Cable Ampacity 1

[cherry2000]

We have a General Cable XT-125 Polyriad 1C-250MCM being used for 125VDC application.See attached Catalog.
Now, this cable is rated for 95degC termination and ambient of 45degC, and for free air laying.
Our conditions are as follows:
75degC termination, 23degC ambient (climate controlled Substation), two 1C cables laid in one conduit.
What is the method of getting the derating/uprating factors? I checked NEC which has uprating factors, but CEC does not appear to have any uprating factor.
Any suggestions will do..I am not looking for the exact ampacity.

 

[dpc]

You're not allowed to uprate beyond what is given in the NEC for a particular conductor and insulation, AFAIK.

If this is an electric utility, you're not bound by the NEC. But since the termination is only rated for 75 C, you will not be able to apply the cable up to its 95 C rating.

I would want to be on the conservative side. The ambient may not always be 23 deg C - things fail.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[cherry2000]

DPC..possibly it would be the correct approach to ask General Cables directly for a correct rating at the given conditions.

 

[dpc]

Yes, that could be helpful. But if you are trying to comply with the NEC, you will need to live with what is in the NEC ampacity tables.

If you are attempting to address an existing condition, it could be helpful if you need to apply for a variance.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[waross]

Just a suggestion, go to the Canadian Electrical Code, Part I and find an appropriate cable from Table D1. These are the types of cables approved for use under Part I of the CEC.
Polyrad XT 125 (CSA C22.2 No. 245 Type X110) does not appear in table DI.
Polyrad does not seem to make a 250 MCM cable.
Free air rating does not apply when installed in conduit.
Cable ampacity is often limited by the termination temperature rating.
Have you considered possible accelerated corrosion of the strands with fine strand cables at higher temperatures?
I'm not saying that you cannot use the cable but you may be on your own to extrapolate ratings.

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

 

[7anoter4]

I agree with dpc and with waross also. But as you did not ask for "exact ampacity" I think if we shall follow the calculation way of IEC 60287 we can get a good appreciation.
As IEC 60287 use metric u/m the following are your data:
Cable type XT-125 Polyriad 1C-250MCM
Conductor cross section= 262 MCM *.5067= 132.7554 sqr.mm
Cable dia =22.1 mm
Conductor dia =0.589 inches= 14.96 mm
No jacket [only insulation] of 0.357 inch=0.1405 mm thickness
Max.conductor temperature TC [degrees C]
Max. ambient temperature TA[degrees C]
Thermal resistance[Roins]: 5 m.K/ w
Calculation:
Insulation resistance Tins= Roins/2/PI() =5/2/pi()= 0.79577
FOR CABLES OR CONDUITS IN FREE AIR :
For single cable in free air :
H = .21 / (DE) ^ .6 + 3.94
Where DE = Cable dia =22.1/1000 m
H= 6.008
For cable in conduit DE = conduit overall dia.[m]
QS = shield losses factor –no shield losses QS=1
KA = PI() * DE * H / QS * (Tins / NM + Tjacket * QS)
NM=No. of phases NM=1 for single phase
Tjacket=0 [no jacket]
KA=0.332
Factor DDS is calculated through iteration.At the beginning DDS=2 and DDSnew is
Recalculated as follows :
DDSnew = ((TC - TA + DTT) / (1 + KA * DDS)) ^ 0.25
If the cable is not exposed to sunshine DTT=0.
If (DDSnew-DDS)/DDS < 0.01 then the outside resistance will be
Text= 1 / PI() / DE / H / DDS .
If (DDSnew-DDS)/DDS > 0.01 then DDS=DDSnew and DDSnew should be recalculated.
Iadmissible=sqrt((TC-TA)/RCA/ (Tins + NM * QS * Tjacket + NCND * NM * QS * (Tconduit + Text)) * 10 ^ 3)
NCND =no. of cables =2
RCA is conductor resistance at TC temperature including skin effect factor [proximity is neglected].
If is no conduit Tconduit =0
Tconduit= U / (1 + 0.1 * (V + Y * TM) * DEN)
TM =approx TC ; U = 5.2: V = 1.4: Y = 0.011 for metallic conduit or
U = 5.2: V = 0.83: Y = 0.006 for PVC conduit.
DEN =1.65*Cable dia[for 2 cables in the same conduit]
Coclusion:
For 1 cable 262 MCM 60 Hz RCA= 0.158687 ohm/km for TC=95 and TA=45 Iadmissible= 400 A instead of 392 A as per Manufacturer table.[102%].
For the same cable but TC=75 and TA= 45 only 300 A will be permitted.
For 2 cable in conduit TC=75 and TA=23 Iadmissible=170 A per cable then 2*170=340 A.
But, as dpc said:
Quote:
"I would want to be on the conservative side. The ambient may not always be 23 deg C - things fail."
Then for 2 cable in one conduit TC=75 and TA=30 Iadmissible= 159 A per one cable [total 2*159=318 A]

 

[cherry2000]

Anoter..Thanks a lot for the valuable discourse & detailed calculation. Indeed the cable ampacity takes a nose dive when the temp rise is limited to Termination temperatures. I am wondering under what circumstances, the marine XT cables can achieve such high temp as 95 upto 125degC

 

[REDDOG]

See NEC 110.14 C (1)

 

[cherry2000]

Reddog..I am not clear about the temp ratings. NEC is clear in Sec 110.14(c) in following the temp in Table 310.16 which is having a tange of 60-75-90 Deg C. This is definitely the termination temp restriction. Now the XT-125 cable is rated at far higer values. Does this mean that the industrial practice in Marine projects, is to employ terminations with these high temps? I do not see any other way in explaining this. This is specified in NEC Sec 110.14(c)(1) as "..Unless equipment is listed and marked otherwise...."

 

[REDDOG]

The Section indicates that over current devises such as circuit breakers, fuses, and thermal overload relays are calibrated with 4 ' of code sized cable (Ampacity of wire to match rating of over current devise). Applying a cable means that the size of the cable is selected at the 60 or 75 degree column in 310-16.
Once that size is determined you may selected a 90 degree insulated wire, then apply the derating factors at the bottom of the table against the 90 degree Ampacity listed for that cable.
If the value is greater than that from the 75 degree column everybody is happy. If not you must increase the conductor size until you satisfy the ambient conditions?

In summary you may use the higher rated temperature cable, but only at the 75 degree rating.
Remember even if the devise is 100% rated it still has the 75 degree wiring requirement. See 110.3b

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