Voltage drop for boat slip

[ramsy]

A 1-PH x 3 cord to 5-prong, 3-PH plug, or "X" adapter would balance nicely, but Hubbel may have missed it. When using 208Y/120 with "Y" adapters (attached below), that extra phase does seem odd mixing with the next receptical or slip.

About this 44A neutral vector, it seems to deal more with power than current-interference in the nuetral, or the 0.866 resistance-demand factor applied to 3-PH voltage drop. See attachment, and the brief discussion last April.

This 0.866 voltage-drop-resistance factor for 3-PH current, included with Z based on NEC Tbl.9, along with 2x distance conversions is a 1.732 total 3-PH adjustment in my model, and has so far matched your VD results when using the same numbers. We might be using the same proprietary model, which address everything but segment-interference magnitudes.

We havn't agreed on neutral current yet, which alters VD across this segments, but as Smart noted with these dynamic loads, it really shouldn't matter if neutrals are sized for worst-case scenarios near 50 amps.

Also the way I tried to add those segments before was incorrect. Saw multiple loads adding current in parrallel, but this is the same load just changing magnitude with interference and resistence in the neutral.

Without fixed loads between phases, and fixed current-interferance magnitudes, I believe one maximum-current value should be used to calculate VD for the entire length.

 

[Wes G]

Oops

Oops

No, because it's 2 x 120v at 30A.

Otherwise if you figured both separately you get 120v * 30A = 3600VA for the first outlet, then 120v * 30A = 3600VA for the second, for a total of 7200VA. 240v * 60A = 14,400VA total, and that's not the same.

You are 100% right for figuring load. To avoid confusion it would be better to simply state that you always figure voltage drop on conductors based on the maximum unbalanced load that can be placed on them. It is easy to confuse the factors for load calculations and voltage drop calculations.

However if you check the voltage drop (VD=2KID/CM) you will get the same results for 30A@ 120V as you do for 60A@240V.

In this particular calculation, if you use the wire size the engineer specified, #6AWG, your calculation will show the following using ohms law: VD=2X30X165X.000491 or 4.86V. Which if I have 120V at the feeder leaves me just over 115V at the outlet which is not an operational problem. (Remember, your feeder is sized for 50A loads so the VD on it will be much lower than the recomended 3%)

It could be that the cost comparison between #4 & #6 wire would still merit the use of #6 when compared with the lower cost of additional energy used by the #4. You would have to take into account the ammount of use that was expected Etc.

Also remember that the 3% is a recomendation, not a mandate, and it could also be that you are still under the 5% recommended for the combination of feeders and branch circuits.

___________________
Wes Gerrans
Instructor
Northwest Kansas technical College
Goodland, KS

 

[ramsy]

..if you check the voltage drop (VD=2KID/CM) you will get the same results..

Wes, AC formulas are prefered in the field for several reasons.