Voltage drop, total resistance, wire size in parallel circuits

Pete27696

Member
Location
Alaska
If I had to run a cable 1400 ft with a 50 amp drop in 8 locations, how would I calculate the wire size needed to accomodate the voltage drop and the loads wired in parallel?
 

Smart $

Esteemed Member
Location
Ohio
If I had to run a cable 1400 ft with a 50 amp drop in 8 locations, how would I calculate the wire size needed to accomodate the voltage drop and the loads wired in parallel?
First you'll have to determine the minimum voltage (maximum voltage drop) at the end load. If you want equal voltage drop from load to load, divide the total drop by 8. This will be the allowable amount of drop from supply to first load, and then each segment thereafter. In succession using the current amount and distance for each segment of the run, determine the maximum ohms per 1000' for the wire. Use Chapter 9, Table 9 impedance values to size the wire.

This is not the only way to do it!!!

I was succinct because I have to get ready for work. If you need more detail, post back. Others will help if I can't get back to you right away...
 

Pete27696

Member
Location
Alaska
No, this is not for a class. I am trying to put together a temporary power plan for a bridge that is 1400ft long. During the construction, they want a 50A convenience drop at each pier. The distribution boxes are single phase 208V. We can't have any equipment (fuel) on the bridges during the launch, so the power has to generate from land side and go the entire length of the bridge. I am considering putting a transformer in a few places on the bridge to use 480V distribution power and step down to 208V to lessen the lengths and cable size. I will try what has been suggested. It seems that the cable will be of significant size 4/0 or greater to have the proper voltage at the end of the run.
 

junkhound

Senior Member
Location
Renton, WA
OK, the fact that it is temporary makes it a no-brainer to go with your transformer option.

The main reason for not using a transformer would be the cost of having the transformer core losses downstream of the metering for the rest of the life of the bridge. Relatively trivial for likely heavy loads during construction.

You could do a cost/performance trade on wire vs. transformer cost, voltage drop, and power costs.

Everytime I've ever done a trade like that the transformer LOSES for permanent installations with low average load (e.g. <2kW for typical house) but transfomre WINs for temporary, especially if you rent the transformer or use existing shop stock.
 

Smart $

Esteemed Member
Location
Ohio
.... I will try what has been suggested. It seems that the cable will be of significant size 4/0 or greater to have the proper voltage at the end of the run.
Is it possible to have shore power from both sides, splitting the distance per supply run in half?
 

Smart $

Esteemed Member
Location
Ohio
Personally I wouldn?t run a radial circuit. A staggered ring will even out the overall volt drop along the length of the supply.
I assume you mean a ring circuit of sorts, but not sure about the staggered part...???

While POCO's may use a ring (aka loop) on their distribution system, on the consumer side we don't do ring circuits on this side of the pond.
 

Pete27696

Member
Location
Alaska
We cannot run power from both sides of the bridge. We are launching the 1400' bridge with hydrdaulic rams from the east side of the bridge. The other side of the bridge is in the NPRA. I'm leaning towards using transformers, by my calculations I would need 1100mcm cable to get the voltage drop to be in tolerances and carry the load. That is just not feasible. I'm not sure what is meant by radius (ring) circuits.
 

winnie

Senior Member
A 'ring' circuit has a couple of common uses. One use is a circuit fed from both ends. This is common in the UK, but illegal in the US because you effectively have unequal parallel conductors. The other use of the term is a circuit where one conductor is fed from one end, and the other conductor from the opposite end of a long run. This has the benefit of splitting the voltage drop evenly over all of the loads; useful for things like landscape lighting where you want all of the lights to be dimmed by the same amount. Neither of these approaches is relevant here.

Questions:
What are the actual load requirements? It won't be exactly 50A per drop; that is the rating of the drop, not the rating of whatever is plugged in.
What are the actual electrical requirements, eg. voltage, voltage tolerance, phase count, running current, starting current, etc?
How stable is the load. Is it constant or varying?
Will _all_ of the drops be used at the same time, or just transiently at different times?
Can loads with higher voltage tolerance be substituted?
Are you really trying to deliver 208V, or 208/120V?

What are the actual load requirements? Voltage, current, voltage tolerance, phase count, transient (startup) load, etc.? How stable is the load? Are these drops for lighting, or something related to moving the bridge?

Will _all_ of the drops be used at the same time, or just transiently at different times?

Can loads with higher voltage tolerance be substituted?

How did you calculate 1100 MCM cable? That seems overkill to me, but it is very easy to get that as a mathematically correct result if you have incorrect assumptions about your requirements.

-Jon
 

RUWired

Senior Member
Location
Pa.
Have you thought about running a single or parallel set of cables to each of the eight locations. It might limit the size to only needing 2 x 4/0 cables at the last two locations.
 
Last edited:

Smart $

Esteemed Member
Location
Ohio
... What are the actual load requirements? It won't be exactly 50A per drop; that is the rating of the drop, not the rating of whatever is plugged in. ...
@Pete27696

I agree with Jon's assessment to get the wire size reduced to a feasible level. Consider only the maximum ampere draw at any one instant. It could be that no more than 50-100A is all that is required for the entire span.

Additionally, I have to ask if there will be any permanent wiring installed, such as for lighting... and when? The thought is that the permanent wiring could be used to bolster power requirements if installed on an as-it-progresses basis.
 
Last edited:

Pete27696

Member
Location
Alaska
you are right, I don't have all the information required, so I was calculating using worst case scenario, the 1400ft with a 5% tolerance, and a total load of 400A.
The 50A drops are to distribution boxes that are 208V single phase, and the hydraulic heaters are 208 three phase. It is convenience construction power for hydraulic power pacs, hydraulic heaters, grinders, drop lighting, etc... Nothing we are using will have stringent voltage tolerances. These tools will not be used all at the same time, I really think that 10-15A is more reasonable, and still even at that not each location at the same time. There are a lot of unknowns, I am just trying to get them power to work, that is reliable and safe.

I think I have come up with a plan to place 3 480V stepdown transformers at 350', 700', and 1050'. And distribute 208V from those places, making my cords max of 350ft. Then we also have contingency 480V power for the unknowns still.
 
Top