Not your basic voltage drop question.

ADV_Rik

Member
Location
Hesperia, Ca.
300a 480v 3ph load for portable A/C unit that is used on a jumbo 747 jet while work is performed on it.

350' from MSB to 480v Disconnect.

200' of portable cable from Disconnect to A/C unit

Ambient air temp 105-113F .82 correction factor.

Do I need to factor in length of the portable cable for sizing my feeders from the MSB to the EXO?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
There will be voltage drop on both the fixed and portable wiring, so you have to consider both when calculating the voltage at the load.
 

GoldDigger

Moderator
Staff member
To expand on that a bit, you have to look at the cost and practicality of getting the overall voltage drop you need. Two things to keep in mind:
1. If the two sections are roughly equal length, your best results in terms of copper cost for voltage drop will be to keep the same gauge wire in both segments. But,
2. If, for example, you need to reduce the size of the portable section for easier handling, you need to upsize the feeder to a greater degree than you downsize the cord.
A simple spreadsheet will let you play with both sets of numbers while making your decision.

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Fulthrotl

~~Please excuse the mess. Sig under construction~~
300a 480v 3ph load for portable A/C unit that is used on a jumbo 747 jet while work is performed on it.

350' from MSB to 480v Disconnect.

200' of portable cable from Disconnect to A/C unit

Ambient air temp 105-113F .82 correction factor.

Do I need to factor in length of the portable cable for sizing my feeders from the MSB to the EXO?
yeah.... voltage drop is voltage drop....

however, i'd take the 3% on the cord, and 2% on the feeders to the disconnect.
assuming you can pull aluminum feeders to the disconnect, i'd also look at
flexible cord that has fine stranded aluminum wire instead of copper....
easier to drag around, and pay for. you can hypress pin and sleeve stuff
onto it.....

750 MCM aluminum gives you 385 amps and 1.07% on the feeders......

for the cord, it's gonna depend on the insulation type you use to
calc ampacity....
 

GoldDigger

Moderator
Staff member
yeah.... voltage drop is voltage drop...
for the cord, it's gonna depend on the insulation type you use to
calc ampacity....
The other possibility is that the VD rather than the ampacity is the dominant factor, but with that supply voltage and distance it is less likely.


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fmtjfw

Senior Member
What is the frequency? If it is 400Hz then you'll have more skin effect and single conductor will have a higher impedance than paralleled conductors with the equivalent cross-section.
 

kwired

Electron manager
Location
NE Nebraska
300a 480v 3ph load for portable A/C unit that is used on a jumbo 747 jet while work is performed on it.

350' from MSB to 480v Disconnect.

200' of portable cable from Disconnect to A/C unit

Ambient air temp 105-113F .82 correction factor.

Do I need to factor in length of the portable cable for sizing my feeders from the MSB to the EXO?
You mentioned ambient temp and a correction factor. Keep in mind that ampacity and voltage drop are not all that related, but anytime you have to increase ampacity (with the load otherwise remaining the same) you will lower voltage drop.
 

ritelec

Senior Member
Location
Jersey
VD is a concern I understand..........................

But in certain scenarios (like maybe this one) would it be possible to add a buck/boost xformer at the ac unit to boost the voltage...???

Also, maybe a cost factor, but why aren't more boosting transformers added to feeders and circuits ???
 

GoldDigger

Moderator
Staff member
If the circuit where VD is a concern is a branch circuit serving only one load, a boost transformer is certainly an option.
But with other loads on the branch or other branches on a feeder, the no-load voltage may be too high or the voltage step may be a problem.

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kwired

Electron manager
Location
NE Nebraska
VD is a concern I understand..........................

But in certain scenarios (like maybe this one) would it be possible to add a buck/boost xformer at the ac unit to boost the voltage...???

Also, maybe a cost factor, but why aren't more boosting transformers added to feeders and circuits ???
Buck/boost is not a good solution for voltage drop majority of the time. As GD mentioned, when the load is not running, or when load is reduced you could end up with too high of voltage. NEC also does not require us to meet a specific voltage drop level, what is mentioned is in an informational note and is nothing more than suggestion. One really needs to evaluate every situation and determine what is acceptable for each situation.
 
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