Preference to Avoid Large Cables?

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NEC User

Senior Member
Can anyone offer their opinion for what cable sizes are excessive that I should avoid using and thus opt to run multiple sets of cable?

Suppose I have a building with a 480Y/277V 600 Amp Service.

Using Table 310.15(B)(16) is it better to run
4# 1500kcmil?
or
2 sets of 4# 350 kcmil?

I would opt to not use #1500kcmil cable but I'm not sure where the cutoff I should opt to use multiple sets occurs.

Any thoughts?
 

kwired

Electron manager
Location
NE Nebraska
Calculate cost both ways, not just copper but raceways, fittings, LABOR, specialty equipment that may be necessary and I bet you come up with an easy answer
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I do not much deal with raceways, but have found that 1/0 is the cheapest copper I can get.

Sometimes (2) 1/0 is less money than (1) 3/0.

but, the cost of the copper is not always the determining factor. Sometimes where and how it has to be terminated makes a huge difference in deciding what size wire to use.
 

Smart $

Esteemed Member
Location
Ohio
1500kcmil takes two body builders and a trained gorilla to bend by hand for terminations. :happyyes:

500kcmil can be difficult to bend for a very fit person in close quarters.

Double the manpower (and animal power :p) for pulling using powered equipment.
Quadruple or better for manual pulls.

Economics may play a part in the decision, too. :D
 

kwired

Electron manager
Location
NE Nebraska
pound for pound, copper wise, 1/0 is the least expensive copper most of the time.
But still costs a lot more when 14 AWG will get the job done:happyyes:

Have you ever pulled 1/0 to a typical device box containing a 15 amp switch, or tried to terminate it on a 15 amp switch?



Add: does that make 1/0 the least profitable to recycle as scrap metal?
 

FREEBALL

Senior Member
Location
york pa usa
I would stick to parallel 350's. 4 inch, which is the most prevalent conduit will not be suitable size wise. I have always paralleled anything above 400 amp. Is there a reason you cannot.
 

Smart $

Esteemed Member
Location
Ohio
I would stick to parallel 350's. 4 inch, which is the most prevalent conduit will not be suitable size wise. I have always paralleled anything above 400 amp. Is there a reason you cannot.
400A, but more so 500kcmil IMO is a good threshold. Anyone that's worked with 500kcmil and larger won't second guess a parallel installation with smaller conductors.

No electrical reason not to, but other conditions may influence the decision.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
We typically cut off at 600 kcmil Cu for ease of installation. Occasionally 750 kcmil but we haven't used much of those in the past few years. 600 kcmil is still reasonable to pull and terminate.
 
Can anyone offer their opinion for what cable sizes are excessive that I should avoid using and thus opt to run multiple sets of cable?

Suppose I have a building with a 480Y/277V 600 Amp Service.

Using Table 310.15(B)(16) is it better to run
4# 1500kcmil?
or
2 sets of 4# 350 kcmil?

I would opt to not use #1500kcmil cable but I'm not sure where the cutoff I should opt to use multiple sets occurs.

Any thoughts?

Generally I prefer smaller parallel sets whenever possible. I try to stay below 250 to 350 MCM. The ampacity per circular mill really starts to go down rapidly above these sizes. For example take your 600 amp service, and assume no "next size up" or sizing to meet the calculated load, and AL conductors, you could have three sets of 250 MCM or two sets of 500 MCM. That is 33% more conductor area to do with the 500's. Now consider doing it with 1/0 (not that I would do that but just to illustrate the point), and it would take 5 sets which would be a total of 520 circular mills. Compare that to the two sets of 500's and it takes almost twice the metal.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Generally I prefer smaller parallel sets whenever possible. I try to stay below 250 to 350 MCM. The ampacity per circular mill really starts to go down rapidly above these sizes. For example take your 600 amp service, and assume no "next size up" or sizing to meet the calculated load, and AL conductors, you could have three sets of 250 MCM or two sets of 500 MCM. That is 33% more conductor area to do with the 500's. Now consider doing it with 1/0 (not that I would do that but just to illustrate the point), and it would take 5 sets which would be a total of 520 circular mills. Compare that to the two sets of 500's and it takes almost twice the metal.

Even though there are less amps/kcmil, larger conductors require less parallel sets, less conduits, less installation time and less terminations. I would think that smaller conductors aren't always going to save you money on the installation even if they give you more bang for the buck with the amp/kcmil ratio. In many cases they can create larger problems, like in your example, where you will need to terminate 5 sets on a 600 amp CB. :)
 

iwire

Moderator
Staff member
Location
Massachusetts
Even though there are less amps/kcmil, larger conductors require less parallel sets, less conduits, less installation time and less terminations. I would think that smaller conductors aren't always going to save you money on the installation even if they give you more bang for the buck with the amp/kcmil ratio. In many cases they can create larger problems, like in your example, where you will need to terminate 5 sets on a 600 amp CB. :)

When I ran some numbers for a large install I found 200 amp 3/0 sets where the cheapest way to go under the following conditions. On a roof, easy PVC pipe work.

Certainly that can change if the pipe work is hard.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
When I ran some numbers for a large install I found 200 amp 3/0 sets where the cheapest way to go under the following conditions. On a roof, easy PVC pipe work.

Certainly that can change if the pipe work is hard.

I agree there are many variables, labor costs, the length of the run(s), the amount of bending involved, the list goes on.
 

iwire

Moderator
Staff member
Location
Massachusetts
I agree there are many variables, labor costs, the length of the run(s), the amount of bending involved, the list goes on.

Exactly, in my case it was a very large PV array on a number of large roofs. I spent the better part of a day trying to calculate all the variables.

I came up with an installed price per ft for various combinations of raceways and conductors.
 

Fulthrotl

~Autocorrect is My Worst Enema.~
Can anyone offer their opinion for what cable sizes are excessive that I should avoid using and thus opt to run multiple sets of cable?

Suppose I have a building with a 480Y/277V 600 Amp Service.

Using Table 310.15(B)(16) is it better to run
4# 1500kcmil?
or
2 sets of 4# 350 kcmil?

I would opt to not use #1500kcmil cable but I'm not sure where the cutoff I should opt to use multiple sets occurs.

Any thoughts?

yeah. don't use 1500 MCM. at all. ever. just don't.

assuming you are trying to feed a 600 amp load,
(2) parallel 500 MCM aluminum thwn-2 pulls is good for 700 amps......
with a 600 amp load, your wire length on parallel 500's
could be 435' and still only be 2%

what's with the 1,500 MCM?
 
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