Should there be any difference in the way we size a power cable?

Sorry - one more clarification requested.

When I try to get the cable resistance and reactance for my voltage drop calculations, I can get it only for 20 Degree Centigrade.

If our ambient temperature is not the same, how do I get a correction factor. I know that there is a correction factor for ambient temp (50 Deg C) with respect to the maximum conductor temperature(85 Deg C) .

But does this correct the values of the resistance and reactance from 20 Deg C to 50 Deg C also?

The current issue of the Wiring Rgulations, maintained by the by the Institution of Electrical Engineers, gives guidance on this subect.

There are different factors on which cable size is selected like grouping, Method of laying, Voltage drop and short circuit and a size that satisfies all should be selected .You can refer to any electrical Engg.Hand book.

I agree what you said. These Method of cables are very important, since many factors will depend on the way cables are laying and grouping. Especially power cable and control.

From my experience, I do not listen to power cables manufacturers nor to guidance tables in respect of loading cables. The major factor is temperature rise above ambient. It should not exceed 10C degree. Temperature rise of 10C - 20C degrees is acceptable conditioning that you frequently scan IR at both ends of cable and not at insulation. excessive temperatre rise means loosing money and approashing a problem.

That is very correct but you can't sell this here in Europe to the legal inspection prior to start-up permission. You have to have certified calculations, who can differ from method to method.

personnally I had a cable calculated by a so called "notified body" who came to a 3x250 for 1250m after some recalculs with the manufacturer we could , agreed by the same notified body reduce to 3x125 considering all factors for laying venting etc.

Since I always have the cals approved by the legals before ordering cable.

I agree with you, but this technique is not sufficient to rely on, since nowadys there are so many factors which will influence the cable. In any way the voltage drop calculation will depend on the grouping and laying method, since heat dissipated will definetely different in these two methods.

If you operate in America, you need to use the National Electrical Code. Article 430, Motors, Motor Circuits, and Controllers, this article will answer these questions for you. Article 310, Conductors for General Wiring, Table 310.16 has correction factors for Ambient temperature variations. Examples are in the Code Book to help if needed.

Good luck,

James

Thank u all for the inputs. But appreciate if you can address my first part of the question also.

First part:

1) Should there be any difference in the way we size a power cable. I am aware that when we do voltage drop calculations for a DOL motor, we usually take 6 times starting current.But how should we size the cable if I am sizing it for a Star-Delta(S-D) or Variable speed driven (VSD) motor or softstarter driven motor.

Second part:

When I try to get the cable resistance and reactance for my voltage drop calculations, I can get it only for 20 Degree Centigrade.

If our ambient temperature is not the same, how do I get a correction factor. I know that there is a correction factor for ambient temp (50 Deg C) with respect to the maximum conductor temperature(85 Deg C) .

But does this correct the values of the resistance and reactance from 20 Deg C to 50 Deg C also?

Regarding the second part, thank you for the inputs. But I have already taken the correction factor for the difference in ambient temperature from 50 Deg C to 85 Deg C, as given in IEC (correction factor is 0.94).

So what I was trying to find out is whether this also takes care of the cable resistance values per meter length , which is provided by cable suppliers at 20 Deg C. Or do I still need to correct this also by factor? If so, where do I get this.

I also feel that maybe I am derating twice unnecessarily. Once using the IEC values given in IEC tables B.3 of IEC 60092-352 and once using the resistance values/ meter given by the cable supplier at 20 Deg C.

Thank you in advance.

Hi EITM

Cable sizing has many, many variables, their importance varying with each situation. ALWAYS consult your local wiring regulations. The laws of physics being what they are though, we can consider a general situation.

First Part

Take current first. A DOL motor takes a much larger starting current than running. If the start current trips the breaker (or blows the fuse) then the motor won't work. Get a catalogue from the manufacturer of the breaker you intend to use and check to see what the breaker size should be for your motor. Your cable must be large enough to carry at least this current. A SD starter will generate a lower starting current, allowing a smaller breaker, thus a smaller minimum cable size. A VSD will give you the least starting current, but generate all sorts of harmonics which also influence cable choice (for example, screened cable needs to be used to connect to the motor).

Next is fault current. The cable needs to be sized to carry the fault current, or even upgraded to increase fault current to permit the breaker to trip in the time specified by your wiring regulations.

Now we get to volt drop. All the wiring regulations I am aware of allow you to ignore transient currents like start-up for purposes of volt drop. So you calculate volt drop based on running current, not starting current. A big advantage of a VSD is that they can be very volt-drop tolerant on their input, which can give big economic benefits in reducing cable size.

Part 2

You will need a copy of your local wiring regulations to find out your correction factor, which will vary according to the cable type you choose and the installation method also. The correction applies only to resistance; reactance does not vary with temperature (well, too small a variation to count).

Beware you are not confusing ambient temperature with operating temperature. Cable manufacturers give resistance (in terms of volt drop per metre) at the cables rated temperature.

However the current carrying capacity is given at a defined ambient temperature (e.g. 30 degrees for New Zealand, but 40 degrees for Australia). So you see location also matters.

Thank you BG for your detailed reply. It did help to clarify a lot.

But I am not very clear about the answer to my second part.

I am a bit confused on how to derate the cable current rating provided by manufacturer.

1) Based on the installation method on tray. conduit, etc, I pick a derating factor,

2) Based on the grouping I follow, I use another derating factor

3) Based on the ambient temp (say 45 Deg C) and the max temperature a cable can withstand (say 90 Deg C) as per supplier, I use another derating factor.

I guess all this matters and multiply all these factors with the rated current of the cable to get the final derated current for the cable.

But for voltage drop, I am given resistance and reactance values by the vendor. But this is rated at 20 Deg C. So using this derated current I calculate the voltage drop. Is that right?

Here is where my confusion starts ( !!!).

Two things I require clarity –

One is how do I correct the resistance (maybe reactance correction not reqd as stated in your reply) to suit the ambient temperature of 45 Deg C.

Second – should I use the de-rated current or the rated current of the cable. ( I thought using rated current, which is more would help in getting a conservative voltage drop).

Appreciate if you can chip in with your comments.

You really are an Engineer in the making!!

Download this free programme: http://www.generalcable.co.nz/newzealand/NZDownloads/Gencalc.aspx

Remember this is for NZ regulations, which are virtually identical to UK.

Lets take a physical example, 4-core 35mm2 PVC multicore, 100amps load, 3ph over 50 metres. Here are some results:

(a) Unenclosed spaced (on tray), VD=5.22 V, capacity=137amps, temp=54 C

(b) Unenclosed touching (on surface), VD=5.31 V, capacity=125amps, temp=59 C

(c) Enclosed (in conduit), VD=5.57 V, capacity=105amps, temp=71 C

So you can see that as the installation methods trap heat in the cable, the current carrying capacity drops, the cable temp rises, increasing the resistance (but not reactance, which depends on frequency not temperature), which in turn increases the volt drop.

So to answer your questions. If you download the programme, click on Help > Methods of Calculation > Cable route length, you will see an equation for how the temperature is calculated, then the resistance is taken from tables and interpolated. There is an actual equation in the UK IEE Regulations too.

And the answer to your second question is that you put any derating factors onto the max current capacity of the cable for your installation conditions. So yes, derated once again!

Buy shares in copper mines!

Thank u for the reply. It clarified things clearly.

However I am not sure whether your first line was a compliment or not. Anyway trying to be an optimist, I take is as a compliment.

By the way, your foot note one liner about deadlines was awesome.

Thank u.

Absolutely take it as a compliment!!

"To an optimist, the glass is half full.

To a pessimist, the glass is half empty.

To an engineer, the glass is overdesigned and twice as big as it needs to be."

To answer one of your other threads, to calc short circuit current talk to your local Schneider representative about Ecodial software. Also Schneider have a suite of technical papaers called Cahiers Techniques. Check out:

http://www.schneider-electric.com/documents/technical-publications/en/shared/electrical-engineering/electrical-know-how/low-voltage-minus-1kv/ect158.pdf#page=34

Best of luck.