What cable size do I need to feed a 100 amp sub panel that is 550 ft


I'm not sure where you live but US code calls for a minimum of #8 if you're using copper or #6 if you're using aluminum. There would be no advantage in using a larger wire size in this case because at 30ft you will have negligible losses.

I don't know what your voltage is or how many phases you have but for an example of what your losses may be, I will assume a worst case scenario - 100V single phase. If you used the #6 aluminum wire, you would have a maximum of 1.72% voltage drop (loss). If you used the copper #8, you would have a maximum voltage drop of 1.66%. And just remember, when you are calculating losses (voltage drop), the higher the voltage, the better because higher voltages have less voltage drop.

Note: I use the Southwire Voltage Drop Calculator for most of my voltage drop calculations....

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Discussing code with an electrician/diy poster/inspector is like wrestling with a pig in mud, eventually you understand that they like it.

I wonder if within the definition of main power feeder when 'main service disconnect' is used that means the actual wire terminated in said disconnect, or if the intent is the panel in which the main service disconnect is contained. This is the root of the differences of opinion about this code article. If you infer 'panel' or main service disconnect 'enclosure' when the term main service disconnect is used, then any feeder in a residence would fall under 310-15(b)(6) guidelines. This is not the case. If the NEC intended this, they would have used the general term of 'service' or 'service equipment' instead of the specific term of 'main service disconnect'.

In the above example, the 100 amp breaker that will feed the sub panel is a feeder breaker, not a main service disconnect. You can determine this with a simple test. With a main service...

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We did just that here at the house. Actually, I hired an electrician to do it.

The house has 100 amps to the main panel. We put in a 100 amp sub panel right next to the main panel. The breakers in the main panel had to be re-arraranged to allow for a 100 amp breaker to feed the sub panel. The wiring between the panels is either 2 ga or 4 ga, I forget which.

In the sub panel, all breakers feed receptacles that are right off the panel, in conduit. The j-boxes come off 3 sides of the panel. There is a 240v 50 amp breaker feeding a NEMA 6-50R for welders, a 240v 30 amp breaker to a 6-30R for other welders, two 240v 20 amp breakers to two 6-20R's for the compressor and plasma cutter, and two 120v 20 amp breakers feeding two 4-gang boxes of two each 5-20R GFCI's for saws, grinders, drill press, other power tools, etc. The shop lights run off the main panel's 120v outlets that were already in the garage. We dont run all eight 120v outlets from the sub panel at the same time. ...

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Before we get into wire sizes a few questions will be helpful.

1.) Are you sure about 100 amps ? This is huge for a garage unless the garage is a shop. Typical would be 60 amps or less.
2.) Along the lines with question 1 ...what electrical needs will this garage require? Will there be power equipment like table saws , welder , air compressor ....etc..?
3.) Check with your friend and get an accurate list of what he is going to be operating in the garage now and in the future so we can get a better idea for the sub-panel wire size.
4.) Are you wanting a direct burial wire or or you going to use conduit like pvc with individual wires inside it?

To answer your question about the ground wire....YES you need a ground ... you will need to run 4 wires (H-H-N-Grd). The correct drawing is below ..

Last edited by Stubbie; 01-24-2010 at 11:10...
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Ok - you have two ways you can do this.

Option1) you can install conduit from the house panel to the garage panel. The size you want to use for that distance would be 2". (trust me the pull will be much easier) Then you will run #4 thhn copper for 3 of the conductors (2 hots 1 neutral) then you will use a #6 for the grounding wire. You will bury this 2' into the ground. Then at the garage panel you will need to install a grounding bar kit to isolate your grounding bar from the neutral bar. Next you will install 2 grounding rods spaced at least 6' apart and bond them to the grounding bar in the panel using a solid #6 grounding wire.

Option 2) you can purchase #2 direct bury Quadraplex. This is an aluminum direct bury service cable and again - you will bury it 2' deep. Now the advantage of doing it this way is that the wire is cheaper and comes as a complete cable. You will need to use conduit on the end runs where the cable exits the ground and goes into the...

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Also a change in the 2008 NEC has really clarified that to be able to use this table for a feeder the feeder must carry the entire load of the dwelling unit. Here is the new revisions to 310.15(B)(6)

Quote6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units of one-family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service-lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by branch circuits or by feeders, or both, all loads that are part or associated with the dwelling unit. The feeder conductors to a dwelling unit shall not be required...

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They are generally used as sub-panels instead of main service panels, because 100-amp service ... through a conduit. 100-amps ... Size Is...

An underground electrical service looks ... The first thing you need to think about is the size of ... Use 1 1/4...

Find out why the breakers on a 100 amp service panel may add up to more than 100 amps with help from...

Wires are rated for the voltages as well as the wattage they can be used with. A wire with a larger diameter...

100-amp circuit breaker boxes are the minimum size allowed by code. They are generally used as sub-panels instead of main service panels...

Determining the electrical service size for a home is important to understanding how much electricity can be used at ... What Wire...

Mike Holt, National Electric Code (NEC) consultant to Electrical Construction & Maintenance (EC&M) Journal, says, "Rigid PVC conduit...provides many of the advantages...

Size 2/0 THHN...

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Author Information Topic: Wire size for 100 amp subpanel Member

Name: L West
Email: leon_west@porex.com
Location: Georgia
Title: Not_Listed
In Trade Since: 0
Registered: May 2001
Total Posts: 5

posted May 07, 2001 at 11:50 AM I'm installing a 100 amp sub-panel in my basement. The sub panel will be app. 15 ft. from the main panel. Should I use #4 or #3 copper?



Name: Steve Mancuso
Email: electric@vermontel.net
Location: Vermont
Title: Contractor
In Trade Since: 1985
Registered: Oct 2000
Total Posts: 1676

posted May 07, 2001 at 02:34 PM #4 cu or #2 al ,per 310-15(b)(6), if it's a 'dwelling'



Name: L West
Email: leon_west@porex.com
Location: Georgia
Title: Not_Listed
In Trade Since: 0
Registered: May 2001
Total Posts: 5

posted May 07, 2001 at 03:51 PM ...
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In this how to install a sub panel installation I will guide you through the complete installation of a subpanel step by step.

The first question is why are you installing a sub panel. There are either one or two reasons to do this. One because you need more room in your existing panel and in that case if your existing panel is old this would be a great time to just upgrade your current service. Newer 200 amp main panels hold a lot more circuits / breakers.

Two, because you need to bring more power somewhere else like a garage you just built or an addition or even in some cases a finished basement. Either way, you want to install a sub panel. Ok. So let’s get going.

First: Where do you plan on placing the new sub panel? If it’s right next to you main panel that’s one thing, but further away like in that new garage you just built 100 feet away you will certainly want to figure in voltage drop. I can’t get too much into that because ever setup would be...

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Phew, a lot of stuff happening here. Starting with a footnote to this statement:

You cannot assume that you will never use more than 60 amps of power.

You are, of course, making no such assumption. If you or anyone else uses more than 60 amps of power, the 60A breaker in the main panel will trip. And if someone in the future is stupid enough to swap-in a larger breaker without evaluating the wiring, the same can be said for every circuit in your house: There is no known protection against an idiot putting in a larger breaker where it doesn't belong.

Also, it is not a good idea to put these multiple 240v receptacles on one circuit.. What is the reasoning? That your not gonna use two at once??

Yes, that's exactly the reasoning. The principle is called diversity, and it is the reason that the sum of the circuit breakers in a panel can add up to more than the ampacity of the service. It's also the reason why you can put a dozen ordinary 120V receptacles on a...

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100amp circuit - what wire?

I need to run a 100amp 220v circuit from my panel to the garage. About 25 feet.

What size wire do I need for the hots? #3 or #4? What size for the ground?

Can I get 3/2 wire somewhere (no common needed), like I got 10/3 for a dryer circuit?

I've never seen two-conductor w/ground NM (romex) larger than #6 but that's not to say it doesn't exist. For this purpose, you'll probably end up with 3-3-3, 2-2-2 or 2-2-4 type SEU. Around here, 2-2-2 and 2-2-4 are much more common than 3-3-3.

»www.southwire.com/Produc ··· heetOEM8

to jaa
said by jaa:

I need to run a 100amp 220v circuit from my panel to the garage. About 25 feet.

For that you should use at least number two copper, personally I would go with number one.

to Zach
said by Zach:

... 2-2-4 type SEU.

Would that mean two #2AWG hot lines and a #4AWG protective ground (I think it was called machine ground in...

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They sell a quad number 2 2 2 4 at most box stores, either direct burial or piped but its ideal for these garage feeds.


I got to agree with this, over last 30 yrs I must have done a couple dozen of these, have some feeds to my own buildings with number 2 alum, never, never had a cal back because of tripping a 60 including a bud who has a small garage he makes a living out of. I use a 60A breaker cause its cheap, fits the wires. Only reason a guy would need more,,, was a specialized shop providing heavy air demands and simultaneous 50A welding service, and now days with small welding equipment even less of an issue.

As the man said,,, all this V drop stuff is about moot, only an issue when at full load, a number 2 alum wire peaks at half load max in a residential garage for intermittent times, voltage is good most places, so what if there is 5 drop for a split second when the air comp comes on, ain't worth worrying about.

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