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Electrical power requirements question

ron in charleston on Sat July 16, 2005 2:08 PM User is offlineView users profile



I want to buy another generator to power at least one of my house central A/C systems. I’m not sure how big a unit I will need; generators are rated in kw and also max amps and running amps.

I’ve looked at the plates on both of my A/C systems units and the one for downstairs which is one unit says the following:
208-230 volts
197-253 voltage range
16.4 minimum amps
20 fuse amps

The upstairs system is a split system with an air handler in the attic, it says the following on the plate for the part of the unit outside:
208-230 volts
197-253 voltage range
19.3 minimum current amps
30 maximum fuse.
2.6 amps for the air handler in the attic, label also says 40 amps but I think that is for the heat strip thing that I’m not concerned with since I’m dealing with A/C.

Now the downstairs unit cools more space but it seems to require less amps to operate.

My questions are as follows:
Do I need a generator with the running amps rating equal to the fuse amps or the minimum amps?

Are minimum current amps the amps that are need to start the unit or keep it running once it’s started?

Karl Hofmann on Sun July 17, 2005 4:12 AM User is offlineView users profile

What is this? Dont you guys have electricity over there?

-------------------------
Never knock on deaths door... Ring the doorbell and run away, death really hates that!

ron in charleston on Sun July 17, 2005 8:22 AM User is offlineView users profile

Nick,

That backup generator is real nice but I don't want to spend that much cash. I'd be happy with the small 3.6 kw one I have but my wife wants one to run A/C so I figured I could get another generator and use it just for one of the central A/C units.

It seems like if I get a unit that has a continuous amp output of at least 20 amps for one of my units or 30 amps for my other unit I would be fine. Is that right?

Multiquip makes a 6kw unit that has start up watts of 6000 and continuous watts of 5000. The amps for this unit at 240 volts are maximun of 41.6 and continuous of 20.8 With that unit I could run my downstairs A/C. My other small 3.6kw unit connects to the fusebox and runs the refrigerator and other small items.




Quote
What is this? Dont you guys have electricity over there?

Actually we do have commercial power. But we have been know to have bad storms more often than we like; we call them hurricanes. With that in mind many people like to have something that will let them live a little more comfortably until commercial power is restored; that's were the generators come in. Generators also are necessary to make repairs if you plan on using power tools. The longest I've had to use my generator was about 2 1/2 weeks and I sure was glad to have had it.

meaux on Sun July 17, 2005 12:33 PM User is offlineView users profile

I'm thinking about gettin a "LP whole house generator" because, ever since I bought a 5550 portable after Ivan, (ten minutes after I hooked it up, the power was restored) I haven't had to use it...the power never even flickered during Dennis....so, if I had a "whole house", we may never get another Hurricane!!! :-)

-------------------------
Lazy bum who lives off his wife.

01 BMW 530i Sport, 92 Porsche 968, 85 F150, 72 911, 08 GM SUV, 01' Ford Lightnin'

bohica2xo on Sun July 17, 2005 12:46 PM User is offline

Ron:

You can buy a little diesel 10kw unit Like This for about 4k. Certainly enough to run the A/C units.....

When I lived in rural CO, I had an ice storm knock down a bunch of power poles. I was without power for 9 days, and it was january @6200 ft asl. I had a generator big enough to run my deep well pump, and live comfortably. My neighbors did not - none of them had water, and some were without heat.

All my computers run through a UPS - another lesson from REA power. Blown up small appliances were common out there, the power spikes & lightning strikes were awful. People had huge MOV banks that plugged in as a sandwich between the meter and the meter socket. I had a fax machine crap out - opened it up and the fuse was torched, and the MOV was in pieces.

Ron if you are still loking for a frequency meter, I discovered that I have a spare. A JBT brand I think, used with about 1700 hours on it. about 3 inches across. 150 volt meter to match it. Both removed from a military genset we installed permanently. Let me know if you are interested.


.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Dougflas on Mon July 18, 2005 4:28 PM User is offline

I would not recommend buying that Mep 003 military generator for 4K. Parts are hard to find. The system control is overkill and no one will service the electronics. We have a MEP 006 (60kw) at the SHeriff's office and I have to manually monitor the Frequency/speed according to loads. The thing is a diaster to work on..hydraulic governor, etc.

Instead, there is a 15kw continuous(22kw peak) generator out there selling for $2200.00. That will run your A/C and other ckts.

k5guy on Tue July 19, 2005 12:34 AM User is offline

Quote
Originally posted by: ron in charleston
I want to buy another generator to power at least one of my house central A/C systems. I’m not sure how big a unit I will need; generators are rated in kw and also max amps and running amps.

This generator should work for you. Especially in bad weather when you can't buy fuel.



-------------------------


Send me e-mail

JJM on Tue July 19, 2005 1:31 PM User is offline

NickD and Bohica,

Yes, I did install a standby generator and cannot thank both of you enough for your advice, help, and expertise in planning and installing the system.

I don't know if I posted the final installation pics of the genset, transfer switch, panels, and so forth, but when I get home tonight, I'll be sure to post them so you can see the results of your planning "in action" so to speak. You guys are the best!!!

I went with the Onan RS12000 genset and GE ZTX20RM4 200 amp automatic transfer switch, configured to power the entire house load -- not just emergency circuits. This genset powered just about everything I could throw at it without any problem. I ran a 240V 18,000 BTU air conditioner, as well three smaller 120V air conditioners, 5,000 BTU, 6,000 BTU, and 8,000 BTU, not to mention the refrigerator, oil furnace, dehumidifier, attic exhaust fan, three computers, two televisions, toaster, microwave, and just about every single light in the house, you name it (I really wanted to push it for the purposes of testing) without any problem whatsoever -- at a steady 60 Hz. At one point, I was reading a total of 50.6 amps on one leg, and 46.9 on the other leg with my Fluke clamp meter (also thanks to your advice). For the first time, I actually read nice strong voltage too: 240-242V and 120-121V! With our local utility, I'm lucky to get only around 201-218V volts and 106-113V, sometimes even less! I needed to adjust the jumpers in the transfer switch to 220V, so the genset wouldn't "talking over" for the bad power.

Funny you guys should talk about generators over the weekend, as I just did the annual service on mine this weekend. Only 8.3 hours on the unit -- half from the break in, the rest from auto monthly exercising and showing friends and neighbors how it works. I picked a really hot and steamy weekend to do it no less.

As Nick mentioned, there was a quite a bit of problems with the permits and approvals (NYC). I also had a lot of problem problems due to low gas pressure, inherent with natural gas systems, resulting in starting problems. I was only getting 5" WC of gas pressure, and the genset requires 239,000 BTU's at 11" WC nominal, with a 7 - 15" range. The utility wouldn't bump the pressure up at the primary regulator, nor would they install (or allow) a secondary higher pressure regulator for the genset - which is typical with genset installations requiring elevated pressure.

The low gas pressure, believe it or not, was causing the engine to "flood" since there wasn't enough pressure to close the regulator diaphragm. The engine would start fine when cold, but subsequent warm restarts resulted in long cranking times, causing the controller to time out and fault. If I backed off the throttle while warm cranking, engine would start right up. Onan was aware of this issue, and revised the governor controller to crack the throttle open only part way (instead of wide open) during cranking, which they replaced under warranty (out of warranty would've been $655.00, yikes!). The engine now restarted "ok" when warm, but not smoothly as when cold. I then came up with my own fix: replacing the factory regulator spring (3 - 6" WC) with a 1 - 3.5" WC spring. No more starting or fuel flow problems, even at full load!

The only other problem I had was the shrubs the gardener planted to conceal the genset (some stupidly placed in the exhaust path) wound up getting singed and later died just after the first exercise.

By the way, I even went out and bought two 30# propane tanks (also have two 20# for the BBQ) and an adjustable regulator, just in case we lose power and natural gas. All I have to do is install and orifice in the fuel line and adjust the carburetor (and put the original regulator spring back in) for it to run on propane, and I'm ready for anything!

Granted my house is nowhere near as big as Bohica's, so for central A/C I'm considering the Carrier two-stage 38TBD 037-30 (37,000 BTU) with two evaporators, one for the main floor, one for the basement (the house in only 1,100 SF). It has a running load of 14.1 amps, but a locked rotor load of 66.0 amps -- beyond the genset capacity. Bohica gave me the idea of a three phase inverter to cut back on the starting amp draw, but this unit does not come in three phase. I think I should be able to start this unit, however, with a hard start kit and delay for the compressor, and a further delay for the air handlers, this way the condenser fan starts first, then the compressor (with extra kick) and then the air handler. You guy's think this will work? I'll probably need MrBillPro's advice on this one.

If the genset still can't start the central A/C, unit the transfer switch has contactors which I can use cut the central A/C when the genset is on. I'll just run the 18,000 BTU wall A/C (which I'll keep even after the central A/C) and lug in the window shakers.

Once again, I can't thank you guys enough for you help on this one!!! And I'll be sure to post the pics for all to see.

Joe

JJM on Tue July 19, 2005 2:19 PM User is offline

Ron,

Technically speaking, you need a generator that has enough capacity to handle the LRA (Locked Rotor Amps) of any motors in question, which can be anywhere from 3 to 6 times the running amps, as well as enough capacity to carry all your existing "static" loads. That is the only guarantee that the genset will start and run everything you need during a power outage, and the "correct" way to size a generator.

However, "going by the book" you're gonna end-up needing needing a huge generator, and you'll likely have so much extra capacity you can probably run extension cords to all your neighbors to run their stuff. With my little 12 Kw unit I've got power the spare if I don't go crazy, so I gave my elderly neighbor two extension cords she could plug into the outlets on the side of my house so she could power her fridge, a few lights, and the A/C she would desperately need for her health. But don't forget, I don't have central A/C as of yet.

Your downstairs unit requires roughly 3,608 running (16.4A x 220V -- using 220V for an "average" rating) and your upstairs unit requires roughly 4,246 running watts (19.3A x 220V). Even with hard start kits and time delays (another great idea from Bohica) I doubt a 6,000 watt generator will do it, but an 8,000 watt (at 240V) just might. Perhaps you can rent one for a few hours to try (after installing the hard start kit and time delay).

Keep in mind dealers will always try to sell you oversized units because they don't want to have to deal with the unit not meeting your needs. Most dealers told me my 12 Kw whole house configuration would never work, that I needed to install a separate emergency panel without A/C loads, and I'd be lucky to start just 1 window shaker. They all wanted me to go for a 20-25 Kw unit -- for an 1,100 SF house! And I've got oil heat and gas appliances! Meanwhile, I'm running four A/C's with a total of 37,000 BTU's and virtually every appliance in the house without any problem.

When I get around to doing my central A/C as described previously, I'll certainly let you know what happens -- unfortunately it doesn't look like this year though.

Joe

JJM on Tue July 19, 2005 11:57 PM User is offline

Well NickD and Bohica, here's what you helped me with...

Generator itself:



Note the shrubs on the left... they're in shrub heaven now after only one exercise.



Note: The LB and galvanized pipe on the left side coming out of the house is the grounding conductor. NYC requires grounding conductors to be protected in metal piping (galvanized) and bonded at both ends. This was the same for the cold water pipe ground inside too! PITA.

Pipe work:



Left to right:

- 1" aluminum conduit for current carrying conductors (#6) and ground (#10)
- ½" conduit for control wires (#12)
- ¾" galvanized gas line

Here's the whole electrical setup:



I know, those walls need a fresh coat of paint! In any event, left to right: 200A 40 breaker panel, 200A automatic transfer switch, 60A service disconnect, 200A main service disconnect, 200A meter pan. Below the main panel is a junction box for the X-10 amplifier, and next to it is a Leviton whole house surge suppressor. (Structured media center is on the other end of the house.) I know Bohica is really into this stuff; I'd love to see pics of his electrical room!

By the way, that big J-box next to the water meter (circa 1952) that the electric meter is connected to, that feeds four of my neighbors! I get the power first from the street!

Closeup of the automatic transfer switch guts:



Yeah, I know the red in black are reversed (came into the meter pan that way) but does it really make a difference on single phase? That little black box mounted on the inside lower left, that's the trickle charger for the genset starting battery.

Another closeup of the automatic transfer switch (low voltage side):



Note the neutral is switched -- a "separately derived" system.

Closeup of the 60A disconnect:



Since this is a separately derived system, notice the neutral bonding there. Usually the disconnect is placed outside the building, but my electrician thought that would be stupid, since anyone can just come along and pull the disconnect, and bye bye power.

Well, that pretty much covers it... and again my sincere thanks to NickD and Bohica for all their expertise. You guys have probably forgotten more than I'll ever know!

Joe

P.S. Power started flickering a bit tonight with all this heat, checked the voltage -- 201 VAC unreal!

Bigchris on Wed July 20, 2005 7:17 AM User is offline

Beautifully done Joe! I've seen commercial installations (across the river in Jersey) that weren't anywhere near as neatly done! Those conduit runs are some of the neatest plumbing work I've seen too.

With the kind of voltage variations you mentioned I wonder if you've considered adding UPS boxes to protect your delicate electronics? I use a couple of APC Smart-UPS units to protect my computers and AV gear and in the last five years I've not seen a single glitch from power surges disturb either one.

bohica2xo on Wed July 20, 2005 12:38 PM User is offline

Beautiful install Joe!

Glad you got it all worked out. I remember the meter that was installed in paralell, and the j-box from hell - what a mess. If your voltages are that low, just imagine what the house on the end of the straw sees....

Thanks for the compliments, but I don't recall flying to NYC and bending that conduit.... Your work, and very nice work at that! Glad to help the little bit that I did.


As for UPS and protecting electronics, I am sure Nick has a boatload to contribute to the subject. I have only the real-world experience of REA power and lightning.

Good surge protection is important. In my experience most computers and A/V stuff will deal ok with low voltage. The switching power supplies don't squawk about 100v inputs, and will usually just drop out if it goes too low. I never lost a TV to low voltage either, but it may have been luck. Motors on the other hand are not happy with low volts.

My computer network runs on a SOLA transformer, hard wired in at the breaker panel. I just used a large (5kva) unit and put the entire room on it. No backup power, but I have never lost any data either. The faint buzzing noise is in the garage with the power panel. When I had a shop full of CNC machines, ALL of the machine controls were on SOLA's - the service techs thought it was a little nuts, but the never replaced a board either...

I had a whole-house surge protector in CO, sandwiched between the meter & socket. Power company put them in, you paid for the protector, and install was free. It was nothing but a fistfull of MOV's wired phase to ground / phase to phase. I am sure Nick can help with some part numbers if you wanted to make a similar unit for hard wire install. My surge protector ate it once, when a nearby lightning strike took a 12.5kv transformer off of the pole. A couple of my surge protectors inside died too, but nothing else was damaged. Without the big protector, I suspect I would have needed a new refrigerator.... my neighbor lost a diswasher that was running at the time of the strike.


Good to see the pics of the finished product. Thanks for sharing them with us.


.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Dougflas on Wed July 20, 2005 4:43 PM User is offline

If you use a UPs on a generator, you need to pay a little more and use a SMART ups. These are more frequency sensitive. THe regular UPS may not recognize the generator as a power sourc and will stay on battery mode. It'll say to itself, " The power has too much trash in it, I better stay on battery". SEEN it, been there, went smart.

JJM on Fri July 22, 2005 1:10 AM User is offline

Thanks for thumbs up on the install! Coming from you guys, that's the greatest compliment!

I have to fess up though on the pipe work. I must have went ruined a half dozen conduits on just those two runs from the top of the ATS and 60A disconnect. The two conduits (not easily seen) for the grounds coming off the 200A service disconnect, weren't easy either -- a lot pipe ended up in the trash. But I had to get it right. And if electrical work isn't done in a neat and workmanlike manner, in my opinion, it ain't done right!

And yes Nick and Bohica, your contribution to this project was far greater than you're giving yourselves credit for.

Nick, as for the efficiency of this thing, you're right, the efficiency is horrible -- probably because most of the energy is being lost trough heat. This damn thing gets hot -- very hot -- which is why two of the shrubs were reduced to timbers after just one exercise. And you should hear the gas meter howl from all the gas this genset sucks out, accompanied by the dials spinning around and the sound of money leaving my checkbook. I figured out running this thing for a month would probably run about $1,100 to $1,200, so my $445 electric bill now seems like a bargain.

The reason why I'm considering a 3-ton Carrier Infinity is I'm using the old 1 ton for every 500 SF (I know, that's not Manual J). So I figure with my 1,100 SF house (with only about an inch of insulation in the attic and walls, circa 1952) 2-tons is enough for the main floor, and 1-ton should be enough for the cooler basement. The beauty with the Carrier Infinity is I can probably get away with oversizing a bit, since the compressor would run at low speed most of the time to provide good dehumidification, and run on high speed only on those very hot days, or when rapid cool down is required. It's also important to note I like it 68F in the summer.

I can't believe you've only got 3-tons to cool 4,000 SF. You must have Manual J calculated the hell out of it, but then again, you're an engineer so that's to be expected. My friend has a 2,400 SF house, built in the 1960's, with a 5-ton unit and I'm not impressed with its cooling. You must have designed your own system, because I don't know of any typical HVAC contractor that could make that work.

As for the fluctuating power I get from the utility, I haven't had much of a problem with any of my electronics, though my APC Back UPS Pro units do often go into "Smart Boost" as the software would indicate, only now I don't know anymore since the software that came with these units doesn't work in Windows 2000, XP or ME.

It's shame the genset puts out better power than the local utility.

Bohica, speaking of the local utility, it's amazing what they get away with. We get our chops busted over the silliest of code requirements, but for them, even the most basic stuff goes out the door. A 200A service upgrade (from 100A or even 60A) is simply a matter of changing the meter pan, no upsizing of the feeders on their end at all. With that J-box you saw in the photo, this is what's going on in there:

- The power coming from the street enters the box with three #2/0 ALUMINUM conductors.
- Each of those three #2/0 ALUMINUM conductors are bugged to three #4 ALUMINUM conductors (isn't #4 aluminum rated for only 50A?).
- One of those three #4 ALUMINUM wire sets is bugged to my #3/0 copper, then goes to my meter as shown in the photo.
- The other two #4 ALUMINUM wire sets goes out to my neighbors on the left and right of me.
- Then each of those neighbors feeds their neighbors next door (piggybacked in the J-box in their houses) also with #4 ALUMINUM! And if they have 200A service, they've still gotta connect #3/0 copper to the #4 ALUMINUM.

Does this make any sense? So much for code.

Anyway, thanks again guys for the good grade and all your help.

Joe

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