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Discussion Starter · #1 ·
I am a GC trying to better understand wire sizing because it impacts other construction/budgeting decisions.

I have used various wire sizing/voltage drop calculators (such as Southwire) which seem to be relatively consistent with each other, but seem call for wire sizes much larger than are typically used in the field.

A specific example: A 100A supanel, 75 feet away from the load center. The calculators call for a minimum #1 AWG copper wire. This seems odd, because NEC and my POCO only require #4 AWG copper for a service entrance feeder for a 100A main panel. Does not make sense to me.

I experimented with shorter distances (5 feet) and allowing voltage drop of 20%, but the calculators still won't allow anything smaller than #1. What am I missing here?
 

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You aren't doing your calculations correctly.
 

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The Voltage drops allowable are different for different apps--- wires in the air can be loaded more than wires enclosed in a pipe/EMT.....

Do you want hot wires running through a modern super insulated house?

Houses are much larger in all dimensions then even 20 years ago, making the use of code minimum gauges hazardous to your customers.

The science/math behind wire sizing is pretty basic, if 200.00$ of copper is breaking your company, quit now BEFORE the fire sale....

When the lights dim as the AC turns on....cheap wiring.

OP answer, You are missing the V drop from the transformer to the FIRST panel which must be added to the ADDITIONAL 75' of wire to the sub panel, up sizing the wire to the first panel would allow smaller wire to sub panel, less of the allowable v-drop would have been "used up" by the main wires.

If you don't understand it, hire someone that does, and then follow their instructions...
 
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Discussion Starter · #5 ·
richard,
I am a General Contractor. I ALWAYS hire a licensed electrician. I still think it is important to educate myself about the jobs each of my subs do, so that I can make decisions that make the whole project go smoothly.

I seek advice here because I have learned that most subcontractors on a jobsite will give the answer that makes things easiest/cheapest for them, and not offer alternatives that may be a better choice for the project as a whole.
 

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Discussion Starter · #6 · (Edited)
Inner & Fourth,
Thanks for your input. This is all outdoor power in conduit. Specifically:

The client is establishing a new metered electrical service on a vacant lot. The service will be connecting a water pump, an RV and receptacles for construction of a new home.

The power company transformer "Point A" is 100' from "Point B", where power is desired.

Option 1)
Set 100A meter/combo pedestal at "Point B", run secondary feeder in conduit 100' from "Point A" (transformer) to "Point B". POCO calls for #4 feeder (copper)

Option 2)
Set meter/combo pedestal at "Point A" (right beside transformer), set 100A subpanel at "Point B".
Run appropriate subpanel feeder 100' in conduit from "Point A" (meter/combo) to "Point B" (100A subpanel). Calculators call for #1 wire minimum (copper).

It seems to me that the wire requirement should be the same in either instance. What am I missing?

(Please don't bring in other options, I am really just trying to better understand how wire sizing applies to these two situations)
 

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Why not figure new house wiring size required, and location/length needed, USE THAT WIRE for temp pole, loop up temporary excess at temp panel.... cheapest answer.

Water pump or well hundreds of feet underground?

If your subs need this level of control, maybe you should upgrade?

It is a fine line between OCD and detail orientated.

Your post is a near perfect example of "black box" tech mixed with "shopping for a positive outcome behavior".

Your ignorance combined with greed will place the users of your cheapest legal products at risk, Study electricity more, shopping out the various codes less.
 

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Discussion Starter · #9 ·
Wow.....you don't even know me, and you've jumped to all kinds of conclusions. I am not trying to shop codes, micromanage subs or find the cheapest way to do this. I am merely using this EXAMPLE to try to UNDERSTAND this concept.

My ignorance is exactly what I am trying to fix, so I can do a better job for my clients and work more efficiently with various trades. If you are just here to insult me please go away. If you really want to help, can you please explain why option #1 and #2 require different wire sizing?

I asked not to bring in other options because that just distracts from the pertinent question about wire sizing. Since you took the time to suggest an alternative, I will explain. The POCO provides the wire to connect new homes, but does NOT provide wire for temp or utility services. Considering that the new house will be 400A and 300' away from the transformer, it would be a very expensive temp pole when all that is needed now is 100A, 100' away. Once the house is built, the POCO will run the 400A all the way to the house at no cost.
 

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Discussion Starter · #10 ·
tjbnwi,
Thanks for the references. I missed it before because the numbers all changed in 2011 NEC. The way I read 310.15(b) it, both of my "Option" circuits are considered feeders as the paragraph defines it, so it makes no difference.

BUT...NEITHER of my two options qualify for smaller conductors permitted by 310.15(b) because it only applies if the feeder is serving a dwelling. For example, in my home, the the meter/main disconnect is on one side of the house and the load center is in the garage on the other end. A feeder sized by 310.15(b) runs through the attic.

So the code is clear, but I am still trying to make logical sense of it. It seems odd they allow this "dangerous" exception ONLY for dwellings, allowing reduced size feeders to run through interior walls, but prohibit it for outdoor applications.
 

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Odds are the calculator your using is just looking up the code.

Yes, it applies only to dwellings, the site you're using probably applies the calcs to just dwellings for simplicity.

If you want the engineering formula is shown at the end of 310.15(B). Just do the calcs and submit.

Tom
 

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Miranda, What is your goal/? How to game the Utility to lower your costs as a GC? Wouldn't the cheapest TOTAL cost eventually be the cheapest cost for your POCO service area customers????


Hundreds of millions of $ are spent annually on Network designs, Math prizes have been issued for small improvements on the existing level of art.

For your original question, moving the transformer to as close to the service entrance as allowed would be the cheapest solution set, Just as only running ONE SET of wires for the new building would be cheaper then TWO SETS.......:censored: You might pay more, but you saved the customer hundreds of billable dollars. Are you making the pizza better, or just an another pizza cutter?

You seem to be seeking to stick your customer(s) with some of your normal overhead expenses by gaming the utilities temp wiring rules...

If I have misinterpreted your goals, forgive me. For the math and science voids in your CV, I'd start at the local community college night classes for an A.S. degree.
 

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Discussion Starter · #13 · (Edited)
tgbnwi,
Thanks very much for the info. Doing the calculations with the formula helped give me a better understanding of how the sizing works out in different situations.

Fouth,
The only one gaming here is you, in your mothers basement, in between trolling online looking for strangers to insult. If you learned how to interact with people in a positive and professional manner, perhaps you would get more work and be able to afford to move into your own place.
 

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Miranda7: Did you read your original post?:blink:

The wiring codes and tables evolved over the last 120 years, can your business survive your learning the ins and outs of the NEC etc....?

Voltage drops create heat, wiring enclosed inside a super-insulated wall cavity will exceed the wiring insulation's temperature rating and fail, many times starting a fire INSIDE THE HOUSE.

In a few hours anyone with two years of algebra should be able to cobble up an Excel spread sheet to find the least cost solutions that meet code.
I'd googgle it, there are probably existing free ware solutions available....:eek:

I just can't see how squeezing a hundred or two dollars out of your electrical sub is the first, best use of your time.(maybe your electrician is marginal?

The more information disturbs you, the greater its value.... No one likes to be told they are wasting time on a trivial issue. Hate me, fine but don't ignore the info.:jester:

My basement is paid for several years now, with safe wiring. that I understand how the electrons and positrons flow back and forth...:thumbup:

I act in a positive professional manner with "professionals" Honey, You didn't make the cut... Keep trying.:thumbsup:
 

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Miranda7,

The power company is basing their wire size off of a load demand factor, in the case that you will not need the full 100A at any giving point of service. So it is in their best interest to run the smallest/cheapest service wires they can to reduce cost and labor.

A lot of times the power company don't factor in voltage drop because they usually are within a certain footage of their load they are servicing.

Other than services you are required to size feeders in respect of all de-rating factors to the full load of the over current protection device.

I hope this clears up some stuff and helps.
 

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I don't understand the quandry here. The new home builders I've worked with just have a temp service they use for every new home. Same wire, same box, and some even have poles. Buy a set up once, and reuse it - amortize it over a few homes. If you aren't going to reuse it, people will buy temp services off of Craigslist, so sell it when you're done.

I could see this being an issue of you're only building ONE new house.
 

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I don't think there is a quandary here. I think that she simply wanted to understand why the Power Company used smaller sized wire which did not reflect her research of what she thought to be the right sizing of wiring and voltage drop.
 

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The wire size calculators give you a very simple and quick calculation, these calculator is generally used to calculate the size of wires. Below is the formula to do calculation for wire size:-

"For single-phase circuits:
Wire Circular Mils = (Conductor Resistivity)(2)(Amps)(One Way Distance in Feet)/Allowable Voltage Drop"
 

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Discussion Starter · #19 ·
Thanks CB and H and Peter,
As CB said, there is no quandry. I was just using this situation as an example to gain a better understanding of wire sizing per code.
 
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