It is not an uncommon practice. The parallel runs should be of the same type, gauge, and length to ensure that the current divides equally.

If building codes exist and are enforced where you are, be sure multiple wire runs in the same circuit are allowed.

Take care that the terminations are all well made.

Regarding the economics it takes less manpower to run multiple cables. Terminating is easier, I know I'd rather bend a couple of 240mm's than a 500mm.

"I'd rather bend a couple of 240mm's than a 500mm."

But of course, a couple of 240's will not carry the same current as a single 500...

Its pretty common on very high powered applications like in industrial settings where the main power source is a 277/480 system supplying several megawatts of power from one central transformer at thousands of amps per phase.

Running a single conductor that can handle 2000 - 3000 amps or setting up a buss bar and conduit system may not be practical or as cost effective as simply pulling 5 or more smaller sized wire conductors ganged together.

I worked at a coal mine years ago as a student co op electrician and we installed a 277/480 volt 3 PH service out to a hot start line for the big mine trucks that ran 3 or 4 500 MCM lines for each phase.

Cost wise it was cheap being it was a common size of wire the mine carried in bulk but mostly it just came down to it was the largest wire size we peons could physically work with by hand given the run locations and bends involved between the main service point and where it went to outside the building.

A single run of 1500 MCM cable (we actually had wire that size!) for each phase would have worked as well but there was no way to physically work with it due to its shear weight and thickness with the man power and space given.

DLO cable is a strong suggestion along with the bus bars for your tight spots where you can not route the cable. Call me, Triton Manufacturing's product base is based on custom sizing for all of your electrical distribution needs.

Franklin E. Bulley
Product Development Specialist
(Laminated Bus Bars)
Triton Manufacturing
5700 Triton Way
Monee, IL 60449
Office Phone: 708-587-4019
Cell Phone: 708-941-5336

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Running multiple cables is very common where high current is required, however all conduits must contain all three phases plus a neutral where required, plus a ground(green) wire.

This practice prevents inductive heating of the conduit that would occur if they carried only one phase each.The individual ground in each conduit is to trip the breaker if a fault occurs within the conduit, and is sized according to the upstream breaker size.

There is also a maximum fill of conduits, which differs for new work or existing work, so familiarize yourself with the NEC regulations before beginning.

Most states in the USA require licensing of electricians with annual renewal contingent upon completion of a certain number of CEU's to keep them updated of changes.

If every cable carries all 3 phases, they have no net magnetic field 3-10 cable diameter away. If every cable carries a single phase, like the 240kV overhead lines, the individual cables are kept 15 - 30 cm apart. For cooling, for sure, but more importantly to keep them magnetically apart.

I witnessed a 12kV subpanel in a large welding shop. The connecting cables carrying single phases each were not secured except by their terminal. On a sort of short the breaker was a bit slow. The cables ripped themselves off the terminal by magnetic forces. Denting the cabinet badly. I was told, it sounded like a big gun going off.

It is common practice as others have noted.

When the practice is utilised measures must be taken to ensure that the parallel conductors cannot be easily disassociated. Disassociation may lead to a resultant smaller conductor path being protected by a too large device. Fires may ensue.

If the risk of disassociation is high then consider properly dimensioned protection for each conductor.

Do it properly and you'll be right.

Refer publication by "Copper development association".

My thanks to everybody. An interesting useful learning exercise for me.

I must admit I was thinking along the lines of smaller size cables where running two 2.5mm2 cables to reduce volt drop in a 12 vdc system, and to feed a cooker in a 250vac systems, instead of one 4mm2 cable (because I do not happen to have any).

Which might appear as laziness and a temptation to take short cuts, and an attitude from which of bad practice stems - leading to rules and regulations to prevent such practice.Hence the question.

Thanks again.

Hollo Crabtree

You are probably right. The insurance company would not take much notice of sound engineering arguments - they would just latch onto trivial non-compliance to delay paying out or to avoid it completely.

That's why they employ Loss Adjusters - to adjust their loss downwards.

Multiple conductors on a single circuit breaker is prohibited in most cases because they are not rated for connection to more than one conductor, and is a violation of design and code requirements.

You can't just wedge it under the screw and tighten it up, even if it will go.Long term security of the connection is in question,and it is hazardous.

In some places I have seen an extension bus bar is attached to CB terminal and many cables are connected to the busbar.

Absolutely! Depending on the length of the cable and the power requirements, cables are being replaced by Bus Bars in order to meet these power requirements. A bus bar does what a cable can not and this is where I come in! I have worked with Bus Bars for over 25 years and even though the cost of a bus bar is somewhat higher than a cable there are many advantages.

Franklin E. Bulley
Product Development Specialist
(Laminated Bus Bars)
Triton Manufacturing
5700 Triton Way
Monee, IL 60449
Office Phone: 708-587-4019
Cell Phone: 708-941-5336

CR4 ADMIN - email address removed

From the CR4 Rules: Do not post phone numbers or email addresses. The CR4 Admin will delete all phone numbers and email addresses posted in threads or comments. You can share this information via the CR4 internal messaging system.

The other ends aren't then jammed into a single screw terminal, they'd be terminated with lugs and held on with bolts (or a single srew terminal per conductor) onto a busbar (single fingered bus comb if you like) at the breaker end.

But yeh, if you look around you will see it alot especially at high load points. If it's done properly it doesn't stand out as being obviously lashed up. If it does stick out like dog's nads then it probably hasn't been done quite right.

Temporary power reticulation setups use it a lot because it is easier to run lots of smaller conductors and then pack it up when finished than it is to run single large conductors.

There's a right way to get around most obstacles. Material availability can be a serious obstacle in some parts

FWIW 2 2.5mm cables have a smaller cross-section, and therefore a smaller current-carrying capacity, than a single 4 mm cable.

Conductors are not sized according to their diameter but by their cross sectional area.

2x2.5mm² is 5mm²>4mm², so to say that the current carrying capacity of a 4mm² is greater than a 5mm² has an odd ring to it.

The current carrying capacity of 2x2.5mm² is actually higher than a single 5mm² due to lower derating at higher temperatures (ie better cooling). The conductivity of 5mm² is the same regardless of whether it is 1x5mm², 2x2.5mm² or 5x1mm². Conductivity, which is a physical property, is not the same as current carrying capacity which is a contrived attribute. You will find that conductor size "ampacity" tables vary between jurisdictions, the actual properties of materials do not.

To phph001

Yes I agree if it is diameter, but in my case it is the cross section area of the cables that I am thinking of using.

Sorry if there is confusion.

Horace40,

I don't know the codes in England. However in the USA the almost universally used NEC has rules for the smallest size conductor that can be run in parallel. This is because at larger sizes for the fuse or CB, there can be significant heating of a cable before power is cut off, and larger cables can absorb the heating better than smaller ones.

The enemy you are fighting is heat. As cable size increases, the relative ratio between surface (for removal of heat by any method) and area (for conducting the current) tends to be worse. For DC circuits the current is carried equally well anywhere in the conductor, but for AC the oscillating magnetic field tends to push the current flow towards the surface. These two effects favor multiple runs of reasonably large cables.

Don't be afraid to continue asking questions. We all are learning. --JMM

Hello Jmuelle

Interesting. Would the alternating current in a multi-strand flex tend to flow on the surface of the combined core or would the current tend to flow on the surface of each strand.

The surface of each strand in a prorated manner with concentration at the periphery of the bundled strands.

There is a finite resistance between each strand even though they are in intimate contact.

At 50/60 Hz the skin effect doesn't become much of a design issue does it? If it was we'd have hollow transmission lines yes?

Now in RF systems it matters and is measurable.

Wal,

Aside #1--With the large number of electronic loads these days and the resulting significant portion of currents in harmonics as high as 33rd or higher, the skin effect has a small but real contribution to where the current flows in a conductor nominally running at 50 or 60 Hz. I know that in connecting VFD's to motors, the substantial impedance mismatch between the motor leads and the internal windings does result in a reflection of the VFD's signal, which is the cause for the up to 2x voltage imposed on the first few turns of the winding. One mitigating method in use is an impedance matching transformer or network at that point!

Aside #2--There are hollow transmission lines and more commonly lines with two, three, or four strands spaced apart by many centimeters. These are not due to the skin effect, but are to minimize corona discharge. The hollow conductors are used to reduce the weight.

Back to the topic--I suspect that a 2.5mm² cable is a little small for running in parallel. Also, contrary to some other posts, at the larger sizes of conductors it is significantly less expensive (labor only, material only, or combined) to run conductors in parallel--double the cross-section but only add 50% to the ampacity.

--John M.

Thanks for that.

Although laying a single cable of higher size will be cheaper than multiple cables of smaller sizes in parallel, think about higher bending radius of large size LV power cables and the difficulty in termination of such cables within the panel or inside terminal box of motors. In motor feeders, however, you may consider running one power cable and terminating in a power junction box and using multiple flexible cables from the JB to motor terminal box for ease of termination.

Follow British Standard 7671. That is why it is there.

Thanks Wal

Spot on! I appreciate the detail because it gives a direct hit. I have posted a GA.

Here in the USA, NEC says any circuit requiring a 1/0 kcm or larger can be parallel conductors as long as all 3 phase conductors are in every pipe used ( 3 phase has to have 3 pipes with each phase in each pipe). Any circuit requiring less than 1/0 kcm is not permitted. Most wires that large are very hard to bend. That is the main reason for the parallel runs. The ends of the wiring (circuit breaker and device ) usually is set up for the amount of wires in parallel.

The first part is correct, but not the part about requiring 3 conduits.

I was talking about parallel runs not single runs.Try to follow please..... : )l

Only conductors of 1/0 (53.5 MM Square) and above size are allowed to be paralleled.(In USA,per NEC).

Here is a link that explains further requirements:

http://www.iaei.org/magazine/2007/01/equipment-grounding-conductors-for-parallel-conductor-installations/

That would be about the size where single large conductors start getting difficult to wrangle.

Codes are practical guides as well. Usually written around available materials which are made available to match the codes and around we go.

Sometimes and in some places there is no code to follow and the ideal materials aren't available and that's when you have to start using your noggin and experience.

I'm guessing that the 1/0 minimum is saying that if your conductor path needs to be smaller then just use the right size cable...removes a layer of detail that may be prone to improper execution is all. Keeps this type of work-around out of the average domestic sparky's (and competant DIY enthusiast's) realm of discretion.

To Mushtaq Hussainh

Thanks for the detail you have supplied on my account. It is much appreciated and a GA.

Both cases apply to me, albeit that they are trivial in comparison to the greater cause.

Case-1: The source and load are 65ft apart. Not far you might think, but at 12vdc from the batteries at the back of my narrowboat to the toilet macerator pump at the front, there is a bit too much volts drop when the batteries are run down. No problem when the engine is running, but somewhat anti-social in the middle of the night.

On second thoughts running two cables in parallel is not practical now the technical requirements have been clarified. I am going to install a 12v battery at the pump. This will provide the local power and I will use the existing cable to charge it.

Case-2: The source and load are close together. I am moving a 230vac electric cooker and the existing 4mm² cable is too short, so it seems it is alright to replace it with 2 x 2.5mm² cables.

Thanks for all the help.

<...Case-2: The source and load are close together. I am moving a 230vac electric cooker and the existing 4mm² cable is too short, so it seems it is alright to replace it with 2 x 2.5mm² cables....>

Under BS7671, the connection between the two pieces of cable must be accessible for maintenance, which means putting a box in the wall where the joint is to be made. So why not just do it right and use a new piece of 4mm² instead?

If in doubt, consult a qualified electrician.

Thanks for your help but we are going off at a tangent. I am a qualified electrician but long since retired having been initiated in the finer points of the 12th Edition of the IEE Regs - and subsequently up-dated to the 17th Edition. Except in the latter case I prefer not to fork out £80 for a copy that is of no particular use to me.

My basic question is one of 'good or bad' practice. If something does not comply with the Regs' then it could be classed 'bad' practice, but in this instance there seems to be provision for joining cables, so it can't be 'wrong'.

I will do the job 'properly' with 4 mm cable.

In the USA, there are many categories of electrical license.There is a category for single family dwelling, multifamily dwelling, water pump wiring only,swimming pool only, HVAC only, etc, and categories based on total job price:Limited,Intermediate, and Unlimited.

Most large commercial jobs are in an Unlimited category, so unless you have worked for, or been an unlimited licensee, you will have very little exposure to the common practices used in industrial or commercial jobs.Paralleling conductors is a COMMON practice.(Therefor it must be "good").

However, paralleling of small conductors below a certain size is prohibited because the circuit breakers are not designed for multiple conductors,whereas high ampacity commercial breakers are designed for multiple conductors.

I have been licensed for over 25 years, and hold an Unlimited classification,valid in 7 states.I still have not seen it all, and never will, but I have seen quite a bit.

Due to legal considerations, I will only advise in a very general manner, referring to specific areas of the NEC for reference.

There is a phrase in the NEC that refers to "The AHJ" which means the Authority Having Jurisdiction .

This local person has the final say on an installation or practice,even if it seems to disagree with the code.In most areas, it is the local inspector. He, in turn is held accountable for his decisions.There are many ways to interpret the code, and most follow the code closely, but still there is room for interpretation.

No particular licensee is any better or smarter than another.The difference is mainly in their field of work experience.Different licenses require certification of a certain number of year experience at the applicable level before becoming eligible to take the board exam.

I had 15 years experience in industrial environment before I sat for my exam, so I have over 40 total years in the field.I am also retired, which is the only reason I have time to participate in these forums.

I am wiling to help when I can, but within certain limits, as mentioned above.

I check in often, so if you have any electrical-field related questions, I am willing to help out.If I do not know, I can probably find out.

Now if you will excuse me, I have a fish on my line, and it feels like a big one.....

Verbatim from my link post#27 regarding parallel conductors.

Don't forget to check the short circuit capacity of the cables.

Instead of large multicore or single core cables, laying smaller multiple cables is economical and easy to handle(bend). Don't forget to check the short circuit withstand capacity.

While designing a feeder size having number of parallel multi-core cables for a load current from 400Amps to 1500Amps or above for three phase system, the following things are required to check:

1. Cable size in feeder shall be preferable to 240mm² but not to exceed 300mm² for handling point of view and current per mm²

2. Estimate approximately number of multi-core cables in parallel by dividing the total load current with selected multi-core single cable current at 90C⁰.

3. Estimates the feeder current by applying the current de-rating factors as per the method of laying the cables. If the feeder current meets the load current then check additional factors

· Percentage voltage drop is within 5%

· Cost of the feeder multi-core cable is economical

· Does it meet the short circuit capacity?

What I gather from this discussion is: If I need to increase the capacity of a circuit, say in order to use a larger motor, it is permissible to add another cable (same size, same termination) in parallel with the existing wire(cable) and change the breaker with a suitable one. The only problem I could see would be termination at each device. You don't want 2 wires under a screw post, right?

Pretty much right there mate.

About using screw post s or not would depend on the conductor and screw terminal sizes.

A pair of 2.5mm2 conductors consolidated (twisted together) would work fine.

A pair of 16mm2 conductors less so unless it was a large enough terminal with two socket headed (allen key) grub screws.

Larger conductors work well with crimp lugs and studs.

You have to be able to achieve a strong connection with low risk of disassociation.

it is absolutely fine to run cables in parallel to meet the desired current carrying capacity. its common to run multiple smaller cables in parallel due to limitation at the cabel termination end.

check any large distribution transformers it would have multiple cables in parallel from the secondary side to the switchboard or MCC