This is an Electric Project!!!! Need to be school on Compressors.

bob58o

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Somebody please tell me why I can or cannot run a 5k-8k btu window AC and a 35-50 pint per day dehumidier on a single 15amp 110-120v circuit. Both probably say to use a dedicated circuit if using other large appliances, but I want to find out how large "LARGE" really is.

Actual power measurements of similar sized units were showing about 400-600 watts for 5k-8k btu window units on full blast and about 550-600 watts from the dehumidifier. 5kbtu with CEER of 12.1 means it averages 413 watts right? 8kbtu, 12.1 CEER, 661 watts???

Actual In Home Power Testing shows right around 600 watts and 5.0 amps with 8kbtu window AC.... If starting current is 1.5X normal operating current then it would be 7.5 amps.

450 watts for a 50 pint dehumidier should be around 3.75 average amps..... If starting current is 1.5X normal operating current then it would be 5.6 amps.

So lets say if both are running, I am using 1050 watts and pulling about 8.75 amps. Cool.
If the starting current is 1.5x the normal operating current, then combined they would use 13.1 amps during starting. Cool.
If the starting current is 2.5x the operating current, the combined they would use almost 22 amps during starting. Is this cool?


My concern is about starting current. What happens if both compressors kick on at the same time? How long does starting current last? Milliseconds? Seconds? How do I find starting current (does it go by a different name? What is Lock rotor current vs full load current vs starting current ...? What is Rated Current? Seems to be larger than average current. -sorry i stupid

How much current can a circuit breaker pass for how long before it trips? Will a breaker trip immediately if a certain amount of current passed. Can a 15 amp breaker pass 20 amps for 1 second? 30 Amps? For 10 seconds? What about 60 amps for 1 millisecond? -double sorry still ignorant.

Will a breaker trip if both compressors kick on at same time?
Hopefully you can get an idea of where my head is at so you know how to best educate me.

Chart showing Actual Wattage vs Advertised Wattage

This model Dehumidifier

GE ADEL45LY

546 real world average watts (about 4.6 amps)
Advertised as 6.2Amps and 713 watts.

Frigidaire FFAD2233W1
Advertised as 3.5 Amps and 403 watts but pulls an average of 272 watts from the wall.

So these DH's use about 70-75% of the "Rated Power" on average during long term operation. Cool.

Now I'm looking into NEC code letters and HP and kVA/HP but getting a little lost.
Any help on starting current and how it applies to circuit breaker?
Thanks
 
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bob58o

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So I discovered time vs current trip characteristic curves for circuit breakers....
Seems about 5x the rated current for about 10 seconds before tripping. So a 15 amp could do 75 amps for 10 seconds? And a 20 amp could do 100 amps for about 10 seconds.

And looks like 1 second at 9x the rated current. So 135 amps for 1 second on a 15 amp breaker??
I think 10-20X the rated current causes a delay then a trip.
And more than 20x the rated current causes an immediate trip??

Does this sound somewhat reasonable?
I was looking at the first two images of time - current trip curves here.
 

bob58o

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Found LRA (locked rotor amperage) might be same as start up current.
LRA = 1000*(kVA/HP)/Voltage
Voltage is 120V
HP of a 6500 BTU Air Con about 2.5 HP
2.5 HP should have Locked Rotor Code around "L"
"L" means 9.5 kVA/HP

LRA = 1000* (9.5)/120 = 79 amps
Not sure how accurate that is, but makes "order of magnitude" sense.
However the same website has a calculator that when I input 2.5 HP, 120 nominal volts, and code letter "L".... it tells me max LRA@full voltage is 208 amps.

Still don't know how long the start up current will last. If you leave me here alone long enough. I will figure something out. LOL

Code Letter "L"
kVA per HP with Locked Rotor = 9.0-9.99
9.5 = kVA / HP
9.5 * HP = kVA
HP =2.5
9.5 * 2.5 = kVA
23.75 = kVA
killowatts =23.75
watts = 23750
Volts = 120
Amps = 23750 / 120 = 198 AMPS.
 
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bob58o

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So what I discovered (what seems to be a very simple model that might work for cases like this)....

For an appliance...
Inrush Current for first 1/2 cycle (1 second / 120 )=
Normal Operating Current x 20
This inrush lasts for one half cycle, which is 1 second / 120 (at 60Hz). = 0.0083 seconds
For the next 5 cycles (5 * (1 second / 60)) the inrush current will be 4-8x the normal operating current.
This smaller, but longer lasting Inrush current lasts for 5 seconds /60. = 0.083 seconds.

So an appliance that pulls 600watts on average pulls an average of 5 amps.
During 1st half cycle it can pull 100 amps for around 0.008 seconds.
The rest of the inrush current is 5amps x 8 =
40 amps and lasts for 0.083 seconds.

If two 600watt average appliances kick on at same time. Inrush could be as much as 200 amps for 0.008 seconds and 80 amps for as long as 0.083 seconds. If they kick on one right after the other, could be 100 amps for 0.016 seconds and 40 amps for 0.165 seconds.

Now compare the inrush current to the normal operating current.
100amps = 6.7 x 15amps
6.7 times the 15 amps rated breaker.

40 amps = 2.7 x 15 amps
2.7 times the 15 amp rated breaker.

Now compare these numbers to the circuit breaker trip curves.
Looking at time and (multiples of) current.

15 amp breaker allows 7x rated current for 0.25seconds. That is up to 105 amps for 1/4 seconds.
200 amps would be 13.333 x rated current, which is dealing with the vertical part of the "curve".
 

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karl

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Seems like you got this under control. Lots of numbers and such.

But the solution is still the same, they need to be on separate breakers.

Usually if the breaker keeps triping , and its not worn out, and happens when an appliance starts up,
ya found the problem
 

bob58o

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Do they need sep breakers though? This is not a problem I am having. Half just an exercise to help me learn. Half trying to help a buddy who is renting a space with limited number of circuits.

I'm not entirely convinced yet. Need to learn more about those trip curves. One side of the curve says a 15A breaker can do 12.5 X the rated current for 1/2 a second. I was using the left side of the curve to play it safe, but there is a wide range between the left and right side of the curve.. At 0.1 seconds, the left side of the curve is 7x and the right side of the curve is 15 x the rated current.

Also I was using 8x normal operating current for the inrush current. It might be less. I read 4-8x.

What are the chances that both the compressors kick on within 1 cycle at 60 Hz?
That is a period lasting 17 milliseconds.
Within 5 cycles? 83 milliseconds
Within 10 cycles? 167 milliseconds

I see the potential for a tripped breaker, hence the "dedicated circuit" label.
One running might pull 5 amps by itself. If the other kicks on and adds another 100 amps, we are at 105 amps which is exactly at the 7x mark. Now I need to learn how to read those curves more better.

On the 15A breaker, what does 7X current at around 0.1 seconds tell me?
Does 7x amps trip the breaker right away? Can I run 7x for 0.3 seconds? Does 15x amps trip it right away? What about 15x amps for 0.3 seconds.

I like functions where there is only one "y" value for each "x" value. Vertical lines on graphs don't exist in freshman level HS math, which is where I am most comfortable.
 

karl

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Do they need sep breakers though? This is not a problem I am having. Half just an exercise to help me learn. Half trying to help a buddy who is renting a space with limited number of circuits.

Thank you , makes perfect sense.

Ive never stepped foot in an apartment building, where is the breaker panel, is it accessible , or does building staff need to reset it?
 

Joe-405

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You lost me after the first few sentences but..................

everything pulls more amps upon start up and after it only pulls the minimum. Have this issue at my dads house and that was the reason we found there also. It was the spike amps you can’t see vs. the actual you can see.
 

bob58o

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Thank you , makes perfect sense.

Ive never stepped foot in an apartment building, where is the breaker panel, is it accessible , or does building staff need to reset it?
My apartment has a panel in the bedroom that I can get to. Everything is 20 amps. But when a breaker trips, it isn't any of the ones in that panel. It usually goes out in the whole unit (not just a single circuit) and I need to go down into the basement to where the power meters are to flip a breaker down there.

My buddy is just renting a spare room with a bathroom. I'm fairly certain he only has two circuits. They might be 20 amps, but not sure.
 
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