# Maximum load on 20 amp circuit breaker

I am aware of certain normal maximums. I guess I'm looking for somebody to tell me that I'm pushing things too far, or not. I want to hook up a portable air conditioner (Sunpentown WA-1220E) which is rated at 950 watts (and, mysteriously, at 9.0 amps) along with an HP 4Si printer, which is rated at 1100 watts (9.4 amps, per the documentation) with a 12 amp maximum for 20 milliseconds.

I certainly understand that if both units were operating continuously, the wattage (950 + 1100) = 2050 exceeds the recommended maximums of 80% of capacity (20 amps * 120 volts = 2,400 * 80% = 1,920).

But the printer doesn't operate continuously. Most of the time it is in standby mode. In standby mode it uses about 240 watts (120 if in powersave mode).

Clearly, as long as I don't print (!), I don't have a problem.

I guess my question is: assuming the air conditioner is running, and assuming I start the printer printing, after how long should I shut off the air conditioner (or stop the printer, give it a rest, and if so, for how long, and then start it back up again)?

Thanks

George

 tkrussell Posts: 9,673, Reputation: 3698 Senior Electrical & Lighting Expert #2 Jun 24, 2005, 03:15 AM

The 80% derating factor is only for loads that run continuously , which is considered to be 3 hours or more. Otherwise a circuit can be loaded 100%.

Check the AC documentation. It should tell you how long to leave the AC off before turning back on. If you turn off an AC unit, and try to turn it back on right away, the compressor is still in a locked rotor condition, and may trip the breaker by itself.

According the manufacturers website, your unit is rated at 950 Watts for AC only, and the 9 Amps is if it had the optional heat mode.

Since the printer is only on for a short time, you should have no problems running the AC and the printer together.
 NoCalHomeowner2 Posts: 6, Reputation: 1 New Member #3 Jun 24, 2005, 05:34 AM

George
 esspeajay Posts: 4, Reputation: 10 Junior Member #4 Feb 7, 2011, 03:01 PM
 stanfortyman Posts: 5,198, Reputation: 1401 Electrical & Lighting Expert #5 Feb 7, 2011, 04:13 PM

esspeajay, several things.

1- Your post is very confusing. You are right about receptacle ratings and allowable cord and plug loads. This does NOT mean the circuit is limited to that 80%.
2- Receptacles are wired in parallel, NOT series, regardless if they are residential or commercial. Most all AC wiring is wired in parallel.
3- This thread is almost six YEARS old, and you are answering it like it was yesterday. Please check the dates of the posts you are replying to.
 NoCalHomeowner2 Posts: 6, Reputation: 1 New Member #6 Feb 7, 2011, 06:03 PM
It may be almost 6 years old, but I'm still here and I still have the same Sunpentown A/C unit (a pretty good vote for the quality of the machine, huh?) and the same HP printer (nobody needs my vote as to the quality of HP printers). Six years ago, we ended up running separate circuits for each, notwithstanding the vote(s) of confidence that they can happily co-exist. But the discussion has been very valuable, since we have just recently put them back on the same circuit so that we can plug another unit into the other separate circuit! The metrics are all quite helpful and I thank you for posting them.
 esspeajay Posts: 4, Reputation: 10 Junior Member #7 Feb 7, 2011, 06:54 PM
Comment on stanfortyman's post
Quote:
 Originally Posted by stanfortyman esspeajay, several things. 1- Your post is very confusing. You are right about receptacle ratings and allowable cord and plug loads. This does NOT mean the circuit is limited to that 80%. 2- Receptacles are wired in parallel, NOT series, regardless if they are residential or commercial. Most all AC wiring is wired in parallel. 3- This thread is almost six YEARS old, and you are answering it like it was yesterday. Please check the dates of the posts you are replying to.
I apologize if you where confused, but the idea put forth in my post agrees with your asessment that the 80% rule does not apply. And although you felt compelled to chastise me for posting to an old thread, others who search for a solution to a similar issue just might find that information useful. Perhaps my verbagge was to sylabic for you to understand.
 esspeajay Posts: 4, Reputation: 10 Junior Member #8 Feb 7, 2011, 06:55 PM
Comment on stanfortyman's post
Quote:
 Originally Posted by stanfortyman esspeajay, several things. 1- Your post is very confusing. You are right about receptacle ratings and allowable cord and plug loads. This does NOT mean the circuit is limited to that 80%. 2- Receptacles are wired in parallel, NOT series, regardless if they are residential or commercial. Most all AC wiring is wired in parallel. 3- This thread is almost six YEARS old, and you are answering it like it was yesterday. Please check the dates of the posts you are replying to.
*verbiage typo
 esspeajay Posts: 4, Reputation: 10 Junior Member #9 Feb 7, 2011, 06:58 PM
Comment on NoCalHomeowner2's post
Quote:
 Originally Posted by NoCalHomeowner2 It may be almost 6 years old, but I'm still here and I still have the same Sunpentown A/C unit (a pretty good vote for the quality of the machine, huh?) and the same HP printer (nobody needs my vote as to the quality of HP printers). Six years ago, we ended up running separate circuits for each, notwithstanding the vote(s) of confidence that they can happily co-exist. But the discussion has been very valuable, since we have just recently put them back on the same circuit so that we can plug another unit into the other separate circuit! The metrics are all quite helpful and I thank you for posting them.
Sorry, as Stanfortyman so kindly pointed out your post was quite old. I guess I should have taken notice of that. I am glad it all worked out for you in the end.

Scott
 donf Posts: 5,068, Reputation: 2798 Printers & Electronics Expert #10 Feb 7, 2011, 07:15 PM

Esspeajay,

We have been getting a lot of old questions recycled because the site initiated a new skin. Most of the regular posters in this forum still choose to use the old skin. I know I do.

So when I check the initial date of an item before I post and answer, if it is six months or older, I ignore the question. <Just an FYI for you, nothing more.>

Let me try to explain it this way, please. Just as a for instance, the NEC 2011 is now in use, not necessarily all across the U.S.A. but in some places. Here in Virginia we are expecting the approval 0f the 2008 Code in March.

This thread is three code levels back, which in and of itself may not matter on this thread, however there are threads that were answered with respect to the 2005 or earlier code levels, which if answered today would not get the same answer. For example subpanel questions.

To the best of my knowledge, the 80% circuit loads that you first discussed as load limits. So that if you are planning a 20 amp circuit that will have multiple outlets you calculate the equipment load to be no more than 16 amps. Now in the real world, there is no way you can positively guarantee that a home owner will never go above your anticipated load. You certainly would not calculate in a load of 4 amps to drop the circuit down to 16 amps. The circuit will still be capable of servicing a 20 amp call but you (the electrician) do not an overloaded circuit.

In the case of a continuous load, say for example a water heater, you have to size the circuit for 125% of the needed amperage. So if a circuit required required 18 amps and it qualified as constant use equipment, then you would multiply the 18 amps by 1.25 to get 22.5 amps for the load. In which case you would need to size the circuit for 25 amps and not 20 amps. If you derate the 22.5 amps by 80% your return will be 18 amps. Therefore if you were to derate the circuit by dividing 18 amps by .8 you would end up with a 22.5 amp load.

Clear as mud?

Personally, I do not believe Stan was chastising you as much as he was trying to let you know how we screen items to stay with current problems.

As to his statement about parallel vs. serial connections, that was simply correcting an error on your part. Nothing personal, just getting the correct information disseminated.

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