Electrical Service: Part 1 - Design

Wed Sep 28 2022

Kicking off our series on installing electrical service, we walk through service sizing, transformer location and primary transmission line placement.

Electrical Service: Part 1 - Design

In this series, we'll document our journey to bring electrical service from the local power company to our new house site all by ourselves.

Introduction

Let me start with a disclaimer. I am not an electrician. This blog post series is not intended to be a "how to" guide, but merely a documentation of our experience going through this process.

Disclaimer: Electricity can be dangerous, and mistakes can be lethal. If you are attempting something like this yourself and at any point you don't have absolute confidence that you know what you're doing then stop and find a professional.

While I am not an electrician, I do have some tangential academic and experiential knowledge of electrical systems - enough that I felt confident we could manage this ourselves.

If you are wondering why we decided to connect to the grid and not go off-grid, then check out our blog post all about that decision.

Off-grid or on-grid?

As we're building our dream home in Vermont, there's one big question we have to answer: should we connect to power or go off-grid?

Off-grid or on-grid?

We are also lucky to live in a state, Vermont, where we are allowed to do all of this work ourselves and do not need to hire a licensed, qualified electrician. That's relatively unusual these days, but is one of the factors that led us to buying in this area in the first place.

We Bought Land!

Find out how we bought 40 acres of raw land in central Vermont, and how we plan to build our dream home all by ourselves!

We Bought Land!

Without giving away too many spoilers, as I write this now, we're now finished with this whole process. With the benefit of hindsight I can say that we have absolutely no regrets about tackling this ourselves.

While at times it was challenging, not only did we save a lot of money by doing this ourselves, but I firmly believe we ended up with a better end result than if we had contracted the work out to an electrician. Not because a professional electrician wouldn't do a good job, but simply because we were able to adapt and respond to new information as we went along, thinking to the future and what our needs might be then, to implement the best solution today.

We also had great support from our utility company, Green Mountain Power, who were able to answer our questions and guide us through the process.

In this first blog post in the series I'm going to talk through the design of the system - some of our requirements, constraints and decisions. In subsequent blog posts we'll look at the stages of the installation, including the primary service, secondary service and meter.

Overview of electrical service

Before we jump into the weeds, let me give a very quick overview of how power gets from the electrical grid to your home. There are many nuances to this so I'll simply use our property as an example (I'm also simplifying a few concepts here for the sake of brevity).

At the bottom of our driveway, overhead electrical lines carry (relatively) high voltage electricity from the substation. In our case, this is single-phase with a voltage of 12.47kV (line-to-line or 7.2kV line-to-ground), and is known as primary transmission.

For residential use, this voltage needs to be stepped down to split-phase 120/240V using a transformer - this can either be mounted on a pole (the round boxes you see at the top of some poles) or ground-mounted (typically a big green metal box).

The electricity leaving the transformer is known as secondary service, and from the transformer it goes to your meter socket. A meter installed in the meter socket measures the current passing through and that's what's used to calculate the bill.

From the meter socket, the electricity goes to your service panel. This typically consists of a main breaker and a bank of individual branch circuit breakers known as a load center.

While it varies by jurisdiction, in our case the utility company is responsible for everything up to and including the transformer, as well as the meter (but not the meter socket). We are responsible for the secondary wiring, the meter socket and everything beyond it.

Common service sizes

In slightly simplified terms, your electrical service size can usually be identified by the rating of the main breaker in your service panel. This is the maximum amount of current you can pull from (or push back to - more on that later too) the electrical grid.

The main breaker is matched to the rating of the service panel itself, the service wiring, the meter socket and other components in the electrical installation, so upgrading your electrical service isn't as simple as just installing a higher-rated main breaker - all those other components may need upgrading too.

For us, it's important to get it right first time.

In older homes, 60A or lower is not uncommon. Over time, the norm shifted to 100A, and modern homes may have a 150A or 200A service. Larger homes may even come with a 400A service - beyond this and most utility companies classify it as light commercial rather than residential.

These ratings are measured in Amps, and indicate the amount of current available at 240V AC before the breaker trips. In reality, sustained load should not exceed 80% of the rating. So while a 100A service can provide up to a peak of 24kW (240V x 100A), the sustained power available is 19.2kW.

The question for us was: what size service do we need?

Load calculation

The standard way of answering this question is a load calculation. There are a couple of ways to do this, but in essence the goal is to estimate the expected amount of power you will draw based on either the size of the home or the number and type of appliances you have.

In our case, there are three big problems:

  1. We have only very rough, conceptual plans for our house at this stage;
  2. We know we want to add other buildings in future - barn, workshop, etc;
  3. Electric vehicles change everything.

The first point isn't a big deal. We know the house will be roughly 1,500sqft and can make educated guesses about the number of bedrooms, the appliances, etc.

The second issue presents more of a challenge. A workshop with a handful of large tools (cabinet saw, CNC, welding machine, dust collection, etc) can have a substantial power draw of its own.

The third issue is also a curve-ball. As I talked about in my Off-grid or on-grid? blog post, residential chargers for Electric Vehicles (EVs) can be huge - the Ford Charge Station Pro for the F-150 Lightning is rated at 80A! Now imagine a two-car household where both cars are electric and charging at the same time.

Grid-tie solar

Your main breaker connects your load center to the power grid. It is connected via a bus bar to the other branch circuit breakers, and that bus bar has a rated capacity as well. Typically this is equal to the main breaker rating, so a service panel with a 200A main breaker will usually have a bus bar with a rating of 200A too.

This can cause issues with grid-tie solar because the power from your solar panels is normally back-fed into the panel via a circuit breaker. I'm not going to get deep into details here (have a read through NEC705.12(B)(2)(3)(b) if you want to know more), but essentially this means you have two sources of power feeding the load center (the grid and the solar panels) and the combined breakers can't exceed more than 120% of the bus bar rating.

So even in a 200A service panel, if you have a 200A main breaker then you cannot have more than 40A (because 200A + 40A = 240A which is 120% of the 200A rating of the bus bar) of solar, which equates to just 7,680W once you factor in the 80% maximum rating on the solar breaker itself.

Now, you can get around this by de-rating the main breaker. If, for example, you switched out your 200A main breaker for a 175A main breaker, that would allow you to have a 65A solar breaker, or about 12kW of solar. But now you have less current available from the grid.

Depending on your utility company, there are other ways of tying your solar panels to the grid, but these also have drawbacks, for instance when used in conjunction with battery backup systems.

Choosing a service size

After weighing up all the data, we eventually settled on a 400A service for the following main reasons:

  1. We're building an all-electric home which could have very high peak demands;
  2. We expect to have other buildings which may house high-draw appliances such as the barn and workshop;
  3. We want RVer friends to come and visit, and providing one or more 30A/50A hookups for them is important to us;
  4. We plan to have one or more EV chargers (EVSEs) for our own vehicles and visitors;
  5. We eventually plan to have a moderately large solar system, maybe 15kW or more.

Overhead vs underground

We have a problem - how do we get electricity from the utility pole at the road to our house site, about 1,000ft away up our driveway?

Building a Driveway

An old logging trail won't cut it for long - we need a solid 1,000ft driveway from the road to our house site.

Building a Driveway

There are two options - overhead wires on poles, or underground wire in conduit.

Underground

Going underground would mean burying a conduit more than 3ft underground along the entire length of the driveway - from the road all the way to the house site.

There are advantages to this. Aesthetically, there are no poles to look at, and no overhead wires to spoil the view. Practically, the wires are better protected from winter storms and broken tree limbs or even falling trees. There's a reason that underground wiring is preferred.

But, if you've watched or read about our driveway build, you'll know that we have very shallow rock and trenching more than 3ft deep would be very slow, and very expensive.

Breaking ground... and machines!

We've well and truly broken ground now, hitting some major milestones but also breaking some things along the way.

Breaking ground... and machines!

We spoke with our contractor and in the end didn't even bother getting a quote on it. It would have needed blasting and would probably have end up costing tens of thousands of dollars to dig the trench.

Plus, it would be our responsibility (and expense to lay the conduit). The specifications call for 3" Schedule 80 PVC conduit under a driveway which, with this year's crazy pricing, is sitting around $11 per foot, meaning it'd cost more than $10,000 just in conduit alone.

Underground all the way up the driveway was a no-go.

Overhead

Running the wires overhead on poles is cheaper, but still not cheap. To cover the length of the driveway would require three or more poles, and while the utility company provides the poles and installs them, they do so at our expense.

And as mentioned earlier, this cost saving comes at a cost - both aesthetically and functionally.

Hybrid

In the end, we settled on a hybrid design. We'd use overhead poles to bring the primary service most of the way up the driveway, transitioning to underground for the final stretch.

Bringing the wires overhead most of the way saves a lot of money, but by going underground at the end, the wires won't be too visible from the house site.

Transformer location

Given the hybrid approach we're taking for running the primary transmission lines, there are two places we could choose for the transformer - at the top of the final pole, or pad-mounted on the ground by the utility building.

Pole-mount transformer

The transformer steps down the 12.47kV primary voltage to the 120/240V secondary voltage that we're all familiar with (and, works in reverse when feeding solar back to the grid, stepping up the voltage).

Lower voltage for the same power transmission means more current, and power loss in a wire is proportional to current squared - in other words, higher current means much higher power loss. That's why utility companies use high voltage for transmission lines - to reduce loss.

The distance from our final pole location to the utility building where the meter will be is about 200ft - not a ridiculously long run, but not too short either. For a 400A service at that length we'd have to run giant 350MCM cable - expensive and a pain (for us) to pull.

Even then, we'd still be susceptible to a voltage drop of around 3% in extreme situations - not unmanageable, but also avoidable.

Pad-mount transformer

The alternative is to continue the primary service run underground and locate the transformer close to the utility building, making the secondary (lower voltage) run as short as possible.

While the primary service conduit still needs to be 3", and the 1/0 wire they use isn't any cheaper than the 350MCM cable we'd have to run for secondary, the utility company is responsible for pulling all primary wire and I'd much rather let them handle such a long pull of thick wire!

Additionally, this benefits us with:

  1. Much lower voltage drop - the transformer will be ~20ft from the utility building, and proportionally closer to the other buildings too;
  2. Reduced responsibility - the electric company is responsible for all primary wiring.

On balance, I think either a pole-mount or pad-mount transformer would have worked, but we chose to go with a pad-mount transformer, primarily for reduced voltage drop and also ease of installation.

Temporary backboard

I've said several times that our plan is to have a 400A service, and that's true, but not just yet. For that, we need the utility building finished, but we don't want to wait until then to get power.

In the interim, we're going to pull power to a backboard - a free-standing wooden structure that will house a meter socket and service panel.

While I call this temporary because we're only going to use it until we have the utility building, in reality it could be permanent - there's no time limit on how long we can operate with it for. Specifically, it's not falling under the categorization of what utility companies call "temporary power" for use at a construction site and is usually limited to 30A.

Clearly we don't need anything close to 400A service until we have the utility building (and house, and maybe other buildings) completed, so we're not going to be installing a 400A service on the backboard.

Instead, we'll be wiring up a 100A service with a 50A receptacle, 30A receptacle and a couple of 20A receptacles. This will provide power for our RV, visiting RVers, an EV charger, power tools and more while construction is underway.

5 Basic Electrical Tools Every RVer Should Have

With these 5 easy-to-use and inexpensive tools, you'll be in a good position to diagnose and potentially fix some basic electrical problems in your RV.

5 Basic Electrical Tools Every RVer Should Have

Once the utility building is finished we'll decommission the temporary backboard.

Summary

So that's the plan.

We're going to pull primary overhead service over 700ft up our driveway before transitioning it to 200ft underground where it will terminate at a pad-mount transformer.

From here, in the short term it will connect to a nearby temporary backboard and provide 100A service, but longer term it will have a short secondary run of about 20ft to the utility building to give us 400A service.

400A service will give us more than enough power for not only running our all-electric home, but also powering a workshop and charging multiple electric vehicles at home. It will also give us plenty of capacity to back-feed the grid with solar too.

Join us in the next post as we install the utility poles and conduit for our primary service.

Photo by American Public Power Association on Unsplash.

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