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200 amp underground service wire

fromPok | Posted in General Questions on

Electrical experts, For 200 amp service my electrician recommended 4/0 Urd cable. I noticed the cable is made up of 4/0 4/0 and 2/0 for the neutral vs all wires being 4/0. Then I asked about 250 kcmil. Even this urd cable has 250, 250, 3/0 for neutral. Why is neutral always 2 sizes lower. Is this ok by code for neutral to be undersized? Thank you 

similarly for an aluminum ser cable , that I am using for a 100 amp sub panel from 200 amp main is sized 1,1,1,3.. 3 for ground.  Why is ground sized lower? In Romex, generally all wires seem to be of same gauge. Thanks for you insights.

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  1. Patrick_OSullivan | | #1

    For the service, the utility will typically spec the connectors to the meter. Lower sized neutral is because most loads will be balanced across the split phase and the neutral load will largely cancel out.

    > similarly for an aluminum ser cable , that I am using for a 100 amp sub panel from 200 amp main is sized 1,1,1,3.. 3 for ground. Why is ground sized lower?

    The equipment grounding conductor or "ground" is not there to convey 100 amps to ground for any duration of time. It's there to create low-enough impedance path for fault current so that overcurrent protection devices (circuit breakers) can trigger in the event of a fault. Therefore, it needn't be as large as a conductor sized for a more continuous load.

  2. Expert Member
    BILL WICHERS | | #2

    The way we talk about this in the engineering world is that "the neutral only carries the imbalance current". Imagine that you have three 1,500 watt loads, maybe hair dryers or space heaters. They're all 120 volt loads. Plug one in, you now have 1,500 watts running on one of the two hots and the neutral, since the service provides 120v between either hot and neutral. Plug two 1,500 watt loads in, assume they're on different hots. That means you now have 1,500 watts on each hot, and zero load on the neutral -- the two 1,500 watt loads are in series! They now "look like" a single 3,000 watt load on 240 volts. The reason this works is that the sinewave on each hot is 180 degrees out of phase, so the neutral current cancels with balanced loads on either hot. Now plug in that third 1,500 watt load on the "other" hot -- you now have 1,500 watts on one hot, 3,000 watts on the other, and the neutral carries the "imbalance" part, so only 1,500 watts out of the 4,500 watts total.

    In a normal situation, at least a big chunk of of the loads will be balanced between hots, so the neutral typically carries a good deal less current than either hot is likely to carry. While code stopped allowing undersized neutrals for normal feeders in buildings some time ago, the run of cable between the meter and the utility is "engineered", and has to follow different rules. It IS possible to get cable with all conductors sized the same, but it will cost more, possibly be harder to get, and won't really gain you anything.

    Grounds are sized by a table in the code book that says what size they need to be based on the capacity of the circuit. For copper wire (from memory here), 10 gauge is good to 60 amps, 8 gauge to 100 amps, 6 gauge for 200A circuits and below, and contuining on up. This is for branch circuits though, NOT for ground wires going to ground rods, which are also sized smaller, but are sized based on the size of the service entrance conductors instead of the breaker feeding the circuit. Ground wires, as Patrick mentioned, are not part of the circuit in normal operation -- ground wires are only there to provide a path for "fault current", and need to be low enough resistance to ensure the overcurrent protection device (circuit breaker or fuse) will operate properly when needed.


  3. fromPok | | #3

    Bill & Pat , Thank you

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