Trying to size cable properly 1

[ogbuehi]

We have a contractor submitting a cable that he is requesting our approval on. The load is rated for 300A so he submitted submitted a 300mm^2 (1 run, all phases and neutral in one cable). The voltage is 380V. This cable is to be buried directly in earth and concrete-encased in a electrical duct when the cable runs under the roadway. The conductor is copper and insulation is XLPE PVC sheathed cable. The cable he submitted is multi-core (see attached PDF, page 18 on the document or page 19 by what Adobe says). The code we're using is NEC and I'm having a hard time in 310 trying to find what table applies to this cable. I thought maybe it could be considered triplexed cable but there are 4 current carrying conductors in the cable itself. I don't have access to the cable specifications I would need to do a manual calculation (dielectric loss temperature rise, dc resistance of conductor at temperature, effective thermal resistance between conductor and surronding ambient etc..). I was wondering if anybody had dealt with this type of cable in determining it's current carrying capabilities.

 

[waross]

You don't need to count the neutral for ampacity calculations. (The exception is two phases sharing a common neutral as the neutral carries the same current as the phase conductors when the circuits are equallyloaded.)
Treat it as 3 conductor cable to determine ampacity.
A greater problem may be UL certification. Is this cable listed in the NEC?

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

 

[ogbuehi]

First of all, thanks for the help. I was thinking the same thing since even though the neutral is treated as a current carrying conductor there shouldn't be much current on the neutral to make much of a difference in regards to temperature considerations. I wouldn't be surprised if it isn't UL listed because it's made in Jordan for mostly the middle east. But the other thing I couldn't find is the XLPE insulation in the NEC. Is that considered the same as RHW?

 

[waross]

Barring harmonics, there is only current in the neutral when the phase currents are unbalanced. If two phases are loaded to 100% and the third phase is loaded to less than 10% then the difference will show up on the neutral. Cable heating is mostly I²R. The sum of .9² and .1² is less than 1.0² so this approach is valid and safe.
If you have a lot of third harmonics you must consider the neutral as a fourth current carrying conductor. In the Canadian code that would require the use of the derating factor specified under "More than three up to N current carrying conductors in a conduit or cable."
I am not sure of the NEC equivalent wording.

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

 

[ogbuehi]

These particular cables will be feeding mostly linear loads. Not very much 3rd, 6th, etc.. or higher harmonics. There is supposed to be 2 battery chargers for forklifts when the building is built, but the biggest impact load are motors for some water pumps.

 

[7anoter4]

Since the cable is build according to IEC standard one may calculate the 100% steady stated load current from the IEC 60278 standard.
As the insulation is XLPE the thermal resistance may be 3.5 K.m/w and for PVC jacket 5 will be fair.
No shield or dielectric losses are involved for 0.6/1 KV cable.
The problem is the earth specific thermal resistance. An average-high will be 1.5 K.m/w, but if the soil is dry or rocky could be more.
Another problem may be the vicinity of heat sources as other power cables or hot liquid pipes.
If you have not such a problem, then for a cable buried up to 4 feet depth in a soil temperature of 20 degrees C and 1.5 specific thermal resistance- copper conductor- the maximum permissible will be 500 A per phase.
The short passes under the road [up to 20 feet] may be neglected.

 

[ogbuehi]

Thanks for the help 7anoter4. Where did you get the thermal resistance for XLPE? Also do you know where to find the temperature rating for XLPE? Thanks.

 

[7anoter4]

For conductor maximum temperature for XLPE insulating compound see:
IEC 60502 Table 3 " Maximum conductor temperatures for different types of insulating compound" [XLPE =90 oC].
For specific thermal resistance see: IEC 60287-2-1 Table 1 – Thermal resistivities of materials .See this link:

http://www.upcomillas.es/periodicas/Normas/IIT2902 iec60287-2-1{ed1.2}b.pdf

 

[NAZ55]

Hope this will help.