View Full Version : Subpanel feed wire size

Unregistered

February 17th, 2004, 11:34 AM

I'm planning to add a 100A subpanel in an unattached garage. The location of the subpanel is 150 feet away from the main panel. I plan to feed the subpanel with a 100A breaker installed in the main load center.

Should I calculate the wire size based on 120v or 240v? When I run the calculations at 120v, the wire size calculated to 2/0 copper which won't even fit into the breaker!

Thanks!

marako

February 17th, 2004, 01:07 PM

What did you caculate? How did you get 2/0?

For a 100amp sub you'll need #3 or #4 copper wire (depending on how you read and interpert the code). Most here will say #4. Since your at 150 feet out you'll need to go up a size. It seems to me a rule of thumb is to go up to the next size every 100' of wire length (50' for 120 circuits). So for your application I would go with #3 cooper min. If you think you might actually load the circuit down to 100amps, then might as well go with #2 cooper. It will only cost a few $$$ more and give you better voltage drop.

PS I'm supprised that 2/0 wire won't fit into the breaker. Most breakers take sizes much larger than you would think... like a 15 amp breaker that will hol #8 wire.

ON EDIT: Here is another electrical site:

http://www.electrician.com

They aren't nearly as good as this site, but on the front page they have a "calculators" pull down which has a lot of nice calculators. Use the "voltage drop" calculator near the bottom of the list.

Homer

February 17th, 2004, 01:19 PM

I'm planning to add a 100A subpanel in an unattached garage. The location of the subpanel is 150 feet away from the main panel. I plan to feed the subpanel with a 100A breaker installed in the main load center.

Should I calculate the wire size based on 120v or 240v? When I run the calculations at 120v, the wire size calculated to 2/0 copper which won't even fit into the breaker!

Thanks!

I think that 2/0 copper is a little overkill. You may want to take a look at these two threads for some background on this topic.

http://www.selfhelpforums.com/showthread.php?t=318

http://www.selfhelpforums.com/showthread.php?t=732

As a starting point, look at NEC table 310.15(B)(6). Here you see that a minimum of #4 copper is required for 100A residential feeders. Also, you should add a green or bare #8 copper for the EGC (equipment grounding conductor), see Table 250.122 .

Since you are going 150 feet, you will want to upsize from the minimum to reduce your voltage drop problem. Probably up to #2 copper for the three conductors (2 Hot + Neutral) and keep the #8 copper for the EGC.

You will need to keep the neutral and EGC isolated at the subpanel since you're running the EGC and you must also drive a grounding rod at the detached garage. Use #8 copper for the Grounding Electrode Conductor (table 250.66) connecting the rod to the subpanel.

You could use aluminum conductors instead so just look at the tables above for the equivalent aluminum sizes.

Also, I'm assuming that you've done a demand load calculation based on your needs to confirm the subpanel and main service sizing.

Wait for others to comment before starting.

Homer

Unregistered

February 17th, 2004, 01:49 PM

Figuring 3% voltage drop at 120V = 3.6V

Circular mils = 2KID/Vd

= 2(12.9)(100)(150')/3.6

= 107500 circular mils

2/0 copper is 133100 circular mills

1/0 copper is 105600 circular mils

I'm not sure if that is the correct method though.

What did you caculate? How did you get 2/0?

For a 100amp sub you'll need #3 or #4 copper wire (depending on how you read and interpert the code). Most here will say #4. Since your at 150 feet out you'll need to go up a size. It seems to me a rule of thumb is to go up to the next size every 100' of wire length (50' for 120 circuits). So for your application I would go with #3 cooper min. If you think you might actually load the circuit down to 100amps, then might as well go with #2 cooper. It will only cost a few $$$ more and give you better voltage drop.

PS I'm supprised that 2/0 wire won't fit into the breaker. Most breakers take sizes much larger than you would think... like a 15 amp breaker that will hol #8 wire.

ON EDIT: Here is another electrical site:

http://www.electrician.com

They aren't nearly as good as this site, but on the front page they have a "calculators" pull down which has a lot of nice calculators. Use the "voltage drop" calculator near the bottom of the list.

Homer

February 17th, 2004, 02:35 PM

Figuring 3% voltage drop at 120V = 3.6V

Circular mils = 2KID/Vd

= 2(12.9)(100)(150')/3.6

= 107500 circular mils

2/0 copper is 133100 circular mills

1/0 copper is 105600 circular mils

I'm not sure if that is the correct method though.

I'm told that I do everything in my own way (like that Frank Sinatra song), so here's MY WAY of calculating the voltage drop.

For #4 copper from the NEC tables, it has a resistance of 0.308 Ohms per 1000Ft @75 deg.C.

For 150ft of wire the Rd = 2 x (150 / 1000) x 0.308 = 0.0924 Ohms

At a current of 100A, Vd = I x Rd = 100 x 0.0924 = 9.24V

Voltage drop of 9.24/240 = 3.85%

For #2 copper from the NEC tables, it has a resistance of 0.194 Ohms per 1000Ft @75 deg.C.

For 150ft of wire the Rd = 2 x (150 / 1000) x 0.194 = 0.0582 Ohms

At a current of 100A, Vd = I x Rd = 100 x 0.0582 = 5.82V

Voltage drop of 5.82/240 = 2.43%

So #2 copper would be OK for voltage drop. These numbers are confirmed with the www.electrician.com voltage drop calculator.

I think that your method for calculating the minimum Cmil area would work if you use 3% of 240V instead. You would get 53750 circ mils minimum. (#4 = 41740, #3 = 52620, and #2 = 66360)

You use 240V instead of 120V because the neutral conductor will only ever carry the unbalanced current.

Homer

Wgoodrich

February 17th, 2004, 05:36 PM

The problem with using auto caculators is lack of knowledge and improper data entry.

Example you calculated a 100 amp load. Chances are you won't have near that size amp load.

To calculate voltage drop of a garage you need to do a different type of demand load calculation to discover amp load of that garage.

Decide how many people will be working in that garage at any given time. If only one person working at any given time working then find the largest manually operated tool or machine you will use. Find the amp rating of that one manually operated machine and then increase that amp rating 25% more. Then list the amp rated loads of any auto start machines such as water heaters, a/c, air compressors, etc. and add the full load current to that 25% increased largest manual load. If you wish to add general lighting then add in 25 volt amps per square feet per 220.3. This will be the demand load of that garage telling you what the minimum service size required to serve that garage. Then no matter how big of a service you install start the voltage drop using the demand load calculated [this is your connected load for voltage drop].

Formula is 2 for the path there and back. times [resistance found in NEC Table 8 of Chapter 9 divided by 1000 [exact K per foot] or if copper wire you may use approximate K at 12.8 resistance or if aluminum 21.2 resistance per foot.] times answer of your demand load calc in amps times total feet between the two panels. Then divide that by the circular mill of the wire size you plan to use per Code. This circular mill of that wire size will be found in Table 8 of Chaper 9 of the NEC. This will be the voltage drop if you use the wire size the NEC allows. The answer to the above calculation will be in VOLTS DROPPED. Then to find voltage drop percentage divide VOLTS DROPPED by the apparent volatage served [240volts]. If the answer is less than 5% then you are within reason concerning voltage drop.

Most people install a 100 amp service when they only have a demand load of 20, 30, or 60 amps. To use an auto calculator and post the amp rating of your panel is going to give you a rediculous voltage drop answer requiring much bigger conductor than you really need. This is the bad thing about auto calculators where you can not see what that calculator is doing. The above if an accurate auto calculator is what that calculator is doing for you. Same thing applies to those auto calculators as a computor. They say computers do not make mistakes but then the people entering the data will make mistakes. That is what happened in your case. You entered a 100 amp load when demand load of your garage will most likely be 1/2 that.

Rule of thumb on 125 volts 125 feet before you should be concerned about voltage drop. Rule of thumb on 240 volts 250 feet before you should be concerned about voltage drop. Rule of thumb on 480 volts 600 feet before you should be concerned about voltage drop. This rule of thumb is arrived by an average voltage drop of all conductors listed in 310.16 using the amp load allowed by that chart.

Only difference between the above for single phase and calculating for 3 phase is the 2 is replaced by 1.732 being the square root of the three hot conductors.

Hope this helps

Wg

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