3/c vs 1/c cable

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mull982

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I'm working on a fairly large project building a new plant. There are numerous 480V and 4.16kV distribution and branch circuit being installed throughout the plant. A third party engineering compay sized all of the cables to be installed in the plant and issued a cable schedule for the contractor to install the cables.

The cable schedule in many areas lists a 3/c cable to be used. I have just found recently that the contractor has been ignoring many of these 3/c cable specifications and been installing single conductor cable. There are some areas in the plant where single conductor cables are specified, however there are a number of areas where 3/c cables are specified. The contractor is saying that on the MV-105 medium voltage cable, it is too hard to get the required bend radius on several of the pulls.

My question is a 2 part question:

First, looking at the cable from an electrical integrity aspect, is there a major performance issue between a 3/c and single conductor cable? Anything in the form of ampacity, insulation, etc... What are some advantages or disadvantags between choosing a 3/c cable over a single conductor cable?

Second, from an economical point of view, is there a major cost difference between a 3/c cable and a single conductor cable? Is it possible that the contractor is trying to take a shortcut by purchasing cheaper single conductor cable? Is there a typical multiplier that can be used to compare the two different prices to use to get a rough idea for the total cost difference on the project?

Has anyone ever dealt with this issue on a project?
 
From an ampacity perspective, I think you will find you get more ampacity using single-conductor cables than you would using 3c cables.

Depending on the cable types used, there may be more mechanical protection provided using the 3c cable. Also in terms of routing, there are sometimes issues if you are trying to route single-conductor cables (< 1/0 in size) in cable trays (they can sag between runs, and I think there may be an NEC restriction based on that as well -- you will have to check that).

Most installs I have seen prefer 3c cables for motor loads (3-phase motors I mean). I have seen it done both ways in terms of feeding Motor Control Centers, Switchboards, Panelboards etc). Usually we tend to spec 3c unless there is a good reason (usually ampacity related) to go with 1c.

Your contractor could have a very valid point though regarding the bending radii. Large 3c cables can be a real problem to bend it tight places (and there are restrictions on bending in the NEC and also imposed by the manufacturers).

Thanks,
 
Just looking at tables 310.77 and 310.79, it looks like the single conductor cables have a higher ampacity (which seems to make sense.)

The difference might make a slight difference in fault calculations and arc flash calculations. But it is unlikely it will change anything enough to worry about.

I'm not sure about the cost.

I'd ask the engineer if there is any particular reason he specified 3/C cables.
 
3/c racks up much nicer, (as mentioned).

Cost wise 3/c is probably more, it was last I checked, but you get an extra-layer if protection via the outer jacket.

I also imagine that with twist on 3/c, compared with a bunch of 1/c's in a pile cuts down on induced voltages?


Doug S.
 
Won't let me edit the last post any more... :cool:

96 4/c, mostly 4/4. Can you imagine the logistical mess if that had been done with 1/c... It was entertaining as it was but cost is cost, there times you need to cable it.
drive_racks.jpg
 
Doug S. said:
Won't let me edit the last post any more... :cool:

96 4/c, mostly 4/4. Can you imagine the logistical mess if that had been done with 1/c... It was entertaining as it was but cost is cost, there times you need to cable it.
drive_racks.jpg
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Single conductor is much easier to splice than 3c in the event of a problem you have to separate the conductors to rebuild the stress cones using little wooden chocks a big pita. Single conductors are easier to splice. Bunching high voltage conductors as tight together as possible is begging for more stress. Especially oilostatic when the oil vessel ruptures the cable fails at the high points in the run where the insulating oil has escaped.
 
It might be faster to get single conductor instead of waiting for the factory to fabricate the 3/C. Some of the contractors I have worked with didn't do very well putting together a BOM and ordering material up front when awarded a job, so a lead time of even 3 days caused problems keeping the crew busy.
 
Thanks for the responses.

The other issue that is coming up here involves the EGC size for these circuits. Since the engineer specified a 3/c cable this comes with the EGC in the cable jacket so in the cable schedule he did not specify a size, but rather just said w/Gnd because the ground in the cable would be used.

Now that the contractor is pulling all 1/c cables for these circuits he needs to supply the individual EGC for the circuit. The way the contract is written the contractor is not to size any cables himself without engineering approval. Engineering of course wants extra money to supply the correct EGC size to use for pulling a 1/c since they specified a 3/c cable.

Is there a standard for the EGC size that is installed in for each 3/c cable size? In other words does each 500MCM 3/c cable have to have a certain size EGC with it, or can this EGC size vary?

I was thinking about just telling the contractor to find the EGC size that comes in the jacket for the specified 3/c cable, and then pull this size ECG for 1/c circuits. Any problem with doing this.

I know that EGC's have to be increased in proportion to ungrounded conductors based on 250.122(B) so if the engineer increased the size of the 3/c ungrounded conductors and specified a larger 3/c cable then I'm assuming that the EGC in this 3/c cable increased in proportion as well? Is this true, do the EGC's increase in proportion with cable size?
 
In general the EGC in a mulitconductor cable is sized based on the maximum permitted ampacity of the ungrounded conductors in the cable.
 
If any of these cables are in conduit by themselves, your contractor could be exceeding conduit fill requirements as designed by the engineer.

For the single 3-c cable, the engineer can fill the conduit to 53%.
Split them up into three 1-c conductors, lowers you down to 40%.

Kent
 
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don_resqcapt19 said:
In general the EGC in a mulitconductor cable is sized based on the maximum permitted ampacity of the ungrounded conductors in the cable.
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So would that mean that there can be differences in EGC sizes between cable manufacturers for a given 3/c cable size depending on cable ampacity?

If I understand correctly the EGC has to increase with cable size because a larger cable size would be for a larger circuit and a larger circuit would have a larger avaliable faut current, because of the circuit size, and the lower impedence of this larger cable. It is because of this larger avaliable fault current that the EGC must be increased in order to properly withstand this fault current?

Would you agree that if the engineer increased the 3/c cable size for voltage drop then the EGC in the larger 3/c cable will be proportionally larger as well and satisfy 250.122(B)?

Because of issues with pulling larger 3/c cables is there a cut-off size for specifying larger 3/c cables? Are larger 3/c cable that avaliable for purchase?
 
mull982 said:
So would that mean that there can be differences in EGC sizes between cable manufacturers for a given 3/c cable size depending on cable ampacity?
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No, I don' think that is the case. The EGC is based on the maximum permitted OCPD for the size of the conductor within the cable.
If I understand correctly the EGC has to increase with cable size because a larger cable size would be for a larger circuit and a larger circuit would have a larger avaliable faut current, because of the circuit size, and the lower impedence of this larger cable. It is because of this larger avaliable fault current that the EGC must be increased in order to properly withstand this fault current?
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That is correct
Would you agree that if the engineer increased the 3/c cable size for voltage drop then the EGC in the larger 3/c cable will be proportionally larger as well and satisfy 250.122(B)?
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Not always. You have to look at the numbers and Table 250.122.
Because of issues with pulling larger 3/c cables is there a cut-off size for specifying larger 3/c cables? Are larger 3/c cable that avaliable for purchase?
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I know that multiconductor cable is available up to at least 750 kcmil and probably larger.
 
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