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

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.

Thanks for your quick and helpful response.

George

the National Electrical Code states in article 210.21 b(2) that a receptacle connected to a branch circuit supplying two or more receptacles or outlets, a receptacle must not exceed 16 amps on a 20 amp circuit, and 12 amps on a 15 amp circuit, or if the receptacle is rated 15 amps on a 20 amp circuit, you may plug a device into it up to 12 amps. However, residential receptacles are connected in series not parallel, and in series circuits, ie: xams lights the resistance adds at each unit. In the xmas lights for example each bulbs resistance would add, but in the case of the a/c unit, and the printer, the resistance decreases for each unit because each time you add a reeceptacle you add pathways for the current to flow. Therefore are not really adding the full rated wattage across the circuit. Additionally, the rating in amperes is full load and not continuous. Which means it takes more power to start the compressor motor in your a/c unit than it does for the a/c to run continuously and the amperage does not appear to comply with Ohm's law because of the power factor of your a/c unit. The same is true for you printer to some degree. The wattage it might consume is almost certainly less than it's rated wattage. Additionally your printer, in al probability, does not run for a long enough period. (three or more hours is considered a continuous load) to create enough heat to overload the circuit. So as long as your printer is plugged into a separate receptacle on a separate yoke (not the same duplex receptacle) on the same circuit, and there are no other loads on the circuit, you should not have any issues.

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.

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.

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.

*verbiage typo

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
Licensed Unlimited Master Electrician

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.

Well, if you knew forums you'd know this is highly atypical.

Answering a forum post this old is generally regarded as poor forum etiquette. I was merely trying to point this fact out. This is aside from the fact that esspeajay is trying to use big boy language to try and insult me.

And since you asked, no, it was not your sylabic verbiage. It was your confusing writing style and poor sentence structure, punctuation and capitalization.

NoCal,

I'm really glad you are still here.

Just as an FYI, back before home lasers were really envisioned there were constant complaints when customer's took the "Office Printer" home.

Apart from nuisance tripping during power on cycles there was an insane amount of complaint noise about lights on the circuit dimming every four seconds (In this case I have to limit my expertise to Lexmark printers). The problem was actually endemic to the industry.

Here's why. In offices receptacles are on a 20 amp circuit so the power supplies and other design points were set for a 20 amp feed. Residences, however, are primarily a 15 amp environment.

The inrush amperage to get the motors turning and then heat the fuser lamp easily reached 18 amps, so on a 20 amp circuit it was fine. On a 15 amp circuit it was a nightmare.

So Lexmark and others worked around this by changing design points so that POR cycles were more sequential then instant on.

The next big time villain were fuser lamps. At the initial outset, they were left at a higher holding temperature so that when the printer was called for a job, it took less time to bring the fuser to operating temperature.

The older style lamps pulled an inrush of about 17 amps every 4 seconds. This would cause the lights to dim and appear to be pulsing to a home user. Again design was changed in some cases to vary the warm cycle and in others to reduce the inrush for the warming lamps.

There you have, absolutely much more information than you ever needed.

I am not an electrican but need help. Trying to find how many appliances I can use without tripping a breaker. My appliances do not list how many amps it ues but only how many watts it uses. Is there a converstion factor to translate watts to amps. Thanks Jim

Watts divided by voltage equal amps.

You also have to remember that appliances do not always use what is stated on the rating plate. That is a full load number, and many things have thermostats, timers, etc, that turn things on and off while the appliance is running.