CABLE CURRENT RATINGS

[Edelma_1]

Hello Everyone,

Which of these current ratings is used as the standard current ratings for cable:

Cyclic, Sustained or Distribution.

For example;

A 630mm[sup]2[/sup] Cu cable (dry XLPE type) has a "Winter SUSTAINED Current Rating" of 841 Amperes, a "Winter CYCLIC Current Rating" of 948 Amperes and a "Winter DISTRIBUTION Current Rating" of 1015 Amperes.

Are we supposed to use the worst case scenario? which would be the "Summer Current Ratings" since temperatures are going to be the highest recorded in the season increasing the resistance of the cable.

For Summer we have, Summer SUSTAINED Current Rating" of 756 Amperes, a "Summer CYCLIC Current Rating" of 874 Amperes and a "Summer DISTRIBUTION Current Rating" of 959 Amperes.


N.B: - THIS IS FOR A 33kV SINGLE CORE X.L.P.E. INSULATED LEAD SHEATH & M.D.P.E. OUTER SHEATH CABLES, LAID IN PVC DUCT UNDERGROUND. (Dry design)

 

[SilverfoxUK]

Ask yourself is the cable going to be used in the summer, if yes then all of your winter settings have gone.
Is is connected to a single device such as large motor that gets used once every blue moon. I'm guessing you can't guarantee that.
Go for summer sustained and sleep peacefully in your bed!

 

[Edelma_1]

my point is, why have design calculations based on seasons when winter will eventually go and summer will set in, if someone lays the cables in winter and loads it with say a sustained current ratings of 841A and summer (which has a maximum sustained rating of 756A) comes won`t it have an effect on the cable?

 

[SilverfoxUK]

because you may only use the cable to drive say a very large heater, a permanent installation, in the depth of winter. You have to be very sure if you are using the higher rating. As I said go for summer.

 

[waross]

I would not use those ratings.
Rather I would use the ratings that are laid out in the Canadian Electrical Code.
As for the seasonal ratings;
If the loads include a lot of electric heating, then the winter loads may be higher than the summer loads.
If the loads include a lot of AC then the summer loads may be higher than the winter loads.
Seasonal ratings may allow you to take advantage of seasonal load variations.

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

 

[Edelma_1]

what value does the Canadian Electric Code give for a cable of that type (3 core, XLPE 33kV, 630mm[sup]2[/sup])?

 

[waross]

Single core:

630 mm2 equals 1250 kcmil.
In other tables 1250 kcmil is shown with about 8% higher ampacity than 1000 kcmil.
so, 514 Amps x 1.08 = 555 Amps.
OR

666 Amps x 1.08 = 719 Amps.

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

 

[Edelma_1]

Okay thanks

 

[Edelma_1]

if you are to connect 10 numbers of streetlights of say 60W 230v each along a line of 300m and you are told to give the the voltage drop at each of the lamps. Point of connection of the lamp is an LV distribution panel with a voltage drop of 0.76% and external loop impedance of 0.349 Ohms. how do you calculate it.

 

[7anoter4]

BAHRA, the Manufacturer of this type of cable, recommends 740 A for trefoil and 685 for flat formation but directly underground
-without conduits. Ambient Ground Temperature: 35 C, Soil Thermal Resistivity: 1.2 °K.m/W [according to IEC 60502-2]-one group of 3 single-core cables.

https://www.bahra-electric.com/Downloads/MV_Lead_Sheathed.pdf

 

[7anoter4]

Calculated -according to IEC 60287-2-1 and 1-1- in the same conditions as BAHRA did :
Ambient Ground Temperature: 35 °C
Depth of laying in ground: 0.80 m
Soil Thermal Resistivity: 1.2 °K.m/W
I got:
713 A both ends grounded in 8" pvc duct.
722 A one end grounded 8" pvc duct.
762 A both ends grounded no duct
772 A one end grounded no duct
Three single-core cables in trefoil.

 

[bacon4life]

Rather than a "standard rating" for medium voltage cable, there are various IEEE/IEC standards that describes how to use input assumptions to calculate an approximate cable temperature. It is up to the user to determine appropriate input assumptions that match their application.

We use a similar set of ratings for each type of cable to maximize asset utilization. Using just the most conservative rating wastes capacity under other conditions. Here are examples of how I use the ratings:
[ul]
[li]Winter SUSTAINED Current Rating" of 841 Amperes,-service to an industrial plant that runs 24x7 at approximately constant power for months at a time[/li]
[li]"Winter CYCLIC Current Rating" of 948 Amperes- service to typical residential area that has a large difference each day between on-peak load and off-peak load. During the off peak periods the duct bank will cool off. Weather driven peak loads typically last for only 3-7 days.[/li]
[li]"Winter DISTRIBUTION Current Rating" of 1015 Amperes- maximum current allowed for short durations of 30 -120 minutes. This rating may include assumptions of utilizing the thermal inertia of duct bank to calculate a transient rating.[/li]
[/ul]

My region has much higher electric loads in the winter than summer, so having both summer and winter ratings is important for me. If the OPs cable serves a single fixed load, the seasonal ratings may not be relevant to the OP.

As a slight tangent, in the USA FERC recently mandated that electric utilities must switch from using summer/winter ratings to using ratings based on real-time hourly temperatures, with increments of no more the 5 degrees F between rating.

 

[davidbeach]

The hourly ratings, though, only apply to the BES (Bulk Electric System) and wouldn't impact the OP's 33kV cable. I'm also going to guess, that for BES cable there's not going to be nearly the range of ratings that there will be for bare conductor installations.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[7anoter4]

It's always good to do a check because many times a mistake can creep into the manual calculation. Now, for example, when calculating the external thermal resistance of the duct I used the diameter of the cable instead of the diameter of the duct. Then, the actual ampacities using 8” pvc duct are:
734.8 A both ends lead sheath grounded, in 8"pvc duct.
743 A one end lead grounded, in 8" pvc duct.
[100% load]

 

[7anoter4]

if you are to connect 10 numbers of streetlights of say 60W 230V each, along a line of 300m...

 

[7anoter4]

Sorry.Rwire=300/10 length,1.5 mm^2 copper Rwire=30/1.5/58=0.3448 ohm [ 1.5 copper wire at 20oC].
Rbulb=230^2/60=881.67 ohm

 

[7anoter4]

Sorry[again!] I forgot the return wire. Then I have to change the Rwire=2*30/2.5/58 in order to get total DV less then 3%[2.5 mm^2 copper instead of 1.5]

 

[Edelma_1]

@7anoter4 the 0,76 is the Voltage drop factor which is in percentage so 0,76% of 230 should be

0.0076 x 230 = 1.748 Volts

230 - 1.748 = 228 V

 

[7anoter4]

If you intend to achieve no more than 3% drop from rating voltage [230 V] you need to increase the conductor cross section from 2.5 to 4.However,the ambient temperature has to be more elevated [let's say 40oC]. The reactance may be neglected, still. Since, according to BS 7671 at 70oC has to be 26 A and the temperature rise from ambient 70-40=30oC, for 2.6 A will be 30*2.6^2/26^2=0.3 oC.
So,Rewire [for 300/10=30m] will be now 2*30/4/58*(234.5+40)/(234.5+20)=0.278944 ohm

 

[Edelma_1]

what software did you use for this?

Take a look at this Voltage drop circuit formular