Busbars on edge distance and derating

[patm72]

Hello,
Would anyone have an idea from which distance should derating be applied to busbars run in parallel?
My application is at 480V and busbars would be on their edge, one per phase, and sized for about 900A (that is over the FLA by a factor of 1.25 already).

Thank you.

 

[waross]

Calculate your voltage drop at the end of the circuit.
If the voltage drop exceeds code limits, then you must derate.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[patm72]

@Waross.
Thank you for the advice. I will look at the voltage drop applicable, but my application is using bars for a length of about 10 feet.

I will also precise that it is for a machine (enclosure), and that I was planning on using Flexibars by NVent (Eriflex). Those have insulation. In their manual, they precise that mounted on edge, two run in parallel (no distance stated obviously), the ampacity of two is 1.72 times the ampacity of one. With three, 2.25 times that.

I had one question for them, which was at which distance from each other that derating would apply and they were very unhelpful. Here it is:

"The ampacity derating coefficient is used to help determine the size and quantity of Flexibar needed. We have not evaluated at different separation distances and cannot provide guidance on this question.

Each application is different, so the best way to evaluate the temperature rise of one or more Flexibar in parallel is to install Flexibar into the panel and monitor the temperature rise."

This is why I resorted to ask here in order to know what is done when using rigid busbars, which is probably more commonplace.

For my application, where I should in theory provide conductors of 900A (to be 1.25 times above FLA) without much restriction in terms of heat, Flexibars with 5 laminations of 63mm high would get me an ampacity of 883A at a delta T of 30C, according to their table.

Since I will run three in parallel (ONE per phase), if I am applying the 2.25 factor, I need a set of Flexibars of 10 laminations of 100mm high (over 1900A at same Delta T). It's quite a difference for mounting, space, etc.

I want to do it right, but not overdo it necessarily. I can place the bars (they are insulated by the way) a minimum of 1/2" from each other and possibly widen that to 1 inch.

I hope this information helps in solving my issue.

 

[waross]

Since I will run three in parallel (ONE per phase)

While these may be in parallel physically, they are not considered in parallel electrically.
If you are running one bar per phase, no derating is required.




--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[patm72]

Thank you, Waross. I Appreciate.

 

[che12345]

I have the following opinion for your consideration.
1. In most cases, busbars are of short length say < 30 m. As their resistance and reactance are very low, therefore the voltage drop is of NO significance. For your case 2.5 m (10'); ignore it.
2. For installations in the US, take the UL tinned current rating instead of the IEC.
3. Take the delta T into consideration. Attention: delta T is the bar max. temp - enclosure internal temp, which could be higher than 40 ^oC.
4. In general, limit it to say 4 bars in parallel per phase or use a larger bars, instead of > 4.
5. In general, the air clearance = the bar (including insulation) thickness would achieve good cooling.
Che Kuan Yau (Singapore)

 

[waross]

These are not bus bars.
These are assemblies of parallel flat strips bound into a cable.
The flat strips provide some flexibility and reduce skin effect.
Code ratings for bus bars are not applicable.
Use the manufacturers ampacity ratings.
OP is using one flex-bar per phase.
No derating is needed.
However, it is code in many places and good practice to limit maximum continuous loading to 80% of the maximum ampacity.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

I have following additional opinion for your consideration.
1. If 900 A is FLA x 1.25; that is you had taken "the code in many places and good practice to limit maximum continuous loading to 80% of the maximum ampacity". Attention: The size may base on 900 A. Do NOT derate further by 0.8 factor.
2. nVent published UL amperecity data based on delta T 45 K (NEC 310-16) 75 deg C :
a) n = 5, W = 50 mm, T = 1 mm, amperecity = 911 A.
b) n = 6, W = 45 mm, T = 1 mm, amperecity = 919 A.
c) n = 10, W = 32 mm, T = 1 mm, amperecity = 951 A.
Note: All three are with about the same amperecity.
Attention: Do NOT derate these ratings by 0.8 factor; as you had taken 1.25 factor to arrive at 900 A.
3. For LV applications, "Voltage drop" and operating voltage < 1 kV is of no significance.
Che Kuan Yau (Singapore)

 

[LionelHutz]

If you are applying a UL listing, make sure you follow the UL requirements and not the nVent tables to size the flexbus.