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Electrical Wiring in the Home/Bypass/secondary power to a subpanel



I have a subpanel that is fed by a UPS/Generator combo and services mission critical equipment.  Currently the only way to do maintenance on the UPS is to completely power off the equipment, turn off the 50 amp main breaker that supplies PGE power to the system, move the handle of a double throw enclosed switch from UPS circuit to PGE only circuit, turn the 50 amp back on, then bring the equipment back up.  To get back on UPS power repeat the process.

The UPS/generator power to the subpanel is wired INTO a 30 amp breaker instead of the buss lugs.

What I would like to do is install a bypass 30 amp circuit breaker in my main panel (where the main 50 amp is and located about 3 feet away) and wire that into the subpanel buss lugs.  As far as I can figure as long as I wire L1 to L1 and L2 to L2 then I should be able to turn on the 30 amp main breaker then turn off the 30 amp subpanel breaker and this will supply power to the equipment.  I can then turn off the UPS/generator and 50 main breaker and do maintenance on the UPS without taking down my equipment.

Thoughts?  or I'm I completely loony?

Thanks in advance

ANSWER: Keith,

No can do. In order to have the brief "overlap" you would need, the utility power and the inverted AC power will be operating together for that moment which could have disastrous results on your UPS system. In other words your physically connecting two seperately derived systems UPS and Utilty that you have no way of knowing are in "sync" with each other voltage wise and sine wave wise.

You might be able to do what you want another way but I don't know how sensitive your equipment would be to an automatic transfer switch transfering your equipment in a fraction of a second from your UPS system to utility. It might see it or it might not but thats the way to do this safely but only you know the answer to that question. In other words for you to go off UPS the transfer switch in the blink of an eye would put your equipment on utility. Will that be fast enough to not screw anything up ?


---------- FOLLOW-UP ----------

UPS circuit
UPS circuit  
QUESTION: Hello Bob,

Thanks for the response to my question.  I'd like to clarify a little if I may... The utility power (main panel) is the only power in use when I plan to bypass the UPS/Generator power line.  The UPS will be in bypass mode and the generator will be off.  So I would be essentially tying the utility power into the subpanel at a different location in that panel.

I've attached a drawing with a layout of the circuit to try and better explain what I'm trying to accomplish.

To verify that I'm tying the correct phases together I measured the voltage from the 30amp breaker lugs to the open subpanel lugs. I assume that 0 volts == same leg; 240 volts == opposite phase.

I hope all this  makes sense and it will work like I wish it to.


ANSWER: Keith,

I understand what you want to do but the problem comes back to having your utility power and the inverted AC power operating in parallel for that short period of time before you disconnect power to the UPS system so your equipment see's no interuption. They are two entirely different systems that you have merged together thats the problem for the reasons I stated earlier. It can't be done that way code wise or other wise. Its not safe.

The only way I see you doing this code wise is with  an Automatic transfer switch where you currently have the manual one after the UPS equipment. Remove the manual bypass switch you won't need it. Run your 30 amp line into the the ATS as one alternate source then you'll have the UPS input as the other. As long as your equipment is not effected by the transfer when it occurs in a fraction of a second you'll be good to go. I have installed many whole house generators in the last couple years and once the utilty power returns the ATS switches back to utiltiy so fast its almost completly unoticeable. TVs stay on, computers keep going etc.

BTW   ASCO makes one of the best Auotmatic transfer switches around and in many different sizes.


---------- FOLLOW-UP ----------

QUESTION: Hello Bob,

Again, thanks for taking the time to answer my questions.

In reviewing your second answer I'm still not understanding where the "inverted AC" power is coming from.  There are three power sources in my diagram
1) Main utility power - ON
2) Generator power - OFF
3) UPS power - OFF
Now, in the usage case that I'm looking at the Generator power is OFF, the
UPS power is in bypass mode thus it is OFF also.  So the only power in the system is the Main Utility power which is being fed to the circuit from a 50 Amp in the main circuit breaker panel. Can you please explain where the "inverted AC" is coming from?

I have absolutely no plans to try this while either the generator or UPS are running as that will cause nothing but sparks, smoke, and expensive bill replacing burnt out equipment.

Again, thank you for your help and putting up with us dense amateurs.



To answer your question the "inverted AC" is coming out of your UPS system. Utility power goes into the UPS system where it is changed from AC to DC by a "rectifier", then the rectifier feeds the battery bank, the battery bank then feeds the "inverter" which changes the DC back to AC. If utility power fails the computers never see the interuption because the battery bank keeps things going till the generator comes on to repower the UPS system and keep it going.

Unless I'm reading your intentions wrong, you want to deenergize your UPS system to work on it. So in order to do that and keep your computers going with out interuption you have to connect Utility power directly to the sub panel which is supplying power to your computers then shut off power to the UPS system. So for a brief moment you will have Utility power and UPS power connected simultaeneously to the sub panel until you shut off power to the UPS system. If this is not what your trying to do then please explain what your aim is.  
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"The Frank Williams School of Electrical Construction". My dear old boss is 100 ! and still going God Bless him. I started working for "Tenafly Electric" right out of high school at 19. He was tough but I learned more working for him in 8 and half years than most guys. We did mostly industrial work while doing some commercial and residential so I was fortunate to be exposed to all 3 areas of electrical construction. The guys I worked with were all good guys and I stay in touch with some of them still. Most of us went into our own buisness's which says something about the caliber of the guys I worked with. We had some interesting clients : Wella Corp. of shampoo fame, Farah Fawcett etc, I didn't buy shampoo for 10 years. It was a great place to work. Pan Am the former airline just before they went under, another great place to work, nice clean environment. C&C Metals, the largest button manufacturer in the US at that time, a not so clean environment but a very interesting place to work, lots of machine's cranking out buttons of all kinds but you had to be on your toes, it was a potentially dangerous place to be . All kinds of action going on around you.

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