Parallel lengths in question

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Jminter

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310.10 (H) (2) requires parallel conductors to be the same length. My confusion lies here: Is this the total overall length (ie. sum of line1+load1=line2+load2), or are we talking line/load sides needing to be equal (ie. line1=line2, load1=load2)?

Current example:
Switchgear with 2 UPS units paralleled in. In the drawings, the UPS1 input length is 18ft, output is 30ft; UPS2 input is 29ft, output is 20ft. Totals are 48ft and 49ft, within 3% for voltage drop. However, you've got an 11ft difference in your inputs, and a 10ft difference in your outputs, which would be way off. Alternatively, you can switch the breaker layout so that your UPS1 I/O lengths are 23/25ft, and UPS2 I/O lengths are 24ft/24ft, with totals at 48ft/48ft which is definitely okay for voltage drop.

Are they both right and the alt install unnecessary? Or are the drawings wrong?
 
The master electrician on site is saying that he's seen multiple instances where, when you have two UPS systems installed like this on the same switch gear, if one goes down they won't bypass unless the wire-lengths are within that 3%. He's had UPS units that wouldn't come on until he cut a length off to pull it into that margin, recrimped and terminated it, and it worked fine.
 
That might well be true. If you have parallel UPS units and they are in bypass, then you do effectively have parallel current flow paths.

And if these paths are of different length, then you will get uneven current distribution.

However as you note this causes the UPS systems to not go into bypass. This is why is isn't a code parallel conductors issue. The parallel conductors requirements are there to prevent uneven current distribution with nothing protecting the conductors carrying more current. In your case the protection hardware of the UPSs is protecting the conductors, and their length needs to be adjusted to get proper function.

Jon
 
winnie said:
That might well be true. If you have parallel UPS units and they are in bypass, then you do effectively have parallel current flow paths.

And if these paths are of different length, then you will get uneven current distribution.

However as you note this causes the UPS systems to not go into bypass. This is why is isn't a code parallel conductors issue. The parallel conductors requirements are there to prevent uneven current distribution with nothing protecting the conductors carrying more current. In your case the protection hardware of the UPSs is protecting the conductors, and their length needs to be adjusted to get proper function.

Jon
Click to expand...
I did not know this nor have I ever thought about it.... Interesting.
 
Winnie's response is the correct one. Many manufacturers can provide inductors to accommodate some variation in length from one UPS module overall length to the other, so it would provide flexibility.
 
The document from Eaton below describes the need to have equal impedances through each UPS path when they are paralleled in bypass mode, as winnie and Ron have mentioned. As described in the document, the sum of the input and output conductor lengths must be the same on each UPS to get the required equal impedances.

They describe it as a distributed bypass system which is shown in figure 4 of the document. This physical way of cascading a lineup of UPS's along a busbar can also keep the contribution of the busbar's resistance to each parallel path the same, although often this resistance is small enough to be ignored.

By the way, the individual amplifiers within a wideband RF distributed amplifier are connected up exactly the same way, only they use RF transmission lines instead of busbars. ;)

 
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