electrical questions

[bigjim]

I know some stuff but my brain is really fuzzy the last couple of days so you guys are my short cut.  If something is marked 750 VA I assume that is the limit of the capacity or the amount of power that could zap you. Is that 750 volts?  I think this thing is a battery back up dealy for computers.  It is pluged in to a land line outlet under a desk with at least 3-4 computers. Looks like 2 desk tops and 2 laptops. The item has a total of eight outlets on it.

2nd part of this is not really knowing how much power computers draw in general  I wonder about the likelyhood of overloading a standard modern day circuit guessing 25 amp outlet with computers. Most of these are only full on one at a time but all are on "stand by?".

The brain issue is meds working too good or not good enough today. Thats my story and I am sticking to it.🙃

[D&J]

750 VA is another way of saying watts, at 120 volts it has a max output of 6.25 amps.

Denny

[oldjohnt]

Hi Big Jim, good question here are some basics:

Watts = Volts x Amps   

Power Factor = Watts/Volt Amps 

Volt Amps x Power Factor = Watts

Iffffffffff the Power Factor were unity one as a pure resistive load would be, Watts is equal to Volt Amps.

Here's how Wikipedia explains it:  Power factor - Wikipedia

You ask:  

1 hour ago, bigjim said:

If something is marked 750 VA I assume that is the limit of the capacity or the amount of power that could zap you. Is that 750 volts? 

NOOOOOOOOOOOOO however for an example in the event the Power Factor were ONE (NOT the case for inductive loads) if you think of it as 750 Watts and the output was 120 Volts (like a computer may use) that means it can supply 750/120 or 6.25 Amps

NOTE a Standard NEMA 5-20R 120 Volt 20 Amp household receptacle is rated for 20 amps, but if more than 80% or 16 amps is drawn long enough a 20 amp Thermal Magnetic circuit breaker protecting it may well trip. When I practiced power distribution pursuant to the NEC, I computed the maximum continuous current,,,,,,,,,,,then sized the conductors to be 125% greater,,,,,,,,,,,then sized the circuit breaker to protect the wire. IE if the max continuous current was 16 amps, I specified 20 amp rated wire and protected it with a 20 amp circuit breaker 

How much power any computer or device draws ????????????? see the label I cant answer that from here, check the label. Devices that are inductive in nature (motors,,,,,,,,Transfomer based like a computer power supply might be,,,,,,,,, etc have a less than one power factor) in which case Power Factor Correction Capacitors might be used to balance the load in an attempt to make the PF equal to one.

Clear as mud Im sure lol

 

John T Long retired n rusty power engineer and codes change SO NO WARRANTY

[Kirk W]

1 hour ago, D&J said:

750 VA is another way of saying watts, at 120 volts it has a max output of 6.25 amps.

In layman terms, this is the correct answer. If you want an engineer's view then read what Old John says but like most engineers, John makes it more complicated than need be. The maximum power load limit is often included in the name of the UPS, followed by the letters "VA." For example, the APC (American Power Conversion) Battery Backup 750VA has a load limit of 750VA. However, the maximum wattage a UPS supports is typically 60% of the VA number.

How long will it supply power? Up to 355 minutes for 6 v wifi router, 10 minutes for desktop home computer+router+modem, 66 minutes for notebook computer, 35 minutes for workstation PC according to the specs.

 

[oldjohnt]

2 hours ago, Kirk W said:

4 hours ago, D&J said:

750 VA is another way of saying watts, at 120 volts it has a max output of 6.25 amps.

In layman terms, this is the correct answer. If you want an engineer's view then read what Old John says but like most engineers, John makes it more complicated than need be

 Yo Kirk, FYI   UNFORTUNATELY 

 "750 VA is another way of saying watts" 

  IS NOTTTTTTTTTT BY ITSELF ACCURATE      IN EITHER LAYMENS OR PROFESSIONAL TERMS     PERIOD... Its just inaccurate not knowing what the Power Factor is.   VA may or may NOT be same as Watts !!!! 

  Its  true (750 VA = 750 Watts) ONLY if the load is pure resistive WHICH MOTORS OR SOME COMPUTER POWER SUPPLIES MAY NOTTTTTTTTT BE

 Its not rocket science or actually more complicated for "most" technicians or electrical power engineers like myself to understand these basic NON complicated formulas:

Watts = Volts x Amps   

Power Factor = Watts/Volt Amps 

Volt Amps x Power Factor = Watts

   Iffffffffff the Power Factor were unity one as a pure resistive load would be, ONLY THEN Watts is equal to Volt Amps.

                       Sooooooo simple what's complicated about that ???

    I hope this helps and I will be glad to explain it more for you if needed, just ask here or shoot me an e mail jmn50@msn.com, I'm here to help. Some of my technicians struggled with Power Factor a bit, but in reality its quite simple and certainly NOT complicated once I educated them.

Again VA = Watts ONLY FOR PURE RESISTIVE LOADS which transformers, power supplies, some computers, motors or other inductive loads ARE NOT

 Best wishes, as always it's a pleasure sparky chatting and Im happy to help anytime I can.... You or anyone does NOT have to take my word for it, consult the many texts and educational materials that explain why VA does NOT always equal Watts. PS Im headed to your great State this fall yayyyyyyyyyyyyyyy 

John T   BSEE,JD  Electrical Power Distribution Engineer  

 

 

Edited September 21, 2022 by oldjohnt

[D&J]

Let's face it guys for the average person VA is the same or close to the same as watts.

Denny

[Kirk W]

33 minutes ago, D&J said:

Let's face it guys for the average person VA is the same or close to the same as watts.

Sometimes close really is good enough for common folks. 

[oldjohnt]

14 hours ago, D&J said:

Let's face it guys for the average person VA is the same or close to the same as watts.

Denny

Thats so true Denny I agree with you 100% you get no argument on that one.

  Sure lets face it, but I just hope no one (average person) starts a fire or is injured because they figured VA was the same as Watts !!!!!!!!!!! Im sure everyone here agrees with that !!  

  The "average person" who is not a professional electrician or an electrical engineer can figure VA the same or close to the same as watts and if the load is pure resistive THEY ARE RIGHT. HOWEVER if the PF is less than one, as typical for inductive loads, motors, transformers or some computers THEY WILL BE WRONG AS RAIN (hopefully not hazardous) If components aren't properly and in accordance with the National Electrical Code specified BAD THINGS CAN HAPPEN !!!! Sorry if that offends anyone those are just the facts and at every NEC Seminar I attended I learned of a new code change and often the reason it came about was there was a fire or someone was electrocuted (*&^&^%

 Hey there's a safe correct NEC proper way to do things and there's an "average"  persons way of doing things (maybe even half A__ __ lol)..  Its THEIR RV, THEIR choice, THEIR risk. I enjoy educating my fellow RV lovers, maybe prevent a tragedy along the way !

 Be safe out there and if you're an average person (not a trained electrician) at least consider consulting with the NEC or a trained professional electrician, electrical tech,  or an electrical engineer BEFORE you perform electrical work on your RV IT COULD SAVE YOUR RV OR YOUR LIFE !!!!!!!!!!!!!!!!!!!!!!

 Best wishes I hope this helps, post back any questions and I will be glad to answer. Please do it RIGHT, better safe than sorry... There have been too many RV fires or lives lost because someone took a short cut and didn't perform electrical work SAFELY    

 Lets face it, a person is free to be WRONG (Volt Amps = Watts for average person) orrrrrrrrrrr RIGHT (Volt Amps x Power Factor = Watts) so easy so simple so non complicated and MY choice..I fully support a persons freedom to choose but like to help when I can 

 John T   BSEE,JD  Electrical Power Distribution Engineer.  I prefer to do it RIGHT where electricity is concerned, even if less will get you by ...Yall do as you please.

Edited September 22, 2022 by oldjohnt

[Star Dreamer]

13 hours ago, oldjohnt said:

RIGHT (Volt Amps x Power Factor = Watts) so easy so simple 

So is simple layman's terms, where do we find the Power Factor so we can calculate the watts correctly?

[oldjohnt]

22 hours ago, Star Dreamer said:

So is simple layman's terms, where do we find the Power Factor so we can calculate the watts correctly?

Good morning Star, great question, thanks for asking.  As I recall the PF was available by manufacturers specs or data or on some labels, while on others we had equipment to determine it.  PF IS BASED ON INDUCTANCE, CAPACITANCE AND RESISTANCE (ONLY if the load was pure resistance like a heater or hair dryer etc. would Watts be the same as VA).    FYI on some motors, or transformer based equipment, power supplies, computers etc. we saw a power factor of maybe 0.8. Then it's non complicated and easy to apply the formula    Volt Amps x Power Factor = Watts   to determine the amperage so the proper wire size and overcurrent protection can be safely and correctly computed. REMEMBER to size the wire to have a minimum ampacity of 125% of the maximum continuous current !!!!!!!!! There can easily be situations where that Power Factor of 0.8 is enough to make a difference (IE the "common average person" who says Watts just equals VA doesn't get it) and trust me if I overlooked it my boss (an NEC perfectionist) got all over me lol

 Best wishes Star and everyone here, have a great day now. 

John T  Live in the RV from an Antique Tractor Show in Brookville, Indiana 

Edited September 23, 2022 by oldjohnt

[D&J]

On 9/22/2022 at 7:54 AM, Star Dreamer said:

So is simple layman's terms, where do we find the Power Factor so we can calculate the watts correctly?

Power factor comes into play when you have large inductive loads like in some commercial settings. Then it will have to be figured in when supplying power to the facility. For general residential houses including RV it's basically meaningless. The backup power supply the OP has that rating because it is a inductive load but very small but if a place had a very large bank of computers all on this type of device then it will come into play.

No need to worry about over current protection because everyone of them I have seen have it built into it because it's a 120V rec and you know someone will try and plug something like a electric heater into it so they protect the device from overload. You will also see a lot of rec on them because think about how many USB chargers the average household has anymore.

The power factor is for the incoming power not the power supplied buy the device.

In my 40+ years being a electrician and retiring with a Class A Electrical Contractor license and numerous certifications for the refrigeration end of my business and way to many hours in code classes I did get asked what VA was a few times and some were worried like the OP that it was voltage and I gave the same answer, it's just watts and don't worry about it.

I'm not sure but that rating may have to be on there to get UL approved.

Denny

 

 

 

[bigjim]

Thanks for the help jogging my minimal memory.  I had been thinking of volts, amps and resistance and just was not clicking over to watts and power factor. I got  curious  looking at the mess on the desk at the place I volunteer for the Forest service and I am always concerned with overloaded circuits or extension cords.  I may know more than anyone here and I want to give them good information. This place desperately needs some new circuits and may a couple of dedicated ones. Not going to hold my breath.  I have often groused about stating power in watts instead of amps for my purposes.

[oldjohnt]

  Good info Denny, thanks for posting, I found your experience to be the same as mine in residential applications. Often if the Power Factor is near unity (more typical in residential) Watts was IN THAT CASE close to Volt Amps so not many more calculations were necessary for properly and safely sizing devices, components, conductors and over current protection in accordance with the NEC. Those designs were EASY PEASEY. 

  HOWEVER as you an electrician (NOT the common average untrained electrical user) well knows when designing power distribution where the loads consisted of several computers or a lot of HVAC or motors or transformer based power supplies etc. POWER FACTOR AND WATTS VERSUS VOLT AMP DIFFERENCE BECOMES MORE CRITICAL and MUST be accounted for and dealt with !!!!!!!!!!!!!!!!!!!!!!! Even when we designed Power Distribution for home or business offices that used a lot of computers POWER FACTOR became relative as watts did NOT equal Volt Amps in those situations.

Of course we professional electricians and engineers understand Power Factor and the difference in Watts versus Volt Amps and realize the two are the same ONLY IF the load is pure resistive which computers, motors, HVAC compressors etc ARE NOT !!!!!!!!!  

Sooooooooooooo (for the average common untrained user) simply considering Watts is the same as Volt Amps ABSENT ANY POWER FACTOR INFORMATION OR CONSIDERATION is incorrect and incomplete while   Volt Amps x Power Factor =  Watts    is correct, complete and is soooooooooooo easy, simple and non complicated. No sense in thinking and doing it incomplete or wrong when right is so easy.  YES (as you know) Watts is equal to Volt Amps but ONLY ifffffffffffffffffffff the load is pure resistive. So disregard Power Factor (just say Watts and Volt Amps are the same) at your own peril is my best professional advice for the other untrained electrical readers here !!!!!!!!!!!

PS Denny when you stated "I did get asked what VA was a few times and some were worried like the OP that it was voltage and I gave the same answer, it's just watts and don't worry about it."

GOOD ANSWER, THEY (Non electrical trained common average folks) WOULDNT HAVE KNOWN THE DIFFERENCE EVEN IF YOU TRIED TO EXPLAIN IT TO THEM REGARDLESS  LOL

Its a pleasure sparky chatting with you Denny as one who certainly understands Power Factor and the difference in Watts versus Volt Amps and I hope we have helped educate the others but fear we may have lost and/or bored them to death lol

Take Care Denny, best wishes, its been fun sparky chatting with a knowledgeable professional !!! We have this down maybe we helped others ??

John T  BSEE,JD Electrical Power Distribution Design Engineer  

Edited September 23, 2022 by oldjohnt

[oldjohnt]

29 minutes ago, bigjim said:

Thanks for the help jogging my minimal memory.  I had been thinking of volts, amps and resistance and just was not clicking over to watts and power factor. I got  curious  looking at the mess on the desk at the place I volunteer for the Forest service and I am always concerned with overloaded circuits or extension cords.  I may know more than anyone here and I want to give them good information. This place desperately needs some new circuits and may a couple of dedicated ones. Not going to hold my breath.  I have often groused about stating power in watts instead of amps for my purposes.

 You are so welcome bigjim, glad to help. INDEED if an office uses a lot of computers which can result in a relatively high inductive load, Watts DOES NOT equal Volt Amps and I congratulate you for your justifiable concerns. 

FYI I designed several computer facilities and in those I NEVER specified any Multi Wire Branch Circuits,,,,,,,,Specified many ISOLATED GROUND circuits,,,,,,,,,,,Specified many dedicated branch circuits, and properly well OVER engineered the Equipment GroundING and Bonding circuitry. When designing computer facility power distribution I instructed my electricians Bond to EVERY READILY (even less readily) AVAILABE GROUNDING ELECTRODE.

Do it RIGHT, do it SAFE, do it in compliance with the NEC, consider Power Factor, and you will be rewarded.

Best wishes now thanks for your feedback 

John T   BSEE,JD  

[Kirk W]

Reading these posts about power factor versus vot/amps reminds me of an incident years ago when in Navy Neuclear Power School. One of my classmates was struggling with application of some rules of physics in a practical problem. Finally to explain, a cheif petty officer told the class a story that clearly applies here. 

"If you have a young man and and an attractive young lady sitting at opposite ends of a park bench that have interest in each other and first one moves 1/2 of the distance between them, then the other also moves 1/2 of the remaining distance, if this continues, scientific theory tells us that the two of them will never reach a point where they meet. Practical application denonstrates to us that at some point 1/2 of the remaining difference has no significance in the resulting situation."

The same is true of power factor in this case.

Edited September 23, 2022 by Kirk W

[oldjohnt]

6 minutes ago, Kirk W said:

"If you have a young man and and an attractive young lady sitting at opposite ends of a park bench that have interest in each other and first one moves 1/2 of the distance between them, then the other also moves 1/2 of the remaining distance, if this continues, scientific theory tells us that the two of them will never reach a point where they meet. Practical application denonstrates to us that at some point 1/2 of the remaining difference has no significance in the resulting situation."

Great story Kirk, thanks for posting. Good to see a little humor on here. But I wonder just how attractive she was lol

John T

[bigjim]

Ironically as cheap as I am known to be  I definitly prefer over engineering in so  many applications including electrical applications, big brakes on vehicles, bridges I have to drive over like the "new" bridge over the Mississippi at Memphis and so much more. I am not as smart as a lot but I try to make up for it with a little common sense and care. Heck I even feel some of the cords on items I have plugged in to see if they are getting too warm to suit me.  Some of my hero's are over engineers. 😃

[Ray,IN]

That's a really good analogy Kirk; kinda reminds me of Pi and the never-ending decimal places; just how accurate does the result need to be?

This thread is another educational reading for me, reminds me how much I've forgotten too.

Pi R square, cake R round.

Edited September 25, 2022 by Ray,IN

[oldjohnt]

On 9/24/2022 at 7:53 PM, bigjim said:

I definitly prefer over engineering in so  many applications including electrical applications

Hey bigjim, I'm pretty much the same ESPECIALY in RV applications where you're sleeping amongst all that light flammable plywood tender grrrrrrrrrrrrrrrrrrr  The NEC (my bible for years when practicing and attending their seminars) can be considered as a set of MINIMUM STANDARDS so no harm if an RV is a bit OVER ENGINEERED, in my engineer's opinion. Do it RIGHT (or even more) better safe than sorry !!! Im glad to see there's more than one of us who thinks this way lol

 

On 9/25/2022 at 12:14 AM, Ray,IN said:

That's a really good analogy Kirk; kinda reminds me of Pi and the never-ending decimal places; just how accurate does the result need to be?

This thread is another educational reading for me, reminds me how much I've forgotten too.

 Hey Neighbor Ray, indeed that was a fun analogy and right on the money!! (What if any such little difference matters for near resistive loads) As you know when a load approaches more n more resistive (heaters, hair dryers, insta pots, etc) the two metrics VA and Watts ARE NEAR THE SAME very little difference in accuracy, however, as it becomes more inductive (transformer or computer power supplies, AC, motors etc) they can be quite different.   

  People our age forget SURELY NOT LOL I also learned from this discussion and hope others did likewise.

John T   

Edited September 26, 2022 by oldjohnt