Our utility has a requirement to maintain voltage +/- 5%. For a typical 120/240V 3W 200A service this would mean 114V on either of the 120V legs and 228V between the two. Pretty straightforward ? generally 2/0 copper is used for these services as the conductor is rated at 200A. Assuming the service is at 100% capacity at 200A (bare with me), at some given length of service conductor, voltage will no longer be maintained to within 5%.
The math is the easy part for me, but I?m having a hard time applying this practically. What does a 200A ?mains? rating actually imply? Does this mean that each 120V leg could potentially see the full 200A with 200A on neutral (and therefore must the other leg then have zero load so not to exceed 200A?) Or does each 120V leg see only 100A with the cumulative draw summing up to 200A (zero neutral current in a perfectly balanced system)? 200A across 240V appears to be the same as 200A across both 120V legs in terms of the amount of current flow through the equipment inside the service entrance. So my first question is how you interpret the rating of the service entrance in terms of the currents on each leg.
I think you?ll see my confusion when pointing out that:
200A flowing through a single 120V leg of 2/0 and back out the neutral results in twice the percentage voltage drop as that of 200A flowing across 240V of both 2/0 legs. The voltage drop itself is the same because the I and the R are constant, but it?s the reference voltage that changes the percentage. Practically speaking, because voltage drop is a function of line length, you can run your service conductor twice as far if you use the 240V number then if you use the 120V number.
Would someone mind enlightening me how to interpret the service rating back to the proper application of voltage drop in this case? Thanks.
The math is the easy part for me, but I?m having a hard time applying this practically. What does a 200A ?mains? rating actually imply? Does this mean that each 120V leg could potentially see the full 200A with 200A on neutral (and therefore must the other leg then have zero load so not to exceed 200A?) Or does each 120V leg see only 100A with the cumulative draw summing up to 200A (zero neutral current in a perfectly balanced system)? 200A across 240V appears to be the same as 200A across both 120V legs in terms of the amount of current flow through the equipment inside the service entrance. So my first question is how you interpret the rating of the service entrance in terms of the currents on each leg.
I think you?ll see my confusion when pointing out that:
200A flowing through a single 120V leg of 2/0 and back out the neutral results in twice the percentage voltage drop as that of 200A flowing across 240V of both 2/0 legs. The voltage drop itself is the same because the I and the R are constant, but it?s the reference voltage that changes the percentage. Practically speaking, because voltage drop is a function of line length, you can run your service conductor twice as far if you use the 240V number then if you use the 120V number.
Would someone mind enlightening me how to interpret the service rating back to the proper application of voltage drop in this case? Thanks.
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