an electrical engineer told me (I'm a fourth year apprentice)that we get more amps per circular mil area with two smaller conductors than one larger conductor each have the same total circular mil area due to heat disapation. in short, the parallel setup, dissapates heat better. But then my forman says that the only reason we do it is for pulling reasons. But if that was true wouldnt you get get bigger pipe/ box etc. Who is correct or more correct?

Look at the ampacities of the larger cables in Table 310.16. What is the ampacity of 250kcmil? 500kcmil? Does the cable that has twice the area have twice the ampacity?
Don(resqcapt19)

well, for a 250kcmil has ampacity of 215 amps
two 250 kcmil in parralel have ampacity of 430 amps.
Now, an equivelant area to two 250 kcmils would be a 500kcmil which has a ampacity of 320amps. A 110 amps less than two parralel 250 kcmil. given in same insulation types.

I mean no offense, but am surprised that fourth-year training hasn't covered 310-4.

All things that produce heat can only dissipate the heat through their surface. More surface area equals better heat dissipation. Multiple conductors have more surface area than one conductor. And multiple conduits have better heat disipation than one conduit.

Thank you very much for your responses. I very much like this web site to increase my knowledge. And then pass it on down the road. thanks again

One other benifit of Paralled Conductors would be a reduction of Skin Effect, which is kind of why two Conductors which total the CM's of a single Conductor can handle more Current.

Scott S.E.T.

First, I am not an electrician.

I;'m in HVAC.

My son-in-law bought a new mobile home. THE HOOKUP USE FOUR WIRES.I was told, two hots, one ground and one neutral. Now the neutral and the ground were put under the same lug. Both at the service loop and the distribution box.
HOW CAN THEY BE DEFFERENT, UNDER THE SAME LUG.
I always thought the were derrerent, but maybe not.
What is the derrerence between the ground and the neutral.

Cecil,

Normally, the Grounded Conductor [AKA the Neutral, or Noodle to be more specific ] would only be Ground Bonded at the main service [we will not include the PoCo's Grounding at the Transformer, since this will complicate things]. If there is a Transformer which the customer has connected on the load side of the main service, the noodle for that system will be grounded at that Xformer only [I'm keeping things simple here!]

Some times a noodle will be re-connected to a grounding electrode system for a sub panel [separate buildings]. This one needs to have only the Earth its self as any conductive path between points of Grounding [Grounding Electrode Systems, or connections between EGCs of the two different panels].

To sum it up, having the noodle ground bonded at the service panel and at a sub panel will cause the Equipment Grounding Conductor to carry some current during normal operation - more than it is supposed to be carrying. This becomes an issue.

If the bonding is done at one universal place / panel, this would be the design intention and be proper Grounding for the noodle.

The point is to not have a parallel ["Neutral"] current flowing in the EGC, or any "Large" amount of current flowing on the EGC during normal operation [Large being something over 1 Amp at the most].

I'm not sure if this explains it well enough for you, so feel free to fire some more Q's this way and most likely another member can pick up the subject and explain it much better to you.

Scott S.E.T.

It can also be less expensive. If you have a chance, look at the price difference between 2" conduit, and 2-1/2". The price is almost double. Copper wire gets expensive in the larger sizes. Then, the labor to install the larger pipe and wire is something else to consider.

Why not parallel?,
Doc

Sometimes it's also a matter of convenience.

Earlier this week I had to run a sub-feeder and the required cable size calculated out as 16 sq. mm. That's not a size that's used very often here and I had none available (and a 50-mile round trip to get some). So I used paralleled 10 sq. mm cable instead.

Easier to work due to the smaller size, and reduced voltage drop into the bargain.