engine will buy a lot of sound baffling. That seems to be the trend. And in your situation, $1K of feeder cable will buy a lot of sound loss through distance away from the mastering and sleeping suites (assuming you have the land).

There is another trend. Traditionally an alternator HAD to run 600, 1,200, 1,800, or 3,600 RPM to stay on 60Hz. 3,600RPM is an excellent speed for a 20HP spark engine. But that means when all the A/C kicks out and the lights are dimmed, the engine is still roaring along at freeway speed. However, new/smarter power converters make it possible to slow the engine at low load, yet still stay locked on 60Hz. This could be a big difference when you don't need your entire rated power. My Honda sedan cruises much quieter in 5th at 2,500RPM than it does in 4th at 3,500RPM. Power is 2/3rd but noise may be half, even flat-out.

Ya know, that's an interesting comparison. You already have a lot of 13KW engines going past the house: cars on the road cruise around 20HP. On many cars, the tires make as much noise as the engine. If you can tolerate that, it should be possible to tolerate an enclosed engine, stationary (no Doppler), half as far away as the road.

You could just move. Here outside NYC, on a former rural electric supply, I have not lost power more than 4 hours in the last decade. Knock wood.... last year the town north of me was out for 3 days, and the podunk electric company was in deep trouble. They'd run lines through trees for years, hadn't kept up the pruning, or the redundant routing, and one freak windstorm overwhelmed their repair crews.
 
[quote author="PRR"]You want slow: I know a spark engine that is red-lined at 750 RPM. I understand it is not quiet. It is as big as a house, and pumps a lot of natural gas through the very long skinny pipe from Tulsa to NYC. [/quote]

Oh yeah, that pipeline. Like most of the pipelines that feed the northeast, it runs right near the New Madrid Fault area in southern Missouri. When that fault finally lets go, the northeast is going to have a major-league natural gas shortage. (And it will let go; the largest earthquake ever recorded on the North American continent happened on that fault, back in I think 1809. Rang bells in Baltimore, and was felt in Boston.)

Bob, you have my sympathies. Based on some very limited experiences with radio and TV transmitter supplies, I'd second what people are saying about going diesel. Put the tank well away from the house.

Peace,
Paul
 
> Is 2-phase standard (tranny to house)in America?

240VAC, center-tapped, 3-wire.

This lets us cling to 120V gear, with 240V economics, almost.

> that should yield some thick wiring/cables.

Not really. You have a 12,000 Watt load, on your 2-wire(?) feeder, you need 50 Amp cable. We have 12,000 Watts, we need 50 Amps in the two "live" wires.

The main difference is: you can't have an unbalanced load. We can. I can put 12KW of 120V lamps on all my even-numbered breakers, and pull 100 Amps in one live wire and the Neutral. But that is highly unlikely: I know I could not find 7KW of 120V loads in my house. 120V loads over 1,500 Watts are uncommon, not-quite illegal. The cooker, the hot-water, the big air conditioner (even large window A/Cs), the professional table-saw are all 240V. So just the lamps, TV/hi-fi, PCs, and the occasional spot-heater or toaster give unbalanced loading. And the breakers are supposed to be distributed across both sides of the 240V/120V split so that random plugging of 120V loads will tend to give a balanced 240V load. The Neutral wire, our third wire, is rarely sized for full line current. Traditionally it is the wrapper around the two live wires, and is sized for strengh and protection as much as for current.

One way we do have a lot more wire: we expect a lamp-cord to blow the main fuse. That means general lighting circuits are limited to 15 or 20 Amps, 1,440 or 1,920 Watts. In England I understand they will run a 20A or even 50A ring-main around the house, 4,000 to 11,000 Watts in one run, and fuse lampcords (or just ensure they burn-up without starting fires). So we have a lot more runs, at higher total rated current, and more copper in the walls. I think.

The line from my meter to my fusebox is two round stranded conductors, plus a wrapper. Each live conductor is 10 millimeter diameter Aluminum. I think if it were Copper it would be 70% of this diameter? Do you have 7mm copper for a 3-bedroom house? My wrapper, the Neutral, which carries stray unbalanced loads, when bunched-up for the clamp, is also 10mm Aluminum, so I do have full-size Neutral.

One side effect of 240V center-tap grounded instead of 240V one side grounded is that a fault in one conductor's insulation only exposes us to a 120V shock. We can use cheap rubber. In fact this comes from days of NO good insulation, and Edison guessing that 100V was reasonably "safe". Insulation improved, the economics of higher voltage are compelling, and we've snuck-up 0.2 Volts per year for a century. Irregularly: I get 110V at work, was getting 125V at home.
 
Ah, i see....so all the big loads feed on centertapped 240V?

You have a 12,000 Watt load, on your 2-wire(?) feeder, you need 50 Amp cable

Well, actually.....not going to far back in the distribution chain;
10kV 3-phase into transformer on the pole.
Secondery winding Y-connected, 230/400V 3-phase.

3-bedroom house......hmmm, most are fused 3X16A. Some 3X20A.

So we got a 3-conductor+PEN cable going into the main breakerbox in the house. (Most new installations use a separate N conductor and PE for better EMI-numbers.)

Inside the breakerbox the PEN splits up into PE and N.
All the big loads such as heaters, stove etc. are connected 3-phase.
Outlets 230V singlephase.

Im not going to use your 12kW example because it puts us in nowhere land, right between 16A/2,5mm2 and 20A/4mm2.

But lets work on a 16A fuse:

P=U x I x sqrt3 x cos.

Ignoring cable Z and cos. for inductive loads:

P=400V x 16A x 1,73

That gives about 11kW on 3 x 2,5mm2 + PEN/screen.

The main difference is: you can't have an unbalanced load. We can.

I dont get that.

We can use cheap rubber.

Cheap rubber=low voltage= high current=expensive copper.

But in the end, its hard to say whats better. It becomes a matter of cost VS. safety.
It would be idiotic to feed 230V across the country, but at the same time no one wants kV´s in their outlets.
This is why we dont have a DC distribution net, BTW.
 
Thanks for the great post, PRR.

On a separate subject, I'm still ignorant about how to know there are new posts to a topic. If I hadn't received an email I would not have known about PRRs post here. And when I logged in, the topic was either light or bold, whichever indicates that there are no new messages... I'm just an ignorant guy when it comes to mastering how to use this Prodigy board....
 
Hey Bob

When you login, and enter into any of the places from this forum (Drawing Board, The Lab, Black Market, Brewery) you will see at the left side of any thread a circle with a smal trapezoid thing inside.
example.gif

All the threads that have new posts since your last visit (visit in this case means you have to login) will be indicated by the trapezoid thing in orange colour. If there are no new posts since your last visit the trapezoid will be white. You will not receive any email notification.
Hope this helps.

chrissugar
 
[quote author="chrissugar"]Hey Bob

When you login, and enter into any of the places from this forum (Drawing Board, The Lab, Black Market, Brewery) you will see at the left side of any thread a circle with a smal trapezoid thing inside.

[/quote]

Oh yes, thanks, Chris. I've seen that symbol on other boards. That does it!

Bob
 

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