HV Cable Ampacity

[timm333]

IEC 60287-1-1 provides ampacity calculation methods for HV cables, these methods provide the ampacity for single conductor cables. What would be the total 3-phase ampacity of the whole circuit? For example, if the calculations show that Phase A has 500 A, phase B has 500 A, and phase C has 500 A; then what would be the total ampacity of the 3 phase circuit? Thanks for help!

 

[GoldDigger]

IEC 60287-1-1 provides ampacity calculation methods for HV cables, these methods provide the ampacity for single conductor cables. What would be the total 3-phase ampacity of the whole circuit? For example, if the calculations show that Phase A has 500 A, phase B has 500 A, and phase C has 500 A; then what would be the total ampacity of the 3 phase circuit? Thanks for help!

Terminology is very important here. Often the phases refer to the line-to-line currents in a load at the line-to-line load(s). So: AB, BC and AC
The three wires, A, B, and C, are called the phase conductors.
But sometimes people instead refer to the A, B, and C phases.

Trying to answer your question in a way that is independent of the choice of terminology, if the three phase circuit is capable of 500A on each of the three conductors, it is called a 500A circuit, and would be protected by a single three pole 500A breaker or three individual 500A fuses and a phase loss detector, if needed.
Using the terminology "current per phase" and "total current over all three phases" is sure to cause confusion.

 

[Julius Right]

IEC 60287 provides methods of cable current rating (100 % load factor) calculation
[There are 4 volumes for different calculations]
for n parallel cables or n cores in the same cable. See for instance
IEC 60287-1-1 1.4 Permissible current rating of cables.
The conductor temperature rise:
Δθ = (I2R + ½ Wd) T1 + [I2R (1 + λ1) + Wd] n T2 + [I2R (1 + λ1 + λ2) + Wd] n (T3 + T4) .
n=is the number of load-carrying conductors in the cable (conductors of equal size and carrying the same load)-It does not important the conductor phase.
For cable in free air you have 60287-2-1 2.2.1 Cables laid in free air.
As you can see from above formula n is a multiply factor for T2,T3 and T4 thermal resistance, only.

 

[Sahib]

Current derating for grouping of conductors applies for more than 3 conductors. So the question is how many conductors in each phase. If there is only one 500A conductor in each phase, the total ampacity of the 3 phase circuit is also 500A.

 

[timm333]

Thanks. IEC 60287-1-1 section 2.3.6.1 gives the formula to calculate eddy currents:

λ1"=Rs/R*[gs*λ0(1+△1+△2)+(β1·ts)^4/12×10^12]


Then in the subsection 2 of the same section, formulas are given to calculate the factors λ0, Δ1 and Δ2; these factors are given for 3 cases: (a)-center cable; (b)-outer cable leading phase; and (c)-outer cable lagging phase.

Should we add the values of λ0, Δ1 and Δ2 in all these three cases, and then put the sum in the above equation to calculate λ1"?

 

[Julius Right]

You have to do so if m>0.1 if not you may neglect D1 and D2.
The tricky part it is calculation of Rs [shield ohm/m] from shield cross section area[mm^2] resistivity in ohm*mm^2/m[1/58 at 20oC for copper] corrected for 90oC [234.4+90)/(234.5+20).

 

[Smart $]

IEC 60287-1-1 provides ampacity calculation methods for HV cables, ...

I have a question for you. Why are you even bothering with an IEC standard when you are in the U.S.? As best I can tell, your entire state is under the 2017 NEC.

 

[timm333]

I am actually working on an international project. Thanks for help!

 

[Ingenieur]

500 Amp

if you have a load of 5 MVA at 13.2 kva
i = 5000000/(sqrt3 x 13200) = 219 A
not 656
each conductor handles 219 A, not 656, in fact for a balanced ckt the sum(iabc) = 0
is this being done under engineering supervision?

Why not just use the cable manufacturers data?
an example http://nl.prysmiangroup.com/nl/busi.../downloads/datasheets/Prysmian-Delft-HVac.pdf
gives ampacity for various install methods, spacing, configurations, shielding, etc.
I assume IEC since 50 Hz
but you can find the same for cables sold under various codes/regs/etc

 

[Smart $]

I am actually working on an international project. Thanks for help!

If the project is within U.S. borders, there are only a few states which have not adopted the NEC into law. While your project spec's may demand IEC compliance, NEC compliance is mandatory by law, regardless of what the project spec's may say.

Amperes of current on each line (L1, L2, L3; aka ØA, ØB, ØC; A, B, C for short) require the conductor to have an equal or greater ampacity. In some cases, the required ampacity or portion thereof may require a 125% factoring of the load current.

The circuit rating is the lowest of the conductor ampacities or OCPD rating.

 

[Julius Right]

You are right, Smart $. However, NEC does not present any way to calculate the ampacity if it is not intabulated. The part (C) Engineering Supervision actually it is not a part of NEC [except the reference to IEEE 835-1994 (IPCEA Pub. No. P-46-426), Standard]. In this standard is presented a way of calculation based on The Calculation of Temperature Rise and Load Capability of Cable System by Neher and McGrath. This is the U.S.A. variant of IEC 60287.
By the way, I read somewhere that before the first edition called IEC 287, a delegation came to the U.S. to consult with Neher and McGrath.

 

[Smart $]

You are right, Smart $. However, NEC does not present any way to calculate the ampacity if it is not intabulated. The part (C) Engineering Supervision actually it is not a part of NEC [except the reference to IEEE 835-1994 (IPCEA Pub. No. P-46-426), Standard]. In this standard is presented a way of calculation based on The Calculation of Temperature Rise and Load Capability of Cable System by Neher and McGrath. This is the U.S.A. variant of IEC 60287.
By the way, I read somewhere that before the first edition called IEC 287, a delegation came to the U.S. to consult with Neher and McGrath.

I recognize 310.15(C) as an alternative to the NEC tables (and derating)... but OP'er lists his occupation as an electrician, not EE.

 

[timm333]

Yes it will be under engineering supervision. It has been good discussion, thanks.

 

[timm333]

Sahib, sorry but I find your posts irrelevant. And I am not looking for opinion from India anyway due to the quality issues. So you don't need to bother answering my questions.

 

[Sahib]

I do not think the post4 is irrelevant. India is an IEC country and this is an open forum.

 

[timm333]

Right, I know anybody can express opinion here, but still there has to be some quality of opinions. There is a reason that in middle east, people from India are paid 2 times less than people from other countries, and the reason is the quality of work.

 

[Sahib]

Right, I know anybody can express opinion here, but still there has to be some quality of opinions.

Okay, but you have not explained how the post4 is of poor quality. Don't say it is from India and so of poor quality.

 

[timm333]

Just not interested...

 

[Sahib]

It seems neither NEC nor IEC requires current derating for a group of 3 conductors. May be installation quality issue.

 

[Ingenieur]

Right, I know anybody can express opinion here, but still there has to be some quality of opinions. There is a reason that in middle east, people from India are paid 2 times less than people from other countries, and the reason is the quality of work.

This is BS