Wiring Advice for Feeder

[BamaRenegade]

I'm planning on wiring my workshop to the main panel in my home. They are separated by 275 feet (so 550 feet total circuit distance). I'd like a 100A feeder.

Calculations from http://www.csgnetwork.com/wiresizecalc.html indicate I'll need at least 350 mcm for AL. I've seen guidance on this site recommending a smaller DIA wire for similar installations. Why the discrepancy? Am I missing something? 350 mcm would push me to XHHW while I'd like to use Mobile Home Feeder if I could get by with 4/0.

If I go with 350 mcm XHHW I'd like to purchase one long spool of black wire and use this wire for the 3-conductors (maybe ground too). Can this be done according to code as long as the ends exiting the conduit are color coded appropriately?

 

[ishiboo]

Usually these things are sized with a voltage drop calculator, yours is more of a fixed DC load calculator.

4/0 is fine for 100A at 275', you would be under 3% voltage drop.

You normally don't have a lot of color options in those sizes, they are usually all black when you buy individual conductors... they can definitely be reidentified at the ends. You can also parallel 2 conductors once you reach 1/0 IIRC.

 

[rockwithjason]

what he said

 

[BamaRenegade]

Appreciate the insight. I think I discovered why my calculations were off after reviewing this thread: http://www.garagejournal.com/forum/showthread.php?t=170378&highlight=voltage+drop

Seems these calculations are done with a 240V source so that 3% of 240V is 7.2V versus 3.6V for 120V. Theoretically.. on a 100A feeder circuit couldn't all the current be pulled from a single leg so that the 3% VD requirement applies to 120V?

 

[BamaRenegade]

bump. Seems my follow-up question (above) took a few days before it was visible.

 

[offroadsteve]

Theoretically its possible that the entire 100A could be drawn at 120V, but in practice its highly unlikely. If the panel is balanced properly, the loads should be shared between L1 and L2, and if the two legs are balanced perfectly, effectivly you have is 2, 120V loads in series, summing to 240V. This makes the Vd calcs for 240V valid.

Also remember, that calcuated voltage drop is assuming you are using 100A of current, which is a lot! If you are only using the normal stuff (lights, moderate tools, etc), you are going to be more like 30-40 amps tops, so the voltage drop is significantly lower at your normal useage levels.

 

[BamaRenegade]

Will 2.5" conduit work for the 4/0-4/0-4/0-2/0 MHF? According to the spec sheet it has an outer diameter of 1.553".

 

[pattenp]

2.5" conduit will do just fine for 4/0.

 

[toler234]

4/0 Al
1/0 CU

 

[wyliesdiesels]

U say the buildings are 275' apart but then double that for your cable length? Why double? What will your total cable length be?

Am I missing something because the sizes I got are:

@ 275': 1/0 cu. or 3/0 al

@ 550': 4/0 cu. or 350 kcmil

 

[pattenp]

U say the buildings are 275' apart but then double that for your cable length? Why double? What will your total cable length be?

Am I missing something because the sizes I got are:

@ 275': 1/0 cu. or 3/0 al

@ 550': 4/0 cu. or 350 kcmil

Your sizes are right. The calc length is 275', service to load. The only reason I stated the 4/0 was because he mentioned MHF and I believe it doesn't come in 3/0.

 

[theoldwizard1]

U say the buildings are 275' apart but then double that for your cable length? Why double? What will your total cable length be?

This proper practice for DC current/loads.

 

[wyliesdiesels]

O I c. That makes sense. I've only done some minimal DC wiring in cars....

 

[BamaRenegade]

The reason for the number '2' in the voltage drop equation (see link in post #4) is to account for the voltage drop that will occur in both the hot leg and the neutral leg (120V circuit) or both hot legs on a 240V circuit. On a 275' run between two buildings your electrical circuit length is 550' when running a single leg of 120V or 240V; however, the voltage drop equation takes this in to consideration.

120V ------ resistance of 275ft -----><100 Amp Load>
Neutral ----- resistance of 275ft ----<

If you run two separate 120V circuits pulling 100A and their phase difference is perfect at 180 degrees then the return currents on the Neutral leg should cancel each other out resulting in zero VD on the Neutral leg. This is one reason why it's important to balance 120V loads across the two hot legs.

Hope this helps.

 

[BamaRenegade]

I’m working through the details for this feeder and could use some guidance. I’ve added the graphic below to help illustrate my plans.
My house has two 200A panels located on the left exterior wall where the red line starts. It is a two story house with crawlspace. My plan is to come up, over, and down with Southwire Alumaflex SER to get under the crawlspace. Fortunately the area above the first story is unfinished attic space. Somewhere within the crawlspace I plan to transition to MHF inside 2.5” conduit in order to make the underground run to my workshop. How and where should this transition be made?
I plan to install a 100A breaker in one of the 200A panels. What’s the best way to transition from the 4/0-4/0-4/0-2/0 SER to the 100A breaker which I assume will accept 2/0 wire? I’ve read where some folks install a separate recessed junction box to splice the wire. Is this box typically installed above the existing panel to avoid drilling through the vertical studs? Suppose I could come out with 2/0 wire and transition to 4/0 inside the attic since the short run of 2/0 shouldn’t effect voltage drop. What size ground wire should be used for the smaller cable? Probably more to follow as feedback is received…. Thanks in advance.

View media item 27070

 

[BamaRenegade]

Bump. Post delayed due to moderator review.

 

[wyliesdiesels]

The reason for the number '2' in the voltage drop equation (see link in post #4) is to account for the voltage drop that will occur in both the hot leg and the neutral leg (120V circuit) or both hot legs on a 240V circuit. On a 275' run between two buildings your electrical circuit length is 550' when running a single leg of 120V or 240V; however, the voltage drop equation takes this in to consideration.

120V ------ resistance of 275ft -----><100 Amp Load>
Neutral ----- resistance of 275ft ----<

If you run two separate 120V circuits pulling 100A and their phase difference is perfect at 180 degrees then the return currents on the Neutral leg should cancel each other out resulting in zero VD on the Neutral leg. This is one reason why it's important to balance 120V loads across the two hot legs.

Hope this helps.

I fully understand VD in A/C electrical. It was the doubling of distance for D/C electrical that I didn't know about!

 

[Aceman]

Somewhere within the crawlspace I plan to transition to MHF inside 2.5” conduit in order to make the underground run to my workshop. How and where should this transition be made?

I would do it in the crawlspace, with the box mounted to the floor joists. I would use an 18x18x4" or 6" deep box.

I plan to install a 100A breaker in one of the 200A panels.

Since your wire size is 4/0 AL(180 amp rated) I would install a subfeed lug kit in your 200A panel and run the 4/0 right to it. Then I'd install a 200 amp MB panel in the garage.

What’s the best way to transition from the 4/0-4/0-4/0-2/0 SER to the 100A breaker which I assume will accept 2/0 wire? I’ve read where some folks install a separate recessed junction box to splice the wire. Is this box typically installed above the existing panel to avoid drilling through the vertical studs?

See above.

Suppose I could come out with 2/0 wire and transition to 4/0 inside the attic since the short run of 2/0 shouldn’t effect voltage drop. What size ground wire should be used for the smaller cable? Probably more to follow as feedback is received….

See above.

You will spend far more time and money trying to transition from 4/0 to 2/0 with a box, connectors, and insulated taps then using a simple subfeed lug kit.

 

[BamaRenegade]

I would do it in the crawlspace, with the box mounted to the floor joists. I would use an 18x18x4" or 6" deep box.



Since your wire size is 4/0 AL(180 amp rated) I would install a subfeed lug kit in your 200A panel and run the 4/0 right to it. Then I'd install a 200 amp MB panel in the garage.


You will spend far more time and money trying to transition from 4/0 to 2/0 with a box, connectors, and insulated taps then using a simple subfeed lug kit.

Appreciate the feedback. Definitely see the simplicity of this approach. Wouldn't there be a safety issue with loads between 180 and 200 amps? Is this a code approved method?

Should the junction box between SER and MHF be grounded and if so how is this usually done when the ground conductors are large?

 

[Aceman]

Appreciate the feedback. Definitely see the simplicity of this approach. Wouldn't there be a safety issue with loads between 180 and 200 amps? Is this a code approved method?

As long as the load is less than the wire ampacity you are allowed to round up to the next breaker size when the ampacity doesn't correspond to a standard breaker size.

Should the junction box between SER and MHF be grounded and if so how is this usually done when the ground conductors are large?

I would use a lay-in lug.

 

[pattenp]

Appreciate the feedback. Definitely see the simplicity of this approach. Wouldn't there be a safety issue with loads between 180 and 200 amps? Is this a code approved method?

Should the junction box between SER and MHF be grounded and if so how is this usually done when the ground conductors are large?

Are you using metal conduit and Jbox? I'd use PVC conduit with PVC Jbox and not worry with grounding.

 

[BamaRenegade]

As long as the load is less than the wire ampacity you are allowed to round up to the next breaker size when the ampacity doesn't correspond to a standard breaker size.

Does this imply that the MB at the load should be 100A in my case?

Are you using metal conduit and Jbox? I'd use PVC conduit with PVC Jbox and not worry with grounding.

Haven't bought anything yet... I'll stay all PVC.

 

[Aceman]

I would use a 200A MB panel.

 

[BamaRenegade]

I've started to work the details and I've come up with another option to run by you all that I think is cheaper.

Background:
Both 200A panels in my home are EATON 3/4". A 100A breaker will accept up to 1/0 wire. Their subfeed lug will accept 3/0 wire. My panels are located on an outside wall and I'd like to minimize the hole size I drill in the top plate.

Approach:
Run 50' of 1/0-1/0-1/0-2 AL SER from 100A breaker in my home's panel to a junction box located in my crawlspace. Splice with 4/0-4/0-4/0-2/0 MHF to run the remaining length in conduit to my workshop. Do you all see any problems with this option? The max current of the 1/0 SER is 100A at 60deg so I assume it is ok to run this cable at 100A in my application.

Thanks again.

 

[pattenp]

If the 1/0 SER is not run within insulation it can be sized based on the 75C column which is 120A. Either way you're okay on the amps.

 

[BamaRenegade]

Sorry to bring an old thread back to the front.... finally getting around to finishing the feeder. Is there any reason I can't use 4/0-4/0-2/0-4 MHF instead of 4/0-4/0-4/0-2/0? Both are rated for 200A while I'm only going to pull 100A. Thanks.

 

[pattenp]

Using 4/0 MHF with the reduced neutral and ground is okay. Actually the 4/0 when used as a sub feed it's max over current rating is 180 amps.