Load bank conductors

[garybenz8]

I recently returned from a power monitoring project in which we were tasked with monitoring the a 480 V, delta-connected load bank. The electrical contractor had already connected the load bank prior to our arrival, and what we found was that they neatly lined up (as if racked), and separated by phase, the fifteen (15) 4/0 conductors along the ground. I explained that although it made for a neat and tidy looking connection, it really wasn't the "best" way to have arranged the conductors. I showed him that the "outer" conductors were carrying much more load (350 A) than the "inner" conductors (150 A), and that the outer ones may end up overheating. While I knew this to be true, I struggled to provide a good answer as to why this was occurring - I merely attributed it to the magnetic field, but suspect there is a more complete answer.

 

[chris kennedy]

I showed him that the "outer" conductors were carrying much more load (350 A)

Your 4/0 type W is good for 300A in this install. I find type W to be quite forgiving, don't you?

 

[garybenz8]

Your 4/0 type W is good for 300A in this install. I find type W to be quite forgiving, don't you?

I left after 1.5 hours of a 6 hour burn-in. I suspect I would have heard if the cable failed, so forgiving it must be. But what specifically causes more current to flow on the outer conductors when arranged in a neatly parallel fashion, all the same phase?

 

[donselectric]

skin effect in essence i suspect.....:

 

[brian john]

I recently returned from a power monitoring project in which we were tasked with monitoring the a 480 V, delta-connected load bank. The electrical contractor had already connected the load bank prior to our arrival, and what we found was that they neatly lined up (as if racked), and separated by phase, the fifteen (15) 4/0 conductors along the ground. I explained that although it made for a neat and tidy looking connection, it really wasn't the "best" way to have arranged the conductors. I showed him that the "outer" conductors were carrying much more load (350 A) than the "inner" conductors (150 A), and that the outer ones may end up overheating. While I knew this to be true, I struggled to provide a good answer as to why this was occurring - I merely attributed it to the magnetic field, but suspect there is a more complete answer.

I have done countless load test from 5 KW to 5 meg, never had any issues with cable layout, all grouped or all intermixed. We monitor all load test with a high speed analyzer. The only cases with current distribution issues were the result of a bad element in the load bank.


Have the contractor check his load banks.

 

[don_resqcapt19]

It is the "inductive reactance" caused by the physical separation of the conductors. The impedance of the conductors goes up as they are moved apart. There have have cases of cable damage in cable tray installations installed in the same manner as these temporary cables. The lowest impedance is keeping full sets together...that is one of each phase and the grounded conductor grouped.

 

[garybenz8]

If the inductive reactance goes up as they are moved apart, wouldn't the impedance be greatest on the outermost conductors as they lay? And if so, wouldn't the current be lower in these conductors (due to higher impedance) rather than higher, as measured?
As they lay, the "inner" conductors carried roughly 1/3 of the current as the outermost conductors.

 

[steved]

Proximity Effect

 

[jrannis]

OK I like the idea of using 4/0 at 300 amps. Should the 65 deg rating of the breaker be considered?

 

[chris kennedy]

OK I like the idea of using 4/0 at 300 amps. Should the 65 deg rating of the breaker be considered?

300A is the 60?C rating of 4/0 type W in the application the OP stated. See Table 400.5(B).

 

[don_resqcapt19]

If the inductive reactance goes up as they are moved apart, wouldn't the impedance be greatest on the outermost conductors as they lay? And if so, wouldn't the current be lower in these conductors (due to higher impedance) rather than higher, as measured?
As they lay, the "inner" conductors carried roughly 1/3 of the current as the outermost conductors.

Gary,
Good point. I don't know what the issue is in this installation.

 

[charlietuna]

were the conductor lengths the same(exact)? that would have more effect than placement in my experience...

 

[garybenz8]

My understanding were that they were all the same length, or very close.

 

[charlietuna]

When you Say "VERY CLOSE" ??? How "VERY CLOSE"?

 

[LLSolutions]

What was the surface they were laying on? Metal deck, poured concrete, maybe the issue is the magnetic field; same reason pvc or aluminum is used for iso-phasing. I think that you could still group all the same phase together,but they need to be spaced farther away from the next phase, or just group them per set "abc" "abc"