heat dissipation from cabling

[jimmy2times]

I am trying to work out heat losses in panel and want to include for cabling. 60Hz 3phase system

Am I correct in saying formula is i²R where R is ac resistance of cabling.

We have 9off 95mm.sq conductors per phase and there is approx 300mm between terminations and the sealed deckhead through which they transit into panel. So total cable length is 9 x 3 x 300m = 8.1m

AC resistance of cable is given as 0.23ohms/km, so total resistance is (0.23 x 8.1)/1000 = 1.863mohms.

Current in circuit is 1100A.

That gives about 2.15kW. That seemed high to me have I simplified this too much ?

The panel is IP67, with surface area of 5m2, there are no cooling fans or inlet grills. There is busbar and circuit breakers inside. I am trying to work out the watts dissipated in the panel and whether the surface area (mild steel) can remove the heat without causing excessive rise. This calc alone seems to blow it out of the water suggesting we have an oven from the cabling alone! this is before I even consider busbar losses and circuit breaker pole losses. Hence I'd like to clarify my calcs.

(apologies for the metric units!)

 

[Skogsgurra]

I didn't quite follow that 1100 A and 9 x 3 x 300 m(m) thing.

Shouldn't current be divided between the nine parallel cables? Then you get a lot less current in each cable - and a lot less heat.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

Wait... nine 95²mm cables PER phase was sort of stated with the busted gammer/spelling which would change the results a LOT.

9off 95mm.sq conductors per phase

Each phase would then have 1/9th the current running thru each cable.

1100A/9 = 122A

0.23 ohms/km x 1km/1000000mm = 230nohms/mm

300mm x 230nohms/mm = 69uohms per 300mm length carrying 122A

69uohms x 122A² = 1.027W per wire.

9wires x 3 phases x 1.027W/wire = 28W.

Unless you meant something different...



Keith Cress
kcress -

http://www.flaminsystems.com

 

[jimmy2times]

Arrrgh stumbled on kirchoffs law, doh!

Yes forgot to divide 1100 by 9. Thanks for putting me back on track.

How silly i was taking account of division of current with my busbar calcs but overlooked it on cables.


I thought i was an order out!

Thanks

 

[KuanYau]

to jimmy2times,

1. Many experts had pointed out the wrong concept in your calculation, which I do not wish to further elaborate.
I wish to point out that cable of these size and 300mm in length is of [no heat generation concern]. No one take this into consideration.

2. What you should be more concern is the heat generated [due to contact resistance] at the [cable terminations, busbar joins and heat generated at the ABC withdrawable contacts and their main power three-phase contacts including the MCCBs three-phase main power contacts].
If it is a MCC add also the [wattage of the cable connection terminals and the power loss across three-phase main power contacts]
If there are pf correction capacitor with filter reactor add these heat losses (especially the reactor) which generate very much heat.

3. Surface area of 5m2 seems [rather small unless this enclosure for cable termination only].
The cooling surface should include four surfaces (three if enclosure is placed against the wall) which should include the top areas but exclude the bottom; if the enclosure is floor-mounted.

 

[jimmy2times]

Thanks kuanyau

It is basically a shore connection box, i made a post a couple of days ago (unrelated to this thread) mentioning we are tripping our shore supply. This box is basically a glorified junction box but has breakers, connection plugs (to shore), busbar (various sizes) and the outgoing cabling to ships switchboard.

My thought is the circuit breakers are tripping early on their characteristic due to excessive heat rise in panel. They need derate after 60degC

I have come up with a figure of 580watts (after correcting calc above) this gives a 26-28degC internal rise using graphs i found online. I used a heat dissipation calc for busbar sections and took watts lost per pole of circuit breaker from oem data. Our ambient is 30degC

Within the calcs i estimated 45W for connections of busbars joints and cables contacts. I wasnt sure on this and didnt bother to raise question here as didnt think there would way to calculate it, if there is some guide rule for contact surface resistances i could use i'd be interested to know. Or whether my estimate seems right kind of ballpark area (30 cables connections and 15 busbar joints, i estimate 1W per connection as an average across both busbar and cable)

Either way the answer im getting is leading me to think this could be a problem and think we will conduct thermal survey of box external and place some temp stickers inside to try and capture internal temps around breaker.

One source i found on line cautioned about applying a 25% safety factor in calcs in the temp rise, to allow for the restrictions within the box that lead to the convection/radiation, applying this factor would certainly take the internal temps over 60degC.

 

[itsmoked]

One thing often missed in these cases are waveform distortion. If there are peaky aspects to the current then the RMS values will not be telling the correct power dissipation story. The I^[red]2[/red] will be squaring those peaks which cranks the I²R losses. This happens if you have lots of VFDs or switchmode power supplies, fluorescent lighting, etc.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

This is why there are strict codes regarding sizing of conductors relative to breaker and fuse sizes. If the system is designed to meet applicable codes, you really shouldn't have to be worrying about the heat loss in the cable. It seems far more likely that the breaker is defective or you have loose connection at the breaker that is generating a lot of heat. If possible, I'd do infrared testing around the breaker and perhaps have the breaker tested.

 

[waross]

Hi Keith;
As one friend to another, can you look at this and tell me if I am off base?
The Root Mean Square squares those peaks, however those peaks imply high frequency components. The effective AC resistance of a cable is somewhat frequency dependent, and the I√R losses are greater because of the increased effective resistance for the high frequency components of the current peaks. No problem understanding that waveform distortion increases losses, but a little unsure of the mechanism.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Bill,

I'm not Keith, but the issue with harmonics is determine the effective rms current. It's only 70.7% of the peak for pure sine wave. If you break the waveform down into the fundamental and the harmonics, the effective rms total is the square root of the sum of the squares of the fundamental and harmonics. The ac resistance will increase a bit with higher frequency due to skin effect as well, as you mentioned.

Cheers,

Dave

 

[waross]

Hi Dave;

It's only 70.7% of the peak for pure sine wave.

I have seen a couple of cases where the indicated voltage, based on the average as determined by a de'Arsonval meter and scaled to RMS with the 1.1 form factor was noticeably in error. Both times on the unloaded secondary of a wye connected transformer bank.
I think that we are on the same page, in the I√R of losses due to harmonics The RMS is the RMS and takes care of the current peaks. It's the effective R that increases due to the effect of the higher frequency current components.
Keith did a good job of explaining what happens. Just a little off on why.
I've made much worse mistakes Keith. Grin.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[KuanYau]

To jimmy2times (Electrical)

1. " My thought is the circuit breakers are tripping early on their characteristic due to excessive heat rise in panel. They need derate after 60degC"
Breakers with thermal protection which are tested to IEC Standards are tested to carry 100% rated current at 40 degC. The tripping time shall be within the limits. These limits are also dependent on whether the breaker is in the "Cold" or "Hot" state.
As a rule-of-thumb, the current rating is [de-rating] to 87% at 60 degC and [de-rated] to 80% at 70 degC.
If your incoming breaker current rating is [20-25% higher than the estimated total load], the tripping may not be due to the internal high temperature by the heat generated. It could be due to actual over-loading' for a long duration.

2. "This box .... has breakers, connection plugs (to shore), busbar (various sizes) and the outgoing cabling... One source ...applying a 25% safety factor in calcs in the temp rise, to allow for the restrictions within the box that lead to the convection/radiation,.... the internal temps over 60degC".
A shore connection junction box generally has [neither horizontal non vertical partitions]. A 25% safety factor is good to have but not mandatory unless working under very tight margin.

3. Temperature rise evaluation for enclosure is well established in the IEC world (See IEC Standards).

4. "Our ambient is 30degC"
A simple way to lower the ambient temperature is [to place a simple shade over the top and sides] maintaining a free-air flow space of not less than 300mm.
Another option is to add fins (may be of simple corrugated sheet metal painted dull-black]with area as big as possible attached [tightly on to the outer surfaces] which maintains the IP67 rating.

 

[jimmy2times]

kuanyau.

1. the design seems on the limits, there are 3x400A mccbs in parallel (i made a post regarding these previous thread238-376464). Manufacturer OEM data is derate 0.95 at 60degC and derate 0.9 at 70degC. we are drawing 1100-1150A average

2, it is 1.75x1x0.5m box, there is some perspex partition inside more so as to protect against live parts

3. is this IEC 61439?

4. nice idea but the box is inside the ship! compartment with small porthole opening (classed as weatherdeck by marine rules, hence ip56, meant to say this earlier sorry not ip67)

we may try a trial run with the covers removed and see if problem goes away, will let you know how goes

 

[itsmoked]

Bill; Can't help you with further understanding since now it isn't making much sense to me. I just went thru some numbers and disproved my theory.. Ha!



Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Smoked - you disproved your theory? That is really interesting. What was the theory?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.