Cables Connected in Parallel

Dear Mr.jack of all trades,

Voltage Drop with in the cable is another important consideration.

DHAYANANDHAN.S

It's prettier that way.

Is length of no concern to you? Do you not care about symmetry?

Its exactly for the reason of symmetrical flow of currents.

propagation delay...

_{(can I be credited 20% of your homework mark?)}

Are you sending this question from your phone while writing a test?

Anonymous Poster #1

The first answer is load sharing. If two identical wires are in parallel they share 1/2 the total load. Why? It is much easier to pull two smaller wirers through conduit than it is to pull one large one. Example: #3 THHN is 100 amp max under normal conditions, 2/0 THHN is rated at 200 amps max under normal conditions. Hypothetically for a 200 amp load you can pull either 2ea much easier #3 wires or one hard to handle 2/0.

Problem you can run into is that if one wire should fail the other must take the full load. For the above example you would be pushing 200 amps through a 100 amp rated wire. It would very quickly fail with potential expensive consequences. This 200 amp circuit would probably be fused for 200 amps so the fuse won't help for stopping the flow.

Another reason is that the total number of wires necessary for your circuit can be installed in a smaller size conduit thus a cost benefit.

If you are unfamiliar with electricity, working with it and the standards involved don't! It is suggested that you do not use parallel wires on anything other than a flashlight battery at this time.

Good Luck, Old Salt

GA from me!!

I agree with old salt.

The cables will not shared the current proportional with the conductor cross section but the bigger will be more loaded and the smaller will be less. Let's take an example.

According to NEC Table 310.15(B)(16) a 600 mcm copper conductor

90 o C insulation rated withstand 475 A.

300 MCM only 320 A.

Total=475+320=795 A

Neglecting reactance and skin and proximity effect the resistance for 300 ft. length will be:

R600=0.00674 ohm ; R300=0.0135 Rtot=R600*R300/(R600+R300)=0.045 ohm

I600=Rtot*795/R600=530 A [11.6% more than permitted] and I300= 285 A [17.2 less than permitted]

Sorry, I did not finish the example, actually. At first a correction: Rtot=0.0045 ohm of course, and what I wanted to say we have to reduce the total current of 795 A to 712 A in order to maintain 475 A for the 600 kcmil cable.

What cable size, frequency, amperage you are talking about?

In what application? Normally the question should contain a lot more words than the answer. Put some time in it please. We cannot provide you a vademecum (handbook) for all possible situations

For one of many reasons, you are using 2 cables parallel, you should keep them as identical as possible:

Different lengths create different current density and temperature. (can be critical).

In High frequent power systems also phase shifting occurs (see wave length) what causes distortion of the original shape.

When running in parallel or a greater number it is usual to run identical sized cables. Running like this you know the load is shared equally. It is easier to install 2 or more small power cables rather than a single large cables.

Look to the secondary cables of transformer about 1MVA in an industrial application to see how many single cores are used. More often 3 per phase and 2 for the neutral.

Running cables in parallel you need to remember to use the cable de-rate values that you can get from Mr Slacks copy of the IEE Regs becaue both cable are subject to the same load and will heat up evenly.

More often the cables are of substantial length and a big issue is volt drop. Parallel the cables and you halve the volt drop.

There are many other issues the most important being able to bring out the circuit protective device.

Better to obtain this document directly from the British Standards Institution's regular sales outlets!

I could see one side coming loose. THEN what do you THINK would happen? Should you not have a circuit breaker or fuse link on EACH conductor?

Spare a thought for the currents in the cables under fault conditions. It would be prudent to provide common breaker protection for both cables to prevent circulating fault currents.

Mr Slacks copy of the IEE regulations tells you that this is a must not simply something prudent. Copies of the IEE regulations can be obtained ..etc ...etc...ZZzzzz

silverfox-

With all due respect to those who do follow the applicable codes, sometimes many electricians, engineers and electrical inspectors don't know the applicable sections of the codes that specify how things are to be done. Not berating anyone, just stating what many participants would probably agree with in their professional endeavors. I'm not saying that is wrong to not be able to recite the codes, but many aren't aware of when they should. Fortunately there are some that don't for one reason or another but they are the minority. How many engineers have initiated projects only to find that some things were not done in compliance with codes?

Good Luck, Old Salt

Here.... most of the time.

I hate working on legacy installations. Some malcompliant legacy work I've encountered was performed by those who should have known better.....gone native they have.

One has to set clear responsibility demarcations to avoid scope creep.

G.K.,

I must respectfully disagree with your conclusion. Although the current will share proportional to the cable size, the losses are proportional to the square of the current.

The life of the larger cable will be reduced due to the additional heat generated by the losses. If you oversize the larger cable and don't operate near its ampacity this shouldn't be an issue. To some operators, however, rated ampacity is a concept like a posted speed limit: sort of a guideline to be followed when convenient.

Please give mathematical examples to explain how 'amps squared' losses (within the parameters of permitted design load and parallel cables) reduce cable life.

I guess insulation might break down, or badly made joints might burn, but I'm not aware of any other reason. Perhaps I've lived a sheltered life.

Greetings, Horace.

I can provide the math if you like, but I don't believe the issue rests there.

Unless the parallel cables take different routes between the termination points, we can assume that both cables experience similar environmental conditions, with similar heat-removal capacity. Basic heat transfer tells us that for materials with a similar specific heat capacity and ambient conditions, removal of more heat requires a higher temperature differential between source and sink. The cable generating the additional losses with experience a higher steady operating temperature to enable it to dissipate the thermal energy to the surrounding air or earth.

Please note that I never said a cable would fail prematurely. As long as the cable is operated within design limits, it should last for its design life. However, in my experience we don't replace cables when they reach design life. In nearly all uses, a cable remains in service until it fails. The larger cable is likely to fail earlier than the smaller cable, even if both long outlive their design life.

Hello Pwr2thepeople

What you say makes logical sense. Thanks for taking the trouble to give a full explanation. It is much appreciated.

No... in theory there is no problem,

In fact, no two wires have the same resistance.

There will always be a lower amperage on one of the wires.

The other will always pick it up.

Not a problem unless you are close to full ampacity on both of the wires... in which case one or the other fuse will blow. (Both wires will need to be fused)

I did not bother to find the code ref.... but I am sure it is there.

It makes the standards and the load sharing calculations that much easier to apply...where in all probability parallel cables are likely to be the same size anyway.

If you have to ask that question, then you are on the wrong 101 course!

Try beverage engineering!

Now my question, Am I getting smarter or the questions getting dumber?