VOLTAGE DROP QUESTION -FEEDER CIRCUIT 120240 SERVICE ENTRANCE -MOBILE HOME PARK

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Windclaw

Windclaw

Member
Location
Brevard County Florida
HELLO,
My Dad, the owner of this account has directed me to seek my answers from you.

I need help in calculating voltage drop or more correctly size of wire. My Dad has tried to explain this but I am having difficulty grasping it all.
So here I am. I was tasked to find the correct size of wire for a 175 foot long feed from a meter base in a trailer park to the trailer. The trailer has an 150 amp panel. Dad has tried to explain this .... and I am not getting it, I am only a second year apprentice and 19 years old. So please go easy,

Victor.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Windclaw said:
HELLO,
My Dad, the owner of this account has directed me to seek my answers from you.

I need help in calculating voltage drop or more correctly size of wire. My Dad has tried to explain this but I am having difficulty grasping it all.
So here I am. I was tasked to find the correct size of wire for a 175 foot long feed from a meter base in a trailer park to the trailer. The trailer has an 150 amp panel. Dad has tried to explain this .... and I am not getting it, I am only a second year apprentice and 19 years old. So please go easy,

Victor.
Click to expand...

Why don't you show us what/how much you have so far and then we can step in where necessary, otherwise we have no idea just how much of the process you do or don't understand. If you don't understand much of it at all this can turn into a fairly thorough basic electricity class instead of just help with a voltage drop calculation.
 
G

Gregg Harris

Senior Member
Location
Virginia
Occupation
Electrical,HVAC, Technical Trainer
Windclaw said:
HELLO,
My Dad, the owner of this account has directed me to seek my answers from you.

I need help in calculating voltage drop or more correctly size of wire. My Dad has tried to explain this but I am having difficulty grasping it all.
So here I am. I was tasked to find the correct size of wire for a 175 foot long feed from a meter base in a trailer park to the trailer. The trailer has an 150 amp panel. Dad has tried to explain this .... and I am not getting it, I am only a second year apprentice and 19 years old. So please go easy,

Victor.
Click to expand...

Three things to consider
#1 type of conductor/ copper or aluminium
#2 length of conductor
#3 temperature
 
augie47

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Before we bedazzle you with our collective knowledge and begin arguing about everything from the molecular structure of copper and the coefficient of friction and you go running form the room screaming.. let's back up to post #2 and you tell us how you have approached it so far... are you using a formula or a program? What numbers are you using ?

Keep in mind that in the real world you have one variable that will lead to a variety of answers and that is "load". With a know fixed load you can easily determine a reasonable answer. In this case you are going to have to do some "guesstimating" and look at the answer from a practical standpoint also. If you calculate at the 150 amps, you will find the answer provides a wire that is cost prohibitive in a real installation situation.
 
Windclaw

Windclaw

Member
Location
Brevard County Florida
sorry shows what I do not know

sorry shows what I do not know

1. 175 feet length from meter can to trailer
2. stranded aluminum was thinking 4/0 but dad says ...to check if a smaller size can be used??? he will neither confirm or deny if I am right yet.
3. 150 service.
also does it count if I use single run wire instead of a bundle .... since Dad says it must be in conduit PVC and 18/24 inches deep I guess the correct question is... does the a and b phase of a 120/240 service count as parallel runs when figuring this out?

Thank you,

Victor
 
Windclaw

Windclaw

Member
Location
Brevard County Florida
I have done my calculations from the regular formula

I have done my calculations from the regular formula

I come up with....resistence per foot .0000608 ohms I figure 4/0 good for 660 feet but what about using 3/0? good for good at these amps for 190 feet??

Victor
 
kwired

kwired

Electron manager
Location
NE Nebraska
First thing that you need to establish is how much voltage drop is going to be acceptable.

Next thing is determining what the actual load is, or what load level you want your max allowed voltage drop to apply to. Just because you have 175 amp breaker does not mean you have 175 amps of load. There are many dwellings with 200 amp services that probably have a peak of 50-80 amps much of the time, occasionally may go higher for short times, but maybe run less than 50amps the majority of the time.

If your objective is to stop light dimming when a particular motor starts - things get even more complicated as the current when motor is starting is higher than when running, and the capacity of the supply could factor into this problem on top of voltage drop in conductors.
 
glene77is

glene77is

Senior Member
Location
Memphis, TN
Windclaw said:
1. 175 feet length from meter can to trailer
2. stranded aluminum was thinking 4/0 but dad says ...to check if a smaller size can be used??? he will neither confirm or deny if I am right yet.
3. 150 service.
also does it count if I use single run wire instead of a bundle .... since Dad says it must be in conduit PVC and 18/24 inches deep I guess the correct question is... does the a and b phase of a 120/240 service count as parallel runs when figuring this out?

Thank you,

Victor
Click to expand...

Your Dad is "kindly" trying to help you learn something important.
Calculating feeder size is basic in designing a circuit like this.
Calculating the loads is basic in designing a feeder circuit like this.

You are splitting this project into discrete components, properly.
Regarding whether "the A and B phase of a 120/240 service count as parallel runs when figuring this out? "

here is just a clue:
What are the Loads or "current demands" placed on these feeders ? This can give you a good "estimate" on required wire-size.

here is just another clue:
If the loads phase A is balanced with loads phase B, then current is totally in series. Ideally, loads on phases A and B are fairly well balanced.
Check the size of your residential Main breakers, both are equal, because the (balanced load) currents are in "series".
Kirchhoff and Thevenin electrical laws describe what happens.

I'll check back in a few hours, to see where you have gone with this thinking.
Victor, This is good stuff ! Tickles me. :)
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Windclaw said:
I come up with....resistence per foot .0000608 ohms I figure 4/0 good for 660 feet but what about using 3/0? good for good at these amps for 190 feet??

Victor
Click to expand...
As noted earlier you have to set the acceptable voltage drop value. The NEC, for general cases, recommends a voltage drop no greater than 5%... with 2% for feeders and 3% for branch circuits. The electrical trade in general allows +/- 10% from nominal system voltage… but you have to remember service conductors also figure into the voltage drop, so 5% total drop on feeder and branch circuits is a reasonable number. In the case of a mobile home, the circuits are fairly limited in length, so it is reasonable to assume the branch circuits top off at 2%... so while it is a stretch, an acceptable voltage drop for a feeder is 3%.

A 3% drop of your nominal system voltage of 240V is 7.2V. If you want to take the resistance per foot approach, you simply use Ohm’s Law: E=IR, where E is equal to VDROP, I is your load current, and R is the effective resistance (or if you want to be more technical, impedance, for AC circuits). Let’s start with maximum load current since we already stretched the recommended acceptable drop from 2% to 3%.

7.2V ? 150A = 0.048Ω
​
This impedance occurs over a total circuit length of 350 feet (175 feet each way), so we now calculate ohms per 1000 feet, the units provided in Chapter 9 conductor properties tables.

0.048Ω ? 350ft ? 1000 = 0.137Ω/1000ft
​
Now we simply look to Chapter 9 Table 9 for the smallest size which has an impedance value (effective Z) less than 0.137Ω per 1000ft. For an aluminum conductor in PVC that would be 3/0 at 0.13Ω.

In summary, in must be noted we made no less than three assumptions here in this determination: 1) an acceptable voltage drop of 3%, 2) used the maximum load current of 150A when a lesser value may be appropriate, and 3) a power factor of 0.85. Changing any one of these values may change the result of the determination.
 
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Smart $

Smart $

Esteemed Member
Location
Ohio
Smart $ said:
... Changing any one of these values may change the result of the determination.
Click to expand...
Just wanted to add that if we changed the parameters in my preceding example to an acceptable voltage drop of 2% and a load current of 100A, we'd get the exact same result.
 
Windclaw

Windclaw

Member
Location
Brevard County Florida
Thank you for all your posts....I am still at it however.....Dad says thanks too.

Thank you for all your posts....I am still at it however.....Dad says thanks too.

I was assuming 3% voltage drop from the meter can to the trailer disconnect. So after using vd=2k x I x D/ cmil it is the one dad taught me and I find it useful in most respects. but it seem the real controversy for me was balancing between wire size and acceptable voltage drop, cost is considered but, of less importance in this instance, we even considered copper wire for time but when I went there the cost was prohibitive.

I am starting my own user name on the forum ...I guess DAD is right again ...This forum is the best. any other suggestions are appreciated.
Victor
 
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