Does this formula work for finding the distance in a three phase situation? Distance =CM of conductor x .866 x allowabel E drop divided by 2x K x I . Please note conductor is a #6 Thhn copper cir mill is 26,240 also K is 12.9 also I is 45 amps also voltage is 480 volts and allowabel E drop is 3% of the 480 or 14.4 volts. My answer is 281 feet however I am told that this is incorect it should be 376 feet . Can some one shed some light on this? thanks Ed
Three phase voltage drop and distance equation
- Thread starter ecvolt
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ramsy
NoFixNoPay Electric
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
Using 480vac 3? with 45A on #6cu (cm=26240), and a target voltage of 465vac, after adjusting NEC Tbl.9 for circuit PF, and assuming load PF=1, my results are near 500.Tot.ft in ferrous conduit, 600.Tot.ft in PVC, and a max derating of 0.60.
Using the same Target Voltage spreadsheet, with total PF=0.85 as assumed in Tbl.9, my results are near 250.Tot.ft in ferrous, 300.Tot.ft in PVC.
Using the same Target Voltage spreadsheet, with total PF=0.85 as assumed in Tbl.9, my results are near 250.Tot.ft in ferrous, 300.Tot.ft in PVC.
bdarnell
Senior Member
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- Indianapolis, IN
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- Retired Engineer
I get 376 also using the formula:
D= CM x V
__________
1.732 x K x I
D= CM x V
__________
1.732 x K x I
Ed,
The .866 should be used in the denominator, i.e, multiply .866 by the K, 2, and I, and not by the area and VD in the numerator.
I think this formula is more for resistive loads, don't know if power factor can be incorporated for loads of inductive nature.
The formula without the .866 is one for single phase. The extra multiplier .866 makes it work for 3 phase. 2 x .866 = square root of 3 or 1.73
The .866 should be used in the denominator, i.e, multiply .866 by the K, 2, and I, and not by the area and VD in the numerator.
I think this formula is more for resistive loads, don't know if power factor can be incorporated for loads of inductive nature.
The formula without the .866 is one for single phase. The extra multiplier .866 makes it work for 3 phase. 2 x .866 = square root of 3 or 1.73
1793
Senior Member
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- Louisville, Kentucky
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- Inspector
kthbrwn said:
Would someone explain where the .866 comes from and how it figures into the equation?
I have always used 1.73 for 3 phase calculations.
ramsy
NoFixNoPay Electric
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
Check me on this, but I think we're all on the same page. Whether line current is first vectored * Sqrt(3) for Delta's and not for Y-connections depends on xfrm & loads.
But, in both cases unlike neutral-connected loads, for VD calcs line current phase-to-phase should finally be factored by 0.866, for the odd timing of current peaks, across the lines, in balanced 3-phase systems.
1) With 1-way distance, 3? (VD) can be shown as:
VD = (K*I*L)/CM [travels across a single phase]
With round-trip distance the same VD formula is shown:
VD = (0.866*K*I*2L)/CM same as VD = (1.732*K*I*L )/CM [unrelated to (I) vectors]
2) When solving VD length for round-trip distance:
L = (VD*CM)/(1,732*K*I) [unrelated to (I) vectors]
When solving for 1-way distance:
L = (VD*CM)/(K*I) [travels across a single phase]
3) Round-trip (L) with the effect of PF, XL, & Z from NEC Tbl.9:
VD = Z*I*Sqrt(?) (Use Tbl.9 PF=0.85) [unrelated to (I) vectors]
L = (VD*CM)/(K*I*Sqrt(?)) [unrelated to (I) vectors]
If neutrals paths = 1 phase, this calc. should work across neutrals and lines; just watch the lines for proper (I) current vectors, * Sqrt(3) in Delta xfmrs or Delta-connected loads, before using VD formulas.
But, in both cases unlike neutral-connected loads, for VD calcs line current phase-to-phase should finally be factored by 0.866, for the odd timing of current peaks, across the lines, in balanced 3-phase systems.
1) With 1-way distance, 3? (VD) can be shown as:
VD = (K*I*L)/CM [travels across a single phase]
With round-trip distance the same VD formula is shown:
VD = (0.866*K*I*2L)/CM same as VD = (1.732*K*I*L )/CM [unrelated to (I) vectors]
2) When solving VD length for round-trip distance:
L = (VD*CM)/(1,732*K*I) [unrelated to (I) vectors]
When solving for 1-way distance:
L = (VD*CM)/(K*I) [travels across a single phase]
3) Round-trip (L) with the effect of PF, XL, & Z from NEC Tbl.9:
VD = Z*I*Sqrt(?) (Use Tbl.9 PF=0.85) [unrelated to (I) vectors]
L = (VD*CM)/(K*I*Sqrt(?)) [unrelated to (I) vectors]
If neutrals paths = 1 phase, this calc. should work across neutrals and lines; just watch the lines for proper (I) current vectors, * Sqrt(3) in Delta xfmrs or Delta-connected loads, before using VD formulas.
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