In the next several years, I plan to build a residence approximately 600' from the main service panel of an existing residence. The main service panel is outside (i.e., there is a subpanel inside the existing residence).

There is already 3" undergound conduit installed from the location of the existing main panel to the future home site, with a pull box at the midpoint of the 600' run (i.e., at 300' from the main panel). Also, there are three #1/0 conductors already installed from the location of manel panel to the pull box.

Is there a cost effective way to use the existing conduit, conductors and utility connection point to supply the service needs (e.g., 125-150 amps) of the new residence? For example, what if two #1/0 conductors are installed in the remaining 300' from the pull box to the new home site? Could a second main panel be installed at the locaton of the existing main panel and then a subpanel installed at the new residence? If the utility will supply 480V to the new main panel, won't the #1/0 conductors be sufficient to supply the service needs of the new residence (e.g, once the voltage is stepped down to 120V/240V at the new subpanel)?

At some point, I will definitely have the utility company come out and consult with me, but they are reluctant to do that until I get closer to applying for a building permit. In the meantime, I want to get a rough idea of whether it will be feasible to place the main panel for the new residence in the same location as the main panel for the existing residence, and use the existing conduit and conductors to eventually supply the power requirements of the new residence.

Thanks.

 

At some point is this going to have two different addresses?

 

Running 100A at 480V (200A at 240V) will give you 5.1% voltage drop. Seems good to me.

 

480 volts from the POCO would be ideal but I've never heard of a power company supplying 480 vac to a residence...... and if they did..... it would be the homeowners responsibility to buy a transformer to reduce it.

 

Is the dwelling the only load you have planned. I have done some ranches where we got service at 240V (could not get 480V) then stepped up to 5KV and back down to 120/240 at the dwelling. Then they want a run to a shop and to a barn etc. so having 5KV works well with respect to voltage drop. You will not want much VD getting to your dwelling as it will take away from whats available for the building wiring. I like the oil filled pad mounts (like the utility uses) in open country because of the weather and safety issues. You will have taps available on transformers that can help with some VD compensation.

 

The calculator I'm using says that you can get 100A @ 480V over 600' with #1/0 copper conductors in conduit and a voltage drop of 3.25% or less. Starting with 240V, you can get 15A to the new house with a voltage drop of 9.73% or less.

Southwire Voltage Drop Calculator

 

If it was me, I would consult the power company first and see if they will provide a separate service to the new residence and not even bother with the other service. If the will not, then start looking into transformers and the like.

 

It can't be that bad,

At 480 volts a voltage drop of 3.25% or less means a drop of 15-1/2 volts or less. At 240 volts (or any other voltage) you have the same worst case 15-1/2 volt drop, all other things, like 100 amps through that section of wire, being equal.

So you would get a full 100 amps at the destination at 240 volts and no transformers if you can put up with a worst case 6-1/2% drop. (I came up with 2-1/2% voltage drop at 480 volts and 5% at 240 volts given 0.12 ohms for 1200' round trip.) If you can put the panel in the middle of the new house rather than at one end, and not use any 14 gauge wiring, you can reduce any additional voltage drop within the house wiring.

Now you would get greater voltage drops at 120 volts if the 120 volt loads on the A leg and B leg are somewhat unbalanced and/or you downsized the neutral the rest of the way which you may otherwise do with feed wires of that size.

I am guessing that it would take five poles as well as a utility company transformer (and perhaps 8 or so gauge minimum snow load overhead ground and single phase primary hot) for a new service. You would probably have to pay to have the utility install that particularly if it is going straight into your private property.

I haven't crunched the numbers that finely (ergo you'll need to) but I am guessing that stepping up to 480 volts at the existing house and stepping back down to 120/240 at the new house is viable, especially since you have half of the 1/O cable run already strung. Now, depending on how things are configured at the existing house service entrance, you may need a fourth wire as an equipment grounding conductor (need not be 1/O in size) in the conduit between the houses if you did not use transformers.

Double check the voltage rating of the existing buried wires. Most likely they're 600 volt rated but they could be only 250 volt rated.

 

Joe, the new residence will probably have a different address from the existing residence, but both will be located on the same parcel (for property tax purposes).

PJ, I am waiting for a call back from the power company about whether they can supply 480V for new residential service in my area.

bahtah, your question is on target because the new residence is actually going to be part of a structure that will also be home to a lot of equipment (i.e., used to service a 15 acre vineyard). So we're talking about a combination equipment barn/shop (e.g., for tractors, implements, etc.) and residence (i.e., for caretaker/vineyard manager). I was hoping that it would be sufficient if I could deliver 150 amps to the site within acceptable VD limits.

Alan, I will be finding out soon what the power company estimates they'll charge me for ample service right on site. It's going to have to be undergrond service, because I have already invested to eliminate the overhead service drop at the existing residence. I do not want to reintroduce over head power lines at this juncture. Also, thanks for giving thought to how I might utilize the existing #1/0 cables, which are 600V rated. If the power company will cooperate, my common sense tells me that there ought to be a cost effective way to keep the main panel for the new residence near the existing main panel, and use a #1/0 circuit to supply a fully functional subpanel at the new site. Sounds like you think I may be on the right track. If so, I'll continue to explore it.

 

If the new house main panel (first main disconnect), which can consist of nothing more than a single 100 amp breaker pair, is at the existing house then you definitely need an equipment grounding conductor going to the new house. With transformers you need just 2 current carrying conductors and the step down transformer at the far end needs to be center tapped to give 120/240 volts. So just 3 existing conductors will suffice. Without transformers you need a 4'th conductor for the EGC.

Did you say 150 amps at 240 volts (requires reworking this thread from the very beginning) or 150 amps at 120 volts which presents a lesser voltage drop issue when more or less balanced (worst case 80 amps per leg) ?

 

I think your chances of getting 480 volt service would be greater if you describe this as a commercial building. I'd ask for a 100 amp, 480/277 volt, 3-phase 4-wire service.

 

Allan, I've wondered myself if the amp rating for residential service refers to
120V or 240V when the circuits will invariably be a mix of 120V and 240V. The vast majority of circuits in a residence are 120V, so does that mean that a "200 amp service" is 200 amps at 120V, even though the service supplies 240V? I assume so, but I'm guessing.

Joe, your suggestion about describing the new service as commercial (or agricultural) so that the power company doesn't automatically turn down my request for 480V may be a good one. The service will be for both an equipment barn that supports a commercial vineyard and a residence. Hence, it wouldn't be a stretch to characterize the new service as commercial/agricultural. Should I anticipate though that the electrical rates that will be charged for the "commercial" service will be higher than the rates charged for "residential" service?

 

Since you've mentioned a different address. You could have the power company treat it as a new residence and set a transformer on your property.

 

The service would be 200 amps at xxx volts. The "real" determining factor would be the Volt/Amps (VA) of the transformer feeding the service.

 

A 200 amp service providing 120/240 volts allows up to 200 amps of 240 volt equipment or up to 400 amps of 120 volt equipment or some combination in between. You will have voltage drop issues using the existing 1/O wiring if you did not use transformers.

For a little under 200 amps of 120 volt equipment and no 240 volt equipment a 100 amp 240 volt service or a 50 amp 480 volt service would be adequate. You would be able to use the existing wiring.

 

I can't answer that question, but you can probably find out by checking the power company website.

 

Well, the power company says they won't supply 480V, 3 phase, unless I show them at least one 30HP motor, and they will only supply 240V, 3 phase, if I have at least one 7.5HP. Either way, they would need to install two new transformers at the drop point, and I'd be responsible for 50% of the cost of installing them.

As expected, if I want them to provide a service drop at the new home site, they would charge me to install two new power poles and some rather heavy cabling. If I want them to supply underground service at the new site, they won't use the existing 3" conduit. They would require 4" conduit, special trenching and the like. The charge to me would be 2x of the overhead option.

So, looks like I will have them supply a main panel at the location of the existing main panel, and then I'll deal with getting sufficient power to a subpanel at the new home site 600' away. I'll have to do it utilizing 240V from the new main panel. Given the distance I need to cover to the new subpanel, and my likely load requirements (e.g., 200 amp 120V/240V), should I plan on investing in having the power company deliver 3 phase to the new main panel? They're estimating that I'll have to pay $8K-$12K to get the 3 phase, as opposed to little or nothing for the single phase (i.e., because an adequate single phase transformer appears to be in place already at the drop point).

 

Skip the 3 phase and have them run just a 120/240 single phase service to the new building. The extra cost is just not worth it. If you ever added a 3 phase piece of equipment there are ways to get 3 phase power from a single phase service.

 

Ok. One last question. Let's assume I just install a 240v circuit to supply 20 amps for the couple motors I need to power now, and I add a step down transformer at the end of the run to supply 120v to the convenience outlet. Do I need a ground wire back to the main panel, or can we use the ground rod that will be included with the transformer installation? Thanks.

 

Yes, you need to run a ground wire on both the primary and secondary. A ground rod is for high voltage events such as lightning strikes.

 

Thanks TI. If we're using #6 for the feeds (i.e., 20 amps at 600' with < 5% VD) what size does the conductor for the ground need to be? Smaller than #6, I assume. Thanks again.

 

Further to my last question about the sizing of the equipment grounding conductor, I believe that #12 is required for a 20 amp circuit where the hot conductors are not upsized to address length of circuit and VD. Here, my hot conductors are upsized to #6 to account for the 600' distance and desired < 5% VD. In this situation, am I correct that the code requires that the EGC be upsized "proportionately" to the increased size of the hot conductors? If so, how much larger than #12 do I need to go for the EGC?

Thanks for all of the very helpful input.

 

The ground needs to be upsized by the same proportion as the hot conductors.

 

By "proportionately" I would take that to mean that if you have to up size the #12 to #6, you would have to do the same for the EGC (Equipment Grounding Conductor).

 

That is the way the inspectors have explained it to me based on the circular mils of the conductor.