I'm looking to set a 400 amp meter/main, and run two sets of 4/0 AL SER to a distribution panel. 4/0 is rated for 200 amps. What confuses me is that if I were to run one set of 400 amp sub-feed Al, it would size at 700mcm. So if I take the the circular mills of the 4/0 and multiply it by two, it only adds up to 423,200 circular mills per conductor, not 700, or more. Am I correct in assuming that even though the math does not add up, if the amperage for a certain wire is doubled, and ran in parallel, the job is still done properly? Please explain if you can....thanks
Running Parallel Feeds
- Thread starter Rweenzz
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While it is generally true that a larger wire is permitted to carry more current than a smaller wire, the relationship is not linear. You can?t double the cross-sectional area and expect to get twice the ampacity. Area is not the only characteristic of that determines a conductor?s ampacity. Geometry also has an impact.
But you are right in saying (if I understood correctly what you said) that the ampacity of two conductors in parallel is twice the ampacity of one of the conductors alone.
But you are right in saying (if I understood correctly what you said) that the ampacity of two conductors in parallel is twice the ampacity of one of the conductors alone.
Think of it this way:
1" EMT has an internal diameter of 1.049 inches. Double that, and you get 2.098, very close to the ID of 2", 2.067". Now imagine two 1" pipe resting side-by-side in a 2" pipe.
But since we're talking about area, not length, we need to think two-dimensionally. And it is area, more specifically cross-section area (csa) of conductors that determine ampacity.
1" EMT has a total csa of 0.864 in.?. By doubling that, you get 1.728 in.?.
But 2" EMT has a csa of 2.067 in.?, almost 20% more than the 1".
1" EMT has an internal diameter of 1.049 inches. Double that, and you get 2.098, very close to the ID of 2", 2.067". Now imagine two 1" pipe resting side-by-side in a 2" pipe.
But since we're talking about area, not length, we need to think two-dimensionally. And it is area, more specifically cross-section area (csa) of conductors that determine ampacity.
1" EMT has a total csa of 0.864 in.?. By doubling that, you get 1.728 in.?.
But 2" EMT has a csa of 2.067 in.?, almost 20% more than the 1".
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- retired electrician
480sparky,
Your numbers are wrong. If you double the diameter the area goes up by a factor of 4. Your area for the 1" is correct, but the area for a 2" with a diameter of 2.067" is 3.355 square inches.
That being said, I don't know what it has to do with the question. If you double the area of the conductor, the ampacity does not double. That is one of the advantages of parallel conductors, you get the same ampacity with less copper. Parallel 250's have a bit more ampacity than an 800, but use only 62.5% of the copper.
Don
Your numbers are wrong. If you double the diameter the area goes up by a factor of 4. Your area for the 1" is correct, but the area for a 2" with a diameter of 2.067" is 3.355 square inches.
That being said, I don't know what it has to do with the question. If you double the area of the conductor, the ampacity does not double. That is one of the advantages of parallel conductors, you get the same ampacity with less copper. Parallel 250's have a bit more ampacity than an 800, but use only 62.5% of the copper.
Don
You're right. I read the diameter of 2", not the csa. If this was a test question, I'd be dead wrong (unless I accidently marked the correct answer instead:grin: ).
I guess I was trying to over-simplify an answer. But taking two 1" conduits obviously doesn't replace one 2" pipe.
Not only does paralleling save copper and $$$, but it's a whole lot easier to handle and pull.
As for the OPs question... Would skin effect have any role?
I guess I was trying to over-simplify an answer. But taking two 1" conduits obviously doesn't replace one 2" pipe.
Not only does paralleling save copper and $$$, but it's a whole lot easier to handle and pull.
As for the OPs question... Would skin effect have any role?
The reason the ampacity of the conductors is not linear is a result of the skin effect. Basically AC electricity "prefers" to travel on the outside of the conductors. Increasing the conductors area does not linearly increase the circumfrence of the conductor (Area ~ r^2, Circ ~r). Hope that helps.
72.5kv
Senior Member
Current at 60hz has a skin dept of about 8.57mm. On conductors over 1/2'' current on travels through only a small portion of the condcutor. this were paralleling has the advantage. The added surface of the parallel conductor is what causes an increase in ampacity.
Look at an ampacity table for bus bar Al or CU. notice that the dept. bus is usually about 1/4'' thick. Making is any thicker would be a waste of material.
This is very noticeble if you compare poco conductor to regular wires. They are larger in terms of surface area not dept. Thin layer of Al wrapped around a steel or composite core.
Look at an ampacity table for bus bar Al or CU. notice that the dept. bus is usually about 1/4'' thick. Making is any thicker would be a waste of material.
This is very noticeble if you compare poco conductor to regular wires. They are larger in terms of surface area not dept. Thin layer of Al wrapped around a steel or composite core.
rcwilson
Senior Member
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- Redmond, WA
Heat generated in a conductor has to escape from the surface. More surface area means the cable(s) will run cooler with the same amount of I squared R losses in a cable.
Two 350 kcmil cables have more surface area than one 700 kcmil cable. If you double the cable diameter, cross section area quadrupels, but surface area per foot only doubles.
Skin effect increases the larger cable's resistance creating more heat with the same amps.
Parallel cables can usually carry more amps per pound of copper than a single cable.
Two 350 kcmil cables have more surface area than one 700 kcmil cable. If you double the cable diameter, cross section area quadrupels, but surface area per foot only doubles.
Skin effect increases the larger cable's resistance creating more heat with the same amps.
Parallel cables can usually carry more amps per pound of copper than a single cable.
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