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Thread: Voltage drop for new feeder in an existing building

  1. #1
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    Voltage drop for new feeder in an existing building

    Hi all,
    If I have to add a new sub-panel on the 10th floor of an existing building, and let's say the new feeder to this sub-panel (originating from a distribution board ON the 10th floor) is 100 feet long, how does one know what volt drop percentage to use when calculating the wire size for this new feeder? The voltage drop from the cellar service up to the 10th floor distribution board may be at 2%, or 2.5%, or even already at the max of 3%. How would i know what percentage to use for the new feeder serving the sub-panel, and make sure I don't exceed 3% from the cellar service up the new sub-panel? It's an existing building, and let's say existing drawings and calculations don't exist (as sometimes is the case). Does the NEC cover this?

    Thanks.

  2. #2
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    Other than 2 or 3 very specific cases voltage drop is not a code requirement. There is an informational note that is not enforceable that says the total voltage drop to the finial outlet should not exceed 5%.
    Don, Illinois
    Ego is the anesthesia that deadens the pain of stupidity. Dr. Rick Rigsby
    (All code citations are 2017 unless otherwise noted)

  3. #3
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    Quote Originally Posted by don_resqcapt19 View Post
    Other than 2 or 3 very specific cases voltage drop is not a code requirement. There is an informational note that is not enforceable that says the total voltage drop to the finial outlet should not exceed 5%.
    I know it's not a code requirement, however it is enforced by some of the energy codes, such as ASHRAE 90.1 where you can't exceed a 2% voltage drop for feeders. so if you're renovating the 10th floor of an existing building, and adding that sub-panel, how do you know what percentage to use for that new feeder serving the sub-panel? You wouldn't know what the existing drop is up to the existing distribution board on the 10th floor.

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    Quote Originally Posted by Grouch1980 View Post
    I know it's not a code requirement, however it is enforced by some of the energy codes, such as ASHRAE 90.1 where you can't exceed a 2% voltage drop for feeders. so if you're renovating the 10th floor of an existing building, and adding that sub-panel, how do you know what percentage to use for that new feeder serving the sub-panel? You wouldn't know what the existing drop is up to the existing distribution board on the 10th floor.
    2% for each feeder, not the sum, so 2%
    they are concerned with heat load

    at 100' v drop will not be an issue if ampacity is correct
    what is the current?

    you can measure the v drop ofbthe existing
    measure v btm, v top, current simultaneously
    delta v / i = feeder z
    feeder ampacity x feeder z = v drop
    or calc on the existing wire size
    Last edited by Ingenieur; 04-16-18 at 04:40 PM.
    The difference between genius and stupidity is that genius has its limits.

  5. #5
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    If you're in NYC voltage drop compensation is mandatory by NYC amendment.

    Chapter 2

    Wiring and Protection

    ARTICLE 210
    Branch Circuits

    SECTION 210.19
    Subsection 210.19(A)(1) – Add a new sentence at the end of the paragraph before the first Exception to
    read as follows:
    Conductors of branch circuits shall be sized to allow for a maximum voltage drop of 3 percent at the last
    outlet supplying light, heat or power and the maximum voltage drop allowable for feeders and branch
    circuit combined shall not exceed 5 percent.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  6. #6
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    Quote Originally Posted by Ingenieur View Post
    2% for each feeder, not the sum, so 2%
    they are concerned with heat load

    at 100' v drop will not be an issue if ampacity is correct
    what is the current?
    oh, I don't have a current for this particular case, this is more of a generic question, however you always come across this in renovations in existing buildings.

    hmmm... i think the energy code means 2% for the entire run of feeders. So if you have main feeder A, feeder B, and then sub-feeder C... the sum cannot exceed 2% (it wouldn't be each feeder at 2%). After the 2%, your branch circuit can then be at 3%. Am I interpreting this wrong?

  7. #7
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    Quote Originally Posted by infinity View Post
    If you're in NYC voltage drop compensation is mandatory by NYC amendment.
    Nice! Thanks for that.

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    Quote Originally Posted by Grouch1980 View Post
    oh, I don't have a current for this particular case, this is more of a generic question, however you always come across this in renovations in existing buildings.

    hmmm... i think the energy code means 2% for the entire run of feeders. So if you have main feeder A, feeder B, and then sub-feeder C... the sum cannot exceed 2% (it wouldn't be each feeder at 2%). After the 2%, your branch circuit can then be at 3%. Am I interpreting this wrong?
    just looked at 90.1, correct
    it is total feeders 2%
    branch 3%
    same as nec guidelines

    the only way to do it is measure or calc from as-builts

    a 200 A feeder sized for 160 A load, 3 ph
    v drop / sys v x 100 = (1.732 x L/1000 x Z x i)/ sys v x 100
    assume max 2%
    0.02 = (1.732 x L/1000 x Z x i)/sys v
    L = 11.55/(Z x i) x sys v

    max for 208/3 at 160 A with 3/0 (200 A) Cu, steel cnd
    max one-way L = 11.55/(0.094 x 160) x 208 = 160'
    1% 80', etc
    The difference between genius and stupidity is that genius has its limits.

  9. #9
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    Quote Originally Posted by Ingenieur View Post
    just looked at 90.1, correct
    it is total feeders 2%
    branch 3%
    same as nec guidelines

    the only way to do it is measure or calc from as-builts

    a 200 A feeder sized for 160 A load, 3 ph
    v drop / sys v x 100 = (1.732 x L/1000 x Z x i)/ sys v x 100
    assume max 2%
    0.02 = (1.732 x L/1000 x Z x i)/sys v
    L = 11.55/(Z x i) x sys v

    max for 208/3 at 160 A with 3/0 (200 A) Cu, steel cnd
    max one-way L = 11.55/(0.094 x 160) x 208 = 160'
    1% 80', etc
    Got it. So there's no rule of thumb / formula, other than to actually take a field measurement of the voltage at the point where the new connection is being made, or find as-builts / design drawings and calculate it.

  10. #10
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    Quote Originally Posted by Ingenieur View Post
    max (2%) for 208/3 at 160 A with 3/0 (200 A) Cu, steel cnd
    max one-way L = 11.55/(0.094 x 160) x 208 = 160'
    1% 80', etc
    That is Slick -

    Start new thread for "Conduit fill", & "Point-to-Point SCA"

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