Converting KW/hr to amps

[K8MHZ]

Time flies. It doesn't care if you are having fun or not.

BTW, from context I understand what you mean, but when you write "kW/hr" it means kilowatts per hour, which doesn't make sense. Kilowatt hours is power and time multiplication, not division.

Kilowatt hour = kWh

I am sure most of us know that, but it hasn't been used much in this thread.

 

[K8MHZ]

Really, the only way to get an accurate peak figure is to either do a load calculation, or a measurement with all the loads turned on.

Be advised that the generator can only supply a certain amperage per leg, which the load calcs wouldn't show. A test is always better.

 

[gadfly56]

I suppose that's possible, but it's not likely. Using a little common sense, odds are the air conditioning and water heater loads were on 24 hours. Most everything else was off while people were sleeping (except a few security and night lights) and peak usage was around dinner time when large heating appliances in the kitchen were being used. So it's not crazy random.

I recall a discussion here that suggested unless you have automatic load shedding at the ATS, you have to size the generator for the connected service, depending on jurisdiction. Comments, anyone?

 

[kwired]

I recall a discussion here that suggested unless you have automatic load shedding at the ATS, you have to size the generator for the connected service, depending on jurisdiction. Comments, anyone?

No. Many 200 amp services out there that never see 200 amps. Some maybe never even see 100 amps.

702(4)(B) says "The calculations of load on the standby source shall be made in accordance with Article 220 or by another approved method."

Further down the page (B)(2) says "Where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system." This is your load shedding module(s).

 

[gadfly56]

No. Many 200 amp services out there that never see 200 amps. Some maybe never even see 100 amps.

702(4)(B) says "The calculations of load on the standby source shall be made in accordance with Article 220 or by another approved method."

Further down the page (B)(2) says "Where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system." This is your load shedding module(s).

If there is no load calculation, and it appears that the OP is not in a position to do one, how can you claim with certainty that the load is less than 225 amps? If you haven't calculated it, you don't know it. The only thing you know is that the service is rated for 225 amps.

The OP also indicated that there will be NO load shedding, sinking the 702.4(b) exception.

 

[kwired]

If there is no load calculation, and it appears that the OP is not in a position to do one, how can you claim with certainty that the load is less than 225 amps? If you haven't calculated it, you don't know it. The only thing you know is that the service is rated for 225 amps.

The OP also indicated that there will be NO load shedding, sinking the 702.4(b) exception.

Then you bid a 60kVA generator just to make sure you can deliver 225 amps. In meantime your competition does find a way to figure out the load is under 15 kVA and wins the bid with his much less expensive 15 kVA generator:happyyes:

Now maybe they don't tell him about that big addition they want to add and the generator can't handle it once it is added:blink:

 

[texie]

My only worry is that I give the HO what they want which is the ability to turn on all they normally turn on during a power outage. Money is no object for these people and if I said to get a 225 amp generator they probably would, but I wouldn't feel like I did my job if I let them waste that much money. I'm sure my recommendation will have a healthy reserve capacity. Just not crazy.

A couple of thoughts, especially if you are new to the genset game. Not all gensets are alike, even with the same KW rating. You are likely looking at resi grade machines such as Generac, Briggs and the like that are not exactly real robost machines to put it kindly. This class of machine also typically have fairly poor voltage and frequency regulation. Keep in mind that as a general statement a genset is a "soft" source compared to a "stiff" POCO source and as such can lead to issues, especially when starting inductive loads. This is especially true with these inexpensive machines. There are a lot of factors in the design of a genset that affect the ability to start inductive loads. Having a genset heavily loaded and then suddenly a relatively inductive load starts can lead to unacceptable results.
I would not, especially with an air cooled, 3600 RPM resi grade rattle trap, design to close to it's name plate. And do not underestimate the impact of inductive loads.

 

[junkhound]

... ability to turn on all they normally turn on during a power outage. Money is no object for these people .

Last system I worked on with similar 'very rare' parameters (especially the 'money is no object 'and everything has to work" criteria) was Minuteman silo facilities 50 years ago.
Site normal power was via 480-120 3 phase 45 kVA service -- installed 175 kVA diesel generator ---PLUS a few tons of big ol' led acid batteries and rotary dc-ac converter. Plus another DG set upstairs :happyyes:!!


HO has 225A service nominal = 54 kVA. Pretty sure your HO loads not as critical as a NW site, so a thumb in the air I say sell them 100kVA gen set. The automatic switchover and maintenance over the years will greatly outweigh the initial costs.
Is the HO opting for automatic twice monthly 1 hour running of the genset and automatice refueling service ? regular diesel servicing? etc. ?
If not, once every 5-10 poco service outages the genset will likely fail to start anyway :huh:.

Make sure the HO understands the service, maintenance, regular exercise, etc. etc. implications. If it fails, do you get blamed or reputation tarnished?

That all blathered out, own home has only a 10 kVA gen set driven by 50 HP engine that is 'often neglected'. Last power outage here had to jump the battery to start it
10kVA sufficient to start 4T heat pump if started at a judicious time when clothes dryer or range not operating, etc.

 

[Russs57]

I see you are in Tampa. This means hurricanes. That matters because now you need something that can run for extended times. You need to be concerned with fuel supply. You need elevation above any possible flood level or storm surge. You need physical protection from anything from wind blown debris to falling trees.

Having been through the likes of a hurricane Andrew I would want something that I could count on to last me a month. So big no to 3600 rpm rattle traps. Generous fuel supply, like a huge propane tank buried in the ground. Possibly a concrete enclosure. Maintenance bypass on ATS (mounted inside) so homeowner can switch over if ATS brain dies. Constant float chargers, dual staters and two battery banks. Probably inverter based genset for fuel conservation and clean power/good regulation.

Money is no object is code speak for it better always work and it better not blow up my sensitive electronics. And don’t say anything about reactive nature of my loads, even if I add something like a traction elevator!

I would pay for a load study and factor in that they could have friends and family there. But you don’t want a wet stack problem so make sure you have at least a 30% load.

Way more than you wanted to hear I reckon.

 

[kwired]

I see you are in Tampa. This means hurricanes. That matters because now you need something that can run for extended times. You need to be concerned with fuel supply. You need elevation above any possible flood level or storm surge. You need physical protection from anything from wind blown debris to falling trees.

Having been through the likes of a hurricane Andrew I would want something that I could count on to last me a month. So big no to 3600 rpm rattle traps. Generous fuel supply, like a huge propane tank buried in the ground. Possibly a concrete enclosure. Maintenance bypass on ATS (mounted inside) so homeowner can switch over if ATS brain dies. Constant float chargers, dual staters and two battery banks. Probably inverter based genset for fuel conservation and clean power/good regulation.

Money is no object is code speak for it better always work and it better not blow up my sensitive electronics. And don’t say anything about reactive nature of my loads, even if I add something like a traction elevator!

I would pay for a load study and factor in that they could have friends and family there. But you don’t want a wet stack problem so make sure you have at least a 30% load.

Way more than you wanted to hear I reckon.

Better build a bomb shelter for a house as well - does no good to go above an d beyond to assure you have power for a building then the building gets blown away in the storm

 

[JFletcher]

I agree with russs57…... Some other things to consider beside kilowatt rating is the type of fuel you're going to use. From what I read this morning, Natural Gas in Florida for residential use is second only to Hawaii in terms of expense, around $20 per thousand cubic feet... A 60 kilowatt generator would use $10 an hour in fuel. Dual fuel generators that can use propane also do not have the same kilowatt rating on both fuels, so you have to size based on that. Diesel generators with integral tanks may not have acceptable run time, especially in a hurricane... One model I looked at had a 61 gallon tank with a 40 hour runtime at half load. Having to buy a large propane or diesel tank and have it installed will add more cost.

Afaik, wet sumping only affects diesel generators.

And if your client happens to be in area prone to flooding, it would be prudent to locate the generator at the highest point of the property, or build up some earth under it so it's not under water when you need it.

On an aside, why are the smaller less expensive generators 3600 RPM machines? Is that much more expensive to build a 4 pole 1800 RPM machine that would have substantially less wear and tear on the internals of the engine at the lower RPM?

The truth of the matter is that most residential optional generators, at least around here, will see very little use... my father has a full house generator, is out the country on well water, they lost power for almost a month when Isabel hit in 2003, and probably have run it for another month or two in the past 20 years. For what he paid for it, the maintenance, and the upkeep on it, he could have spent those three months in a $100 a night hotel room, replaced all of the food in the refrigerator, and still come out ahead that way.

If you live in a harsh climate, remote location, are disabled, or have medications that need to be refrigerated, a generator makes more sense than finding shelter elsewhere.

If your clients truly want a generator that can run everything they currently have, and perhaps future loads like an addition or a spa, and are willing to shell out upward of $30,000 for it, by all means. Maybe you can get them to shell out another 30 grand for a PV system while you're at it

 

[retirede]

On an aside, why are the smaller less expensive generators 3600 RPM machines? Is that much more expensive to build a 4 pole 1800 RPM machine that would have substantially less wear and tear on the internals of the engine at the lower RPM?

It’s not just the genny- it take a bigger engine to deliver the same HP at half the rpm.

 

[Russs57]

Kwired, homes down here are bomb shelters, especially ones built after hurricane Andrew. No concern about the structure surviving, just the roof.

Evacuation seems more sensible but isn’t feasible here unless you leave early enough and go far enough. Typically you left for no good reason. So people stop doing that and when the big one barrels down on you it is too late.

I would rather have diesel but don’t know what regulations a homeowner needs to abide by to have what I would consider an adequate supply. That is why I suggested propane.

 

[ggunn]

I agree with russs57…... Some other things to consider beside kilowatt rating is the type of fuel you're going to use. From what I read this morning, Natural Gas in Florida for residential use is second only to Hawaii in terms of expense, around $20 per thousand cubic feet... A 60 kilowatt generator would use $10 an hour in fuel.

A 60kW genny seems excessive to me; does a typical household need something that big?

 

[Ingenieur]

It’s not just the genny- it take a bigger engine to deliver the same HP at half the rpm.

effectively twice as big if rpm 3600 > 1800

T = V x Pme /(4Pi) for a 4 stroke, 2 stroke > 2Pi
V = displacement
Pme ~ mean effective press ~ comp ratio x atm press x vol eff

P = T x n (n = engine speed)
P = rpm/5252 x T (imp units)
assume Pme = same for both engines and include 4Pi, use k

P = rpm/5252 x k x V
so if rpm is 1/2, V must double for same P
or double the speed and double the power for the same engine

 

[JFletcher]

A 60kW genny seems excessive to me; does a typical household need something that big?

Not at all. Typical house needs between 16 and 22 KW. 54 and 60 KW units were referenced here to a post that mentioned a 225 amp service that was more or less fully loaded.

The only things that would overload a generator in the 16 to 22 kilowatt range is an on-demand electric water heater or multiple HVAC units/ electric back up heat. If either of those are present, they can easily double the size of the generator needed to run the house.

 

[Besoeker]

effectively twice as big if rpm 3600 > 1800

T = V x Pme /(4Pi) for a 4 stroke, 2 stroke > 2Pi
V = displacement
Pme ~ mean effective press ~ comp ratio x atm press x vol eff

P = T x n (n = engine speed)
P = rpm/5252 x T (imp units)
assume Pme = same for both engines and include 4Pi, use k

P = rpm/5252 x k x V
so if rpm is 1/2, V must double for same P
or double the speed and double the power for the same engine

So convoluted. Much simpler in SI.

But in either case, something of a digression form the original topic.

 

[Ingenieur]

So convoluted. Much simpler in SI.

But in either case, something of a digression form the original topic.

on the contrary, very straitforward
show us in SI, at least contribute something if you are going to contribute nothing lol
btw we don't use SI, the vast majority of forum members are in the US

 

[Ingenieur]

effectively twice as big if rpm 3600 > 1800

T = V x Pme /(4Pi)
V = displacement
P = rpm/5252 x T
assume Pme = same for both engines and combine with 4Pi
P = rpm/5252 x k x V
so if rpm is 1/2, V must double for same P
or double the speed and double the power for the same engine

very simple

 

[Besoeker]

on the contrary, very straitforward
show us in SI, at least contribute something if you are going to contribute nothing lol

P=Tω

btw we don't use SI, the vast majority of form members are in the US

I fully understand both of those points. Except that there are no Imperial units for Volts, Amps, etc. so you have no choice for those.

But this is off topic so let's stick to that.
Put simply, there is no direct conversion for kWh to Amps.