OCPD on a UPS

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dancase

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We're replacing an old 15 kVA Ferrups (ferroresonant UPS system) with a shiny new dual-conversion online UPS. The Ferrups output feeds a pair of main-lug breaker panels about 25 feet away. There is no other OCPD on the output. The input is protected by a fused disconnect. As I understand it, this installation was deemed acceptable under 450-3a because the output of the Ferrups is a transformer and the unit is located in a supervised location. I've got my reservations, since the Ferrups can continue to produce output even if the input is disconnected -- that's the whole idea! OTOH, by its very nature a UPS is current-limited when on inverter.

Does this configuration comply with NEC?

If this config IS acceptable, is it also acceptable for my new (non-transformer output) online UPS, or do I need to put in an additional OCPD?

Thanks!

D.
 
Re: OCPD on a UPS

I've got to admit that I'm surprised that there haven't been any replies to this query -- Either I've asked a REALLY stupid question, or this is one of those things that nobody wants to take a position on, or it's so obvious that everyone thinks I'm an idiot. Could go either way, I suppose. :D

Well, in the absence of other opinions, I went with my instincts -- since the UPS is capable of supplying output with no input present (that's what it's for, after all), under 240-21 there should be OCP "at the point where the conductors receive their supply." No matter how you slice it, there needs to be an OCP device at the source of supply -- and as I read it, there should have been one on the old installation.

The new installation has a fused disconnect on both the input and output of the UPS.

...just for the record. ;)

D.
 
Re: OCPD on a UPS

This deserves a bump.
But while we're on the subject of UPSs...

I've noticed that a lot of 120 volt UPSes are cord onnected and they have 120 outlets in the front. So what would happen if you unplugged [a fully charged] unit and plugged it into itself? :eek:

~Peter
 
Re: OCPD on a UPS

Originally posted by peter:
I've noticed that a lot of 120 volt UPSes are cord onnected and they have 120 outlets in the front. So what would happen if you unplugged [a fully charged] unit and plugged it into itself? :D

D.
Click to expand...
 
Re: OCPD on a UPS

I should go find an old UPS and try that :D As it could be and interesting boom....

[Edited to remove the double 's' in as. creating a profane word]

[ March 10, 2005, 12:49 AM: Message edited by: bensonelectric ]
 
Re: OCPD on a UPS

Dan,
I agree that a UPS is a SDS and that the output requires OCPD for the conductors pre 240.21. I have also installed a number of Ferrups and the instructions required an OCPD on the output. These were placed into service by a manufacturer's rep and he would not have energized them without an OCPD on the output.
Don
 
Re: OCPD on a UPS

Dan, sorry for the delay, first time I've seen the post. I have done a fair share of designing UPS systems. I treat this type just like you would a transformer. The OCPD doesn't have to be built-in the unit, it can be in the first disconnect device. I follow the rules of 250.30 for SDS, and the manufacture should give you the OCDP size in the specs.

You are correct the inverters limit the current output, even when the input power is present, that is the nature of the design of dual conversion UPS systems.
 
Re: OCPD on a UPS

I am not sure if this applies here, but I have heard that Dale is going to race the big brown truck.

Wait a minute this is a different UPS, sorry
:D
 
Re: OCPD on a UPS

I always find these types of questions to be intriguing. Yes, we can look at the scheme and conclude the an OCPD should be used. But, then overlook the fact that the UPS has a given output capacity and ability to deliver short circuit current. Personally I would start with knowing what overload the UPS and short circuit current is capable of providing. Then I would look at the time current curve of a common UL489 listed OCPD and determine if it would ever trip. I am aware there are OCPDs made specifically for the application with time current curves and magnetic trips that are more in line with the protection of the UPS.
A down stream OCPD may just end up as a simple disconnect and never provide and protection at all.
The same can apply to generators.
Our utilities have a huge reservoir of power where both the UPS and generator are most likely sized to serve an anticipated load. To oversize them so that they have enough current to trip an UL489 OCPD would not make financial sense.
 
Re: OCPD on a UPS

I agree, this could be the answer to our nations energy needs. You could just tap off of it and get free power. I'll bag the idea of getting those 200 watts of PV for my cabin......
 
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