"Interesting" question

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jaggedben said:
I'm absolutely sure of it.

To be clear, I'm not talking about different nominal grid voltages, e.g. the spec current at 208V vs. 240V. I'm talking about what happens when the grid voltage fluctuates higher than nominal, e.g. you're measuring 247.6V on a nominal 240V service. An inverter with max output of, say, 32A, will still output 32A at the higher voltage and thus output a few percentage points more power.
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That's at least not universally true. I remember seeing the specs for some inverters that specified a voltage range and a maximum current range (high, low, and nominal) where the low end of the current range corresponded to the high end of the voltage range and vice versa, and the IV product was always the same. They were central inverters, if that makes any difference.
 
Smart $ said:
The actual phrase is "the inverter continuous output current rating" in 690.8(A)(3), then applied as provided for in 690.8(B) for Conductor Ampacity and 690.9(B) for Overcurrent Protection
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And the difference is... ?
 
ggunn said:
And the difference is... ?
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Using the following image (just one I plucked through Google images), what is the continuous output current rating? I say it's "7.0Arms".

09_nameplate.jpg


I gathered from what you guys were discussing that you were looking to manipulate other ratings such as the maximum AC power divided by the URV of the AC voltage range, i.e. 1650VA ÷ 260V to get 6.3A, and thereby lowering the minimum OCPD rating requirement.

Or was I just imagining that...??? :D
 
Smart $ said:
I gathered from what you guys were discussing that you were looking to manipulate other ratings such as the maximum AC power divided by the URV of the AC voltage range, i.e. 1650VA ÷ 260V to get 6.3A, and thereby lowering the minimum OCPD rating requirement.

Or was I just imagining that...??? :D
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As a matter of fact, you did miss the point of my question. I am well aware of what numbers I need to use to size conductors and OCPD for inverter outputs. My question was whether I could adjust downward the current output of a 480V to 208V transformer by considering the efficiency of the transformer; even half a percent would get me into 500A fuses and smaller conductors and conduit on the 208V side. The consensus was that I cannot do that in a NEC compliant manner. I accept that and I drew the plans with the larger fuses and conductors.

Granted, the discussion strayed a bit, but I was not looking to use some other number for inverter output current. This is not my first rodeo. :D
 
ggunn said:
We have a system under design that uses 480V inverters connecting to a 208V service through a transformer. It just so happens that converting the inverter current using 480V/208V comes out to 401.5A, which bumps our OCPD up to 600A, which then makes us increase the conductor size. My question is this: Can I legitimately use the transformer efficiency to justify the current being less than 400A?
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I am not clear on this, why did the conductor size have to be increased? Was it because you were using parallel 3/0 Cu and that just so happens to be 400 on the money?
 
ggunn said:
As a matter of fact, you did miss the point of my question. ...
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Actually, I did not miss the point of your question... but for some reason, you keep saying I did.

My first comment in this thread was...
Smart $ said:
Wouldn't matter. Code requires using the nominal values for the calculations.
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...and it is on point for the question you asked in the OP. :happyyes:
 
electrofelon said:
I am not clear on this, why did the conductor size have to be increased? Was it because you were using parallel 3/0 Cu and that just so happens to be 400 on the money?
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Conductor size has to at least round up to the standard OCPD size that you are using, if 800A and less. If you are using an OCPD that is significantly larger than you need, you might have to upsize the conductors a lot more than you would need if using the proper OCPD size. If you are using an exotic non-standard OCPD size, then it is my understanding that you need at least as much conductor ampacity as OCPD, even if that exotic OCPD size is less than 800A. Why a non-standard/non-adjustable OCPD exists as part of a product configuration, I do not understand.

A reason you might be in this situation is if you are dealing with equipment that can only accept a breaker from the 225A to 400A "bin". And the OCPD you need per your calculation, is only 150A. You might have a group of inverter input breakers on the same inverter, most of which are 400A, except for the "runt of the litter" zone that only has a third the amount of source circuits. It is my understainding that you need at least 201A worth of wire & terminal ampacity (to round up to 225A) in such a situation.

As for exotic non-standard size OCPD's, I found myself in a situation where 350A fuses and 355A fuses were available for an inverter input, and the next size didn't occur until 500A. If 1.56*Isc were 352A, you'd need the 355A fuse and 355A of wiring. 353A of wiring would not be good enough. However, counterintuitive as it may be, if you had a standard 400A fuse, it would be acceptable to only connect 353A worth of wiring to it.

Please correct me if I am wrong.
 
Smart $ said:
Actually, I did not miss the point of your question... but for some reason, you keep saying I did.My first comment in this thread was...
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Originally Posted by Smart $
Wouldn't matter. Code requires using the nominal values for the calculations.



​
...and it is on point for the question you asked in the OP. :happyyes:
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My question had to do with the transformer and nothing to do with the inverters. If I could have justified reducing the output current from the transformer from 402V to 400V (again, due to transformer losses, not by munging the inverter output current), then (1.25)(400v) = 500V and I could have used 500V fuses. I realize now that I can't do that, so I drew the electrical plans with the larger fuses, conductors, and conduits. That ship has now sailed. Move along; nothing to see here.
 
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Carultch said:
Conductor size has to at least round up to the standard OCPD size that you are using, if 800A and less. If you are using an OCPD that is significantly larger than you need, you might have to upsize the conductors a lot more than you would need if using the proper OCPD size.
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But it doesnt matter how "significantly larger" the OCPD, as long as it is the next size up. Thats why I am confused. The wording of the OP implied that because the OCPD was next size up, he needed larger conductors....I assume it was just because the even 400 plus 125% was 500 which is exactly 2 sets of 350 AL, so anything over that no matter how small puts you to the next size. Just making sure I am not misunderstanding what was going on. (BTW retracting my earlier comment of 3/0 copper, forgot to factor in the 125%).
 
electrofelon said:
But it doesnt matter how "significantly larger" the OCPD, as long as it is the next size up. Thats why I am confused. The wording of the OP implied that because the OCPD was next size up, he needed larger conductors....I assume it was just because the even 400 plus 125% was 500 which is exactly 2 sets of 350 AL, so anything over that no matter how small puts you to the next size. Just making sure I am not misunderstanding what was going on. (BTW retracting my earlier comment of 3/0 copper, forgot to factor in the 125%).
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I apologize if I misled you; I may have not been clear. The 402A instead of 400A current made the larger fuses and conductors necessary. The larger conductors made the larger conduit necessary.
 
electrofelon said:
But it doesnt matter how "significantly larger" the OCPD, as long as it is the next size up. Thats why I am confused. The wording of the OP implied that because the OCPD was next size up, he needed larger conductors....I assume it was just because the even 400 plus 125% was 500 which is exactly 2 sets of 350 AL, so anything over that no matter how small puts you to the next size. Just making sure I am not misunderstanding what was going on. (BTW retracting my earlier comment of 3/0 copper, forgot to factor in the 125%).
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As an example, suppose your inverter output current were 325A, and you are connecting to equipment that only allows 300A, 400A, and 500A breakers. 1.25*that = 406.25A.

Ordinarily, this would be a 450A breaker, with 2 sets of 250 kcmil AL wiring, because that's 410A of wiring which meets the load and rounds up to 450A of OCPD.
However, because you cannot install a 450A breaker, and must round up to a 500A breaker, you need 451A worth of wiring to round up to 500A. That would be 2 sets of 300 kcmil AL.
 
Carultch said:
As an example, suppose your inverter output current were 325A, and you are connecting to equipment that only allows 300A, 400A, and 500A breakers. 1.25*that = 406.25A.

Ordinarily, this would be a 450A breaker, with 2 sets of 250 kcmil AL wiring, because that's 410A of wiring which meets the load and rounds up to 450A of OCPD.
However, because you cannot install a 450A breaker, and must round up to a 500A breaker, you need 451A worth of wiring to round up to 500A. That would be 2 sets of 300 kcmil AL.
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But that is a different situation than the OP - and kind of a strange one: Im not sure why you couldnt get a standard size OCPD.
 
Carultch said:
I've been in that situation before. See the fuse options on this datasheet. I don't know how or why they decided to build 315A and 355A fuses.
http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/br083028en.pdf
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You must have ducked because my comment, or rather the manner in which I intended my comment to be, appears to have flown right over your head. :huh:

I was making light of your use of "exotic" as an adjective to describe any uncommon fuse size. In my mind, any fuse characteristic, not just rating, is far, far, did I say far away from being exotic.
 
ggunn said:
My question had to do with the transformer and nothing to do with the inverters. If I could have justified reducing the output current from the transformer from 402V to 400V (again, due to transformer losses, not by munging the inverter output current), then (1.25)(400v) = 500V and I could have used 500V fuses. I realize now that I can't do that, so I drew the electrical plans with the larger fuses, conductors, and conduits. That ship has now sailed. Move along; nothing to see here.
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And that's is what I am telling you: your intent did not slip by me and my comment was bullseye center on target.
 
Smart $ said:
You must have ducked because my comment, or rather the manner in which I intended my comment to be, appears to have flown right over your head. :huh:

I was making light of your use of "exotic" as an adjective to describe any uncommon fuse size. In my mind, any fuse characteristic, not just rating, is far, far, did I say far away from being exotic.
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I didn't realize you were questioning my use of the word "exotic". May not be the best word to choose, but that's just the word I choose to make my point when dealing with a product spec that deviates from the industry norms.
 
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