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All you electricians

I have a simple question.

How do you "AC" electricians arrive at what size wire to use on a job.


Here is the scoop for DC which isnt really much different from the NEC and Uglys.

To figure the size wire to be used on a DC application,,, here is the formula with constants in black and variables in red.

k*LL*A / Vd
K=11.1
[COLOR="Red"]LL[/COLOR]= loop length
[COLOR="Red"]A[/COLOR]= breaker size
[COLOR="Red"]vd[/COLOR]= allowable voltage drop

so,, if you have a peice of equipment to wire up which is rated at 70A and requires about 100' of cable one way, the size wire would be calculated as followes:

11.1 x 200 x 70 / .75

= 207200 circulair mils or 4/0 cable (with slightly derated breaker size)


For AC,, there is no one formula i have located which makes sense.  WIth a Q factor figured in along with confussing references to single phase vs three phase, it makes no sense.

I have an electrican putting in 120/240 1ph service drop to a 400A meter base and the power company adding a 35k transformer.  We got our preformed building set on a new pad i poured along with a 100k genset, manual and automatic ATS as well as various sub panels in place and genset roll up lug box in place.  The electricians words were "im just gonna use 500mcm and be done with it".  Im asking him how he arrived at this number and he cant answer.  This 500mcm will be the transformer taps to the meter base.

I called several electrical contractors in my area and they all start off with ,,"you pick a wire size and then you see if that wire size will work".
I ask how, and they say, just check to make sure it can carry the current.  I know full well this isnt the complete correct answer but still.
Copper is not cheap now a days and in order to plan, i need to be able to use one formula that computates the wire size with one hand motion just like the DC formula.

So,, i come to a situationw here where I need to figure out what size wire works in the following situation.
In AC calculations,, we do not use LL as our distance, but rather one way distance is substituted.

Distance is 140' from ASCO switch gear to 225A sub panel.
Breaker size is 225A

I have noticed that if i use the DC formula (sub in one way vs LL) and devide by a VD of around 3V, I come very close to what would be typical and observed out in real world instances. 

what wire size for the above situation,
My gut feeling is no less than 2/0 based on experience but 3/0 would exceed standards with RHW / RHH wire at 80% derated breaker.

what formula is a one stop shopping to determine wire size when you do not know what wire size to use?

All you electricians

Reply #1 –
Your 35kVA  transformer is not big enough at 208v/3p for even your 225A panel. (100A @ 208V).  Are you sure that is correct?  Your Genset is 100K. Is that supposed to be a 350K transformer?  That would be 1000A service to the building, which makes more sense.  I'm guessing they are putting in 3 parallel runs of 3-500mcm wires for the transformer to the building (switchgear)?

A 225A panel would require #4/0 wiring w/ a #2 ground.

(Sounds like the electricians need to open up their code book)
Long live the 4-eyes!  - '83 Tbird Turbo - '85 Marquis LTS - '86 LTD Wagon

All you electricians

Reply #2 –
wrong site,, i corrected post 1.  mind was jumping around i guess.

single phase 120/240 for the prefab comcast building project.
the other site with the ASCO switch gear is 277/480 delta with a groomed in neutral and ground making it Y now due to seoncdary sub panel taps and ground ring additions.  Its still delta but ,,, not,, hard to explain but the switch gear has mgb and building grounding rework done. 

the preformed building is a comcast node which will have a continuous load unlike residential transitional loading, this transformer is being upgraded from the 15k it is now.

just ignore the the babbling, i have two different project with two different situations in there.

the question is still the same though,, sorry to confussel you.

All you electricians

Reply #3 –
Quote from: Chuck W;216300
....(Sounds like the electricians need to open up their code book)


I'd agree, or maybe he does since it sounds like the ones he has around are too lazy to actually read it if they do manage to find it. I believe you can check a copy out at your local library but be careful, I hear reading it is a good cure for insomnia. ;)

All you electricians

Reply #4 –
The 225A panel wiring still stands as per the conductor ampacities listed in the code book.
Long live the 4-eyes!  - '83 Tbird Turbo - '85 Marquis LTS - '86 LTD Wagon

All you electricians

Reply #5 –
ok
i confussed you both,,
the example was actually two sites i was jumbling in just for the sake of example,, not one site.

I corrected the information in post 1 but the data is two job sites which i can see now how it could be silly looking.

the prefab comcast building genset is overkill for now based on thier L1/L2 antited draw. The ups feeding a 200a critical panel was tricky to engineer in because it will be single phase 120v input. To prevent the genset from seeing an imbalance on L1 vs L2 when running due to my thoughts of the ups loading down one leg,,, i stuffed a transformer in between the the main service panel and the UPS.  I fed the transformer with 240, the secondary of the transformer is also 120/240 out.  Ill just wire out one leg of the secondary to the UPS but the generator only sees the transformer primary so it feels a balance.


Excluding all the bla bla bla

what is the formula to calculate a wire size in the world of AC>?

daboss,
uglys is all a man needs unless you run into a situation beyond that, its a one stop shoping pocket reference.  I cant really work without it because it it isnt written, its not real.  I just want someone to explain to me how they arrive at wire sizes used in AC.

All you electricians

Reply #6 –
Couldn't tell you the formula.  I just grab the code book....
Long live the 4-eyes!  - '83 Tbird Turbo - '85 Marquis LTS - '86 LTD Wagon

All you electricians

Reply #7 –
Quote from: Chuck W;216308
Couldn't tell you the formula.  I just grab the code book....



ok,, so,, for the sake of conversation,

example:
switch gear breaker size needs to be 225A  / 3 pole 480/277
distance to sub panel is 140ft

you just look up the copper chart "in raceway" not free air, and find the amperage that matches the wire?

if so,, 225A copper wire "in raceway" would arrive at 4/0 based soley on the ampacity of RHW.  (ref- uglys page 67 , 2002 edition)

Im starting the think the one site drilling into the Qfactor is eactly correct and the number of phases has no real relevence.

BTW,, this very question has stumped many people over the past couple weeks at work as well.

One thing is for sure, DC is much easier as there is a set formula.

All you electricians

Reply #8 –
Quote from: jcassity;216312
ok,, so,, for the sake of conversation,

example:
switch gear breaker size needs to be 225A  / 3 pole 480/277
distance to sub panel is 140ft

you just look up the copper chart "in raceway" not free air, and find the amperage that matches the wire?

if so,, 225A copper wire "in raceway" would arrive at 4/0 based soley on the ampacity of RHW.  (ref- uglys page 67 , 2002 edition)

Im starting the think the one site drilling into the Qfactor is eactly correct and the number of phases has no real relevence.

BTW,, this very question has stumped many people over the past couple weeks at work as well.

One thing is for sure, DC is much easier as there is a set formula.



At least one guy in this therad seems to think there is no  "formula". His answer makes the most sense to me but I'm not a high voltage or Ac guy. If I run into any of the EEs here at work I'll ask though, just for the sake of curiousity.

All you electricians

Reply #9 –
Sounds like a job for a design engineer to me.

All you electricians

Reply #10 –
Now my head hurts and I have been doing this for almost 30 years!!!

All you electricians

Reply #11 –
Quote from: softtouch;216371
Sounds like a job for a design engineer to me.


there lies the problem.  Thats my job and discussing this topic with many minds, there is no real answer.  There are plenty of formulas out there but usually based on a known wire size.

Ive been revisiting this question off and on now for a few years.  Its kinda like one of my own punch list items for personal training.

I've never been bashful to ask questions even though most of the time the question is hard.

All you electricians

Reply #12 –
im going to call the sources for the latest NEC and ask.

If it turns out the formula is the same for DC "including LL", there's a lot of wiring out there grossly derated based soley on the fluxuating market rate of copper.

All you electricians

Reply #13 –
well, i spoke to one of the authors of the NEC 2002 edition who works for allegheney power.

In his words which I will simply try to quote:

"there really is not any one forumla located in the NEC for cable size calculation for an AC circuit.  DC circuits are actually a little different when it comes to cable size and there is a formula for that.  In the AC world, all the work has been done for you.  All you do is locate the chart for "cable in raceway" or "cable in free air" ,, go down the column of 70degC and stop at your amperage or breaker size.  Now, scoot to the left and locate the proper wire for that breaker"

My response:  "well,, thats no fun!!  That makes it too easy"

his reply : "yeah, there was a formula for calculating wire size for AC but being as many people are not really good at complicated math, it was determined long ago that the information needs to be more simplified."

so,, there you have it,, too easy!!

All you electricians

Reply #14 –
Just install a small enough gage wire till it sags under the load... Increase the gage a couple sizes, done...:D