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disconnected

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reply to nonymous

Re: Is this calculator accurate? Amp demand for residence

In our household, the bulk of the average load is our PCs. My two PCs draw 1492 VA, according to Kill-A-Watt P3. My 30" monitors vary depending on brightness. I keep them at minumum which draws 64W. At full brightness, they draw 240W. The PC CPUs are the main hogs though. Each PC can heat the room in the winter. And 20 hard drives across the two PCs adds up to a lot of energy. Then there are the graphics cards..
But the real energy hog in my house only gets a workout about 4 hours a month. The sound system. One of the most powerful in the country, putting out a very conservative of 16,500 watts and able to draw 192 amps with a sine wave input at full power (not possible under normal circumstances). What that system is playing hard, the whole neighborhood's lights dim. Basspig.com has photos, for the curious.

Speedy Petey

join:2008-01-19
said by disconnected :

In our household, the bulk of the average load is our PCs.

You may think so but this is almost certainly not the case. Things like the vacuum, hair dryers and appliances all draw much more. People like to think their powerful computers are huge draws, but in comparison to other things they are still not that much. Also, the power varies wildly depending on function, just like your monitor's brightness.
Now, multiple (as in many) computers all working at the same time is a different story.

Bob4
Account deleted

join:2012-07-22
New Jersey
My PC normally draws 60 Watts. If I crank up all 8 threads and max-out the GPU, it draws 200 Watts. No big deal. My dishwasher draws over 5 times that.


Jack_in_VA
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join:2007-11-26
North, VA
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said by Bob4:

My PC normally draws 60 Watts. If I crank up all 8 threads and max-out the GPU, it draws 200 Watts. No big deal. My dishwasher draws over 5 times that.

I make sure I don't have many windows open at the same time on my PC to keep the power usage down. PC's can be expensive to operate.


Draiman
Let me see those devil horns in the sky

join:2012-06-01
Kill Devil Hills, NC
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said by Jack_in_VA:

said by Bob4:

My PC normally draws 60 Watts. If I crank up all 8 threads and max-out the GPU, it draws 200 Watts. No big deal. My dishwasher draws over 5 times that.

I make sure I don't have many windows open at the same time on my PC to keep the power usage down. PC's can be expensive to operate.

Anything that uses power can be expensive to use under the right conditions. More windows open is like using the radio in your car.

Mr Matt

join:2008-01-29
Eustis, FL
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reply to alkizmo
alkizmo: Many years ago I bought a house with a 100 Amp Service. I had the same problem you did. When I tried to run the dryer, washer, central air, water heater and range the main breaker tripped. I was drawing about 100 amps without the AC. Two electricians recommended that I upgrade to a 150 or 200 Amp Service. Here in Florida the main breaker must be installed outside next to the meter can. Upgrading to a 150 or 200 amp service required replacing the main breaker and enclosure and replacing and rewiring the load center. I solved the problem by replacing the meter can with a 200 amp unit with two sets of load lugs. I added another outdoor enclosure with 100 amp breaker and an additional 100 amp load center inside the house. I moved the water heater, washer, dryer and AC to the new load center. The problem was solved.

I wood recommend determining if it would make financial sense in the long run to replace some electric appliances with gas fueled appliances. My electric dryer draws 5.6 KW. If your water heater is electric it can draw up to 6 KW. Consider replacing it with a NG fueled unit. Look at the long term cost. Does electric or NG cost more per BTU. Your gas and electric supplier can supply you with cost comparisons. Make sure you also factor in efficiency.

telco_mtl

join:2012-01-06
said by Mr Matt:

I wood recommend determining if it would make financial sense in the long run to replace some electric appliances with gas fueled appliances. My electric dryer draws 5.6 KW. If your water heater is electric it can draw up to 6 KW. Consider replacing it with a NG fueled unit. Look at the long term cost. Does electric or NG cost more per BTU. Your gas and electric supplier can supply you with cost comparisons. Make sure you also factor in efficiency.

you have to remember here in quebec electricity is only a nickel a KW/h so the conversion to gas isnt a great way to save money. Almost all new construction in quebec, even with our harsh winters, is all electric, the main heating source being baseboards just to give you an idea how inexpensive electricity is here.


Hall
Premium,MVM
join:2000-04-28
Germantown, OH
kudos:2
reply to alkizmo
You may have said this already, but are you tripping breakers ?


alkizmo

join:2007-06-25
Pierrefonds, QC
kudos:1
said by Draiman:

Anything that uses power can be expensive to use under the right conditions. More windows open is like using the radio in your car.

I think he was joking (I hope)
Windows open on a PC uses no extra power. I like to think of it as joking that he's losing heat by leaving windows open

said by Mr Matt:

I solved the problem by replacing the meter can with a 200 amp unit with two sets of load lugs. I added another outdoor enclosure with 100 amp breaker and an additional 100 amp load center inside the house. I moved the water heater, washer, dryer and AC to the new load center. The problem was solved.

I hope this was done by an electrician who checked that your service lines were rated for 200A. In my case, the service lines are only good up to 125A.

said by Mr Matt:

I wood recommend determining if it would make financial sense in the long run to replace some electric appliances with gas fueled appliances. My electric dryer draws 5.6 KW. If your water heater is electric it can draw up to 6 KW. Consider replacing it with a NG fueled unit. Look at the long term cost. Does electric or NG cost more per BTU. Your gas and electric supplier can supply you with cost comparisons. Make sure you also factor in efficiency.

My water heater is NG as well. If I had an electric water heater, I'd definitively need an upgrade to at least 150A.

said by Hall:

You may have said this already, but are you tripping breakers ?

As I have said before, no I am not "tripping" (popping) the main fuses.
However the situation I've mentionned where multiple heavy loads are active is very new. My in-laws arrived from China two weeks ago.

Look at it this way:

There are only a couple of electrical appliances that operate the same regardless of how many adults are in the house (AC, lights, TV/PC, fridge, washer/dryer and dish washer come to mind)

Those are the potential unattended loads (For the 120V I will only add them to the total at 50%):

20A Dryer
10A (120V) Washing machine
20A AC(with blower)
5A (120V) dishwasher (Averaged 5A motor only & 15A heating element)
4A (120v) fridge
3A (120V) TV/PC
2A (120V) lights (Mostly CFLs but a few halogens)
-------
52A total of stuff that usually won't all run at the same time, but COULD.

With only my wife and I, we'd potentially use 35A (Range @ 25A + ONE 120V 10A appliance). So 35A + 52A we have 87A potential simulatenous load. That's a good buffer for LRA surges.

We now have 4 adults potentially using 55A (range @ 25A + 3x 10A appliances on the same leg). So 55A + 52A = 107A (Purely coincidental that I reached the result of the calculator).

Of course, again, this requires that all unattended loads be running at the same time. A fridge doesn't run non stop, the washing machine is mostly around 2AM (start delay). However the washer/dryer are MORE likely to be running (As there is twice as much laundry to do).

To summarize my point: Our electricity load has increased significantly, but it has only been 2 weeks. It's not enough time to say that we've experienced a moment where we were all attending heavy load appliances at the same time that unattended loads synchronized to run simultaneously (And survived).


whizkid3
Premium,MVM
join:2002-02-21
Queens, NY
kudos:9
reply to Hall
said by Hall:

LRA = locked rotor amp
That's the amount of amperage a motor will pull at startup, typically in the 3x range of normal pull. I'm not sure if you have to size circuit breakers or fuses for LRA or for normal load. I thought breakers or fuses were designed so they wouldn't trip for this scenario.

LRA is generally the highest instantaneous inrush current of a motor at startup. Its more like 6 times the full load (normal) running amps (FLA) of the motor, and can be up to 12 times as much or even more with high efficiency motors, which are much more common today. Breakers and fuses must be sized to accommodate both the normal running current and the inrush current of motors (or they will trip). For typical motors with an LRA of 6x of the FLA, an HACR rated circuit breaker or time-delay fuse rated at about 125% of the motor's FLA will work fine.

alkizmo See Profile, as for the demand calculations, the NEC (and other codes) base them not on the possibility that everything will be running at once; but rather on the probability that at sometime over a full year, for a contiguous period of 15 minutes, one will reach this peak load. 15 minutes is typically the time that it takes for a thermal circuit breaker or time-delay fuse to trip at its rated current. The NEC has come up with demand adjustment factors based on a very large range of data collected over a long period.

If you look at your electric bill, you may in fact see a monthly 'demand kW' measurement. This is very common for commercial buildings that must actually pay a demand charge related to the peak demand. It is much less common for residences. The 'demand kW' measurement is most typically a 15 minute 'peak' demand value. It is measured by taking a moving average, over a period of 15 minutes, of the power draw in kW; then using the highest 15 minute measurement over the month. I have reviewed the demand data for countless buildings over the years; as well as measured it locally on parts of buildings using a power meter. I must say the numbers come out fairly close to those of demand calculations as required by the NEC when taken over a year (or in the hottest part of the summer). See section 220.87 of the NEC regarding required methods of determining existing loads (for where the NEC applies).

As for motor start-up & demand, it hardly affects a 15 minute demand measurement; nor is it included in demand load calculations. The only times I have seen issues with motor start-up affecting demand to any degree of concern, is in large chiller plants of commercial buildings. When you have multiple chillers and pumps all starting up in the same 15 minutes, it can inflate the demand measurement; and also inflate the electric bill the building will pay for the entire month. This is much more likely due to the equipment running simultaneously at full bore trying to cool off the building (after it has been heating up all night), than it is a matter of the inrush. However, for this reason, it pays to stagger the start-up of chiller plant equipment and large motors in other similar situations.


alkizmo

join:2007-06-25
Pierrefonds, QC
kudos:1
Interesting info on what this calculation is about.

Considering how close the result is to my panel rating, I guess experience over a couple of years will tell if my 100A feed is enough. Sure enough though, I can't add any extra large electrical appliances.

Fortunately I have no demand kW measurement on my bill.

Bob4
Account deleted

join:2012-07-22
New Jersey
reply to alkizmo
nm