Sub Panel wiring question

[MapleLeafFarmer]

my neighbour installed a 100 amp sub panel in his new workshop.
Lowes recommended 2-2-4 alum URD service entrance cable.
He is now worrying about the length of run and wondering if he should have gone to a larger size. He already bought the wire and waiting for me to dig the trench.

So it is a 100 amp sub panel, wired with 2-2-4 alum wire (90c) at 125 foot run of wire buried 36" deep.

I said I think size its fine but I be no expert.

opinions?

 

[85Hokie]

Unless he is running 3 welders at a time, he will be fine.

The voltage drop - the "thing" that we judge via loss of voltage per foot is approx 6% AT 100 AMPS!!!!!

SO IF he got near that 100 amp limit - hopefully the main would trip.

At the 80% rule - he is at 5% voltage drop.

I hope he did NOT bond the neutral and ground at the subpanel AND he ran a new ground rod (some places require two 6' rods at min. 6ft apart)

 

[MapleLeafFarmer]

I hope he did NOT bond the neutral and ground at the subpanel AND he ran a new ground rod (some places require two 6' rods at min. 6ft apart)

thanks! I will mention this advice to him but since I havn't yet dug the trench (ground still frozen here) if he knows what this means he probably still has time.

 

[North Idaho Wolfman]

Unless he is running 3 welders at a time, he will be fine.

The voltage drop - the "thing" that we judge via loss of voltage per foot is approx 6% AT 100 AMPS!!!!!

SO IF he got near that 100 amp limit - hopefully the main would trip.

At the 80% rule - he is at 5% voltage drop.

I hope he did NOT bond the neutral and ground at the subpanel AND he ran a new ground rod (some places require two 6' rods at min. 6ft apart)

Yea 85Hokie said it really well!

Depending on your location, Some locations require 2,2,2,4 to be used for a subpanel, Hot, Hot, Neutral and Ground.
Some locations you can do the 2 ground rods at 6' apart to make up for it, but it might not meet code.

Code here required, I have 3 panels, 6 Ground rods a UFER ground too (bonded pad) for my house.

 

[Yooper]

Rewired my shop in 2013 and code was everything back to the main panel. Good information 85Hokie!

 

[skeets]

Naww he aint got nothing to worry about the line loss is minimal I dont have my NEC right handy, but I doubt there is anyway he could max it out with out popping breakers first

 

[skeets]

Voltage Drop
Maximum Circuit Distance
3.00%
613.50 ft

A maximum distance of 613.50 feet will limit the voltage drop to 3% or less with a 2 AWG aluminum conductor delivering 20 amps on a 240 volt single phase system.

Download PDF

Engineering Information
75
Amps Rated ampacity of selected conductor
0.3042
Ohms Resistance (Ohms per 1000 feet)
0.0450
Ohms Reactance (Ohms per 1000 feet)
0.9
Power Factor
7.20
Maximum allowable voltage drop at 3%

 

[skeets]

Voltage Drop
Minimum Conductor Size
1.95%
1/0 AWG

1 conductor per phase utilizing a 1/0 AWG aluminum conductor installed Cable, Conduit (non-Steel), & Direct Burial will limit the voltage drop to 1.95% or less when supplying 90 amps for 125 feet on a 240 volt single phase system.

Download PDF

Engineering Information
120
Amps Rated ampacity of selected conductor
0.2100
Ohms Resistance (Ohms per 1000 feet)
0.0440
Ohms Reactance (Ohms per 1000 feet)
0.9
Power Factor
7.20
Maximum allowable voltage drop at 3%
4.684
Actual voltage drop loss at 1.95% for the circuit

So yeah he is good to go

 

[GreensvilleJay]

do both of yourselves a favour... run the cable IN 4" conduit ! Adss a little cost up front but allows for other wires to be pulled in, later and adds a layer for detection/protection in the future.

 

[SRRGC1]

do both of yourselves a favour... run the cable IN 4" conduit ! Adss a little cost up front but allows for other wires to be pulled in, later and adds a layer for detection/protection in the future.

I am always in favor of big is better on the conduit size as it makes for pulling cable easier. Why would you want additional wire/s later to the subpanel through the same conduit? I do not believe it is allowed to have any additional LOW VOLTAGE TYPE conducts or cable with 240 stuff. Should a person anticipate wanting low voltage or cable now or in the future, just bury an additional conduit along side the subpanel conduit. My suggestion.

 

[85Hokie]

do both of yourselves a favour... run the cable IN 4" conduit ! Adss a little cost up front but allows for other wires to be pulled in, later and adds a layer for detection/protection in the future.

Amen brother ................ Amen.

I shoved some 6g wire down a 1.5" plastic fiber conduit one time........ the amount of "DAWN" I used got my ass chewed by my wife! AND it was only 75 feet!!!!

 

[85Hokie]

I am always in favor of big is better on the conduit size as it makes for pulling cable easier. Why would you want additional wire/s later to the subpanel through the same conduit? I do not believe it is allowed to have any additional LOW VOLTAGE TYPE conducts or cable with 240 stuff. Should a person anticipate wanting low voltage or cable now or in the future, just bury an additional conduit along side the subpanel conduit. My suggestion.

Excellent point ........

run a big 4" and a separate 2" for ethernet or cable - always nice to have ONE open ditch and have everything buried when the fun starts.

 

[GreensvilleJay]

re: Why would you want additional wire/s later to the subpanel through the same conduit?

cause it's easy and simple to pull several cables at the same time..... if you have polypipe ,it works fine as 'conduit'. Actually preferred over 10' pcs of PVC.

while codes vary from place to place, I tyrapped TTY cables to 440/3ph for over 1500', zero issues.

 

[DustyRusty]

do both of yourselves a favour... run the cable IN 4" conduit ! Adss a little cost up front but allows for other wires to be pulled in, later and adds a layer for detection/protection in the future.

You have to use a very specific wire when you run it in conduit. Always use the proper wire and you will not have problems. It is also against the code to run any other wire inside of the same conduit as an electrical wire. If you don't know the code and fully understand it, then it is a bad practice to be giving any advice about electrical installation. The sub-panel must have the grounds and neutrals separated and both ground and neutral bars must be isolated from the breaker box. The grounds should be hooked to an approved grounding rod and if there is a water well casing nearby, it should be a continuous uninterrupted bare grounding wire from the bus bar to the ground rod and then to the water well casing. If I remember correctly the code requires 2 grounding rods or one of the other approved grounding methods as outlined in the NEC (National Electrical Code). All of this is done to protect the users from electrical shocks.

 

[North Idaho Wolfman]

While I'll agree with putting wire in pipe is a good idea, there is some downfalls.
4" is a lot excessive and extremely expensive ~$1460 USD to do a 125' run.
2" would be fine to run that wire size and length.
5 Number 2 wires can be run in a 1 1/4 PVC pipe.
Note: Putting wire in pipe lowers it's ampacity over wire direct buried in the ground.

 

[Henro]

Yea 85Hokie said it really well!

Depending on your location, Some locations require 2,2,2,4 to be used for a subpanel, Hot, Hot, Neutral and Ground.
Some locations you can do the 2 ground rods at 6' apart to make up for it, but it might not meet code.

Code here required, I have 3 panels, 6 Ground rods a UFER ground too (bonded pad) for my house.

I should remember why, but I don't. So I will ask.

IF one were to run 4,4,4,4 wire to the sub panel (or whatever size was needed, but all the same), why should ground rods at the sub panel be needed/required? 4 individual conductors, not a built up wire that is not likely available in that configuration.

The safety ground would carry any ground current back to the ground rods connected to the incoming service entrance panel. And would have the same capacity as the power carrying conductors.

I am pretty sure I was not required to use a ground rod(s) when installing a sub panel in my home, in the basement at the opposite side of the house. So if this is correct, why would a sub panel in the shed not far from the home require ground rods, if all four conductors connected to it are the same size?

I can see a possible reason if the forth wire is smaller in size...But in the case of my question, all four conductors are the same gauge.

 

[ctfjr]

As other posters have suggested, run a separate pipe in that trench. I would use PVC as the only thing I would ever run would be low voltage.
PLUS leave a pull rope in it

 

[North Idaho Wolfman]

I would have to go through the code book to find it but it has to do with separate buildings sharing a power feed.

I should remember why, but I don't. So I will ask.

IF one were to run 4,4,4,4 wire to the sub panel (or whatever size was needed, but all the same), why should ground rods at the sub panel be needed/required? 4 individual conductors, not a built up wire that is not likely available in that configuration.

The safety ground would carry any ground current back to the ground rods connected to the incoming service entrance panel. And would have the same capacity as the power carrying conductors.

I am pretty sure I was not required to use a ground rod(s) when installing a sub panel in my home, in the basement at the opposite side of the house. So if this is correct, why would a sub panel in the shed not far from the home require ground rods, if all four conductors connected to it are the same size?

I can see a possible reason if the forth wire is smaller in size...But in the case of my question, all four conductors are the same gauge.

It's in the NEC code, 240-23, because the service feed it to a separate building.
Why this is so important.
If you loose a hot or neutral in the cross over it's measurable and obvious, where loosing a ground is not.
So if the underground crossover ground is compromised ( like high resistance) or lost (broken) the ground on that building is not lost.

 

[85Hokie]

I should remember why, but I don't. So I will ask.

IF one were to run 4,4,4,4 wire to the sub panel (or whatever size was needed, but all the same), why should ground rods at the sub panel be needed/required? 4 individual conductors, not a built up wire that is not likely available in that configuration.

The safety ground would carry any ground current back to the ground rods connected to the incoming service entrance panel. And would have the same capacity as the power carrying conductors.

I am pretty sure I was not required to use a ground rod(s) when installing a sub panel in my home, in the basement at the opposite side of the house. So if this is correct, why would a sub panel in the shed not far from the home require ground rods, if all four conductors connected to it are the same size?

I can see a possible reason if the forth wire is smaller in size...But in the case of my question, all four conductors are the same gauge.

Earth "grounds" are different ......even though that sounds CRAZY as hell. In a house, there is typically a "ground" and the meter base or a ground via an underground "pipe" or both.

A ground at the house is not the same as a ground 125' away - I know that sounds weird..... the point really is this - electricity is a lot like water, spill water- it will FIND the lowest point on the floor/ground ....
Electricity will FLOW to the "easiest" way back to ground.......

for that reason you want to have a separate ground at a "new" sub panel.

Here is more on the subject manner:

copy and pasted



Rule #1: The grounded conductors (neutrals) and the equipment grounds must always be isolated after the main service panel. There are two essential reasons for this rule:

To provide a low-resistance path for current travel to the transformer in the event of a ground fault. This path should not be energized to ensure that it is fully available for the fault current in the event of a malfunction.

To prevent the ground wires from conducting returning neutral current during normal operation. Neutral current in the equipment ground wires can energize the casings and enclosures of appliances and equipment.

Rule #2: In a subpanel, the terminal bar for the grounded conductors (commonly known as the neutral bus) should always be insulated from the enclosure. The reason for this rule is to prevent the enclosure from conducting current.

 

[Henro]

I would have to go through the code book to find it but it has to do with separate buildings sharing a power feed.



It's in the NEC code, 240-23, because the service feed it to a separate building.
Why this is so important.
If you loose a hot or neutral in the cross over it's measurable and obvious, where loosing a ground is not.
So if the underground crossover ground is compromised ( like high resistance) or lost (broken) the ground on that building is not lost.

View attachment 76553

I will need to digest this. BUT is it a probability thing?

I mean, what are the chances that a ground wire will be compromised when it runs in the same conduit as the power conductors? As compared to the probability that the same thing could happen within a structure? AND given that the ground wire (forth conductor). is the same size as the others.

Again, I am referring to a situation where all four conductors are the same size, so a parallel safety path through the earth between ground rods should not be required.

I am not disputing anything, just trying to see the logic.

And yes, I do have a bit of electrical background, just NOT residential stuff.