• Sign In
  • Create Account
  • Sign Out
  • My Account
  • NEWS
  • PRODUCTS
  • CONTRACTORS
  • ENGINEERS
  • RADIANT & HYDRONICS
  • INSIGHTS
  • MEDIA
  • RESOURCES
  • EMAGAZINE
  • SIGN UP!
cart
facebook instagram twitter linkedin youtube
  • CONTRACTORS
  • BATH & KITCHEN PRO
  • BUSINESS MANAGEMENT
  • HIGH EFFICIENCY HOMES
  • TECHNOLOGY
  • WATER TREATMENT
  • PMC COLUMNS
  • PMC COLUMNS
  • Dave Yates: Contractor’s Corner
  • John Siegenthaler: Hydronics Workshop
  • Kenny Chapman: The Blue Collar Coach
  • Matt Michel: Service Plumbing Pros
  • Scott Secor: Heating Perceptions
  • ENGINEERS
  • CONTINUING EDUCATION
  • DECARBONIZATION | ELECTRIFICATION
  • FIRE PROTECTION
  • GEOTHERMAL | SOLAR THERMAL
  • PIPING | PLUMBING | PVF
  • PME COLUMNS
  • PME COLUMNS
  • Christoph Lohr: Strategic Plumbing Insights
  • David Dexter: Plumbing Talking Points
  • James Dipping: Engineer Viewpoints
  • John Seigenthaler: Renewable Heating Design
  • Lowell Manalo: Plumbing Essentials
  • Misty Guard: Guard on Compliance
  • RADIANT & HYDRONICS
  • RADIANT COMFORT REPORT
  • THE GLITCH & THE FIX
  • INSIGHTS
  • CODES
  • GREEN PLUMBING & MECHANICAL
  • PROJECT PROFILES
  • COLUMNS
  • SPONSOR INSIGHTS
  • COLUMNS
  • Codes Corner
  • Natalie Forster: Editorial Opinion
  • Guest Editorial
  • MEDIA
  • PODCASTS
  • VIDEOS
  • WEBINARS
  • RESOURCES
  • INDUSTRY CALENDAR
  • DIRECTORIES
  • EBOOKS
  • PM BOOKSTORE
  • CE CENTER
  • MARKET RESEARCH
  • CLASSIFIEDS
  • EMAGAZINE
  • EMAGAZINE
  • ARCHIVE ISSUES
  • CONTACT
  • ADVERTISE
  • PME EMAGAZINE ARCHIVES
search
cart
facebook instagram twitter linkedin youtube
  • Sign In
  • Create Account
  • Sign Out
  • My Account
  • NEWS
  • PRODUCTS
  • CONTRACTORS
    • BATH & KITCHEN PRO
    • BUSINESS MANAGEMENT
    • HIGH EFFICIENCY HOMES
    • TECHNOLOGY
    • WATER TREATMENT
    • PMC COLUMNS
      • Dave Yates: Contractor’s Corner
      • John Siegenthaler: Hydronics Workshop
      • Kenny Chapman: The Blue Collar Coach
      • Matt Michel: Service Plumbing Pros
      • Scott Secor: Heating Perceptions
  • ENGINEERS
    • CONTINUING EDUCATION
    • DECARBONIZATION | ELECTRIFICATION
    • FIRE PROTECTION
    • GEOTHERMAL | SOLAR THERMAL
    • PIPING | PLUMBING | PVF
    • PME COLUMNS
      • Christoph Lohr: Strategic Plumbing Insights
      • David Dexter: Plumbing Talking Points
      • James Dipping: Engineer Viewpoints
      • John Seigenthaler: Renewable Heating Design
      • Lowell Manalo: Plumbing Essentials
      • Misty Guard: Guard on Compliance
  • RADIANT & HYDRONICS
    • RADIANT COMFORT REPORT
    • THE GLITCH & THE FIX
  • INSIGHTS
    • CODES
    • GREEN PLUMBING & MECHANICAL
    • PROJECT PROFILES
    • COLUMNS
      • Codes Corner
      • Natalie Forster: Editorial Opinion
      • Guest Editorial
    • SPONSOR INSIGHTS
  • MEDIA
    • PODCASTS
    • VIDEOS
    • WEBINARS
  • RESOURCES
    • INDUSTRY CALENDAR
    • DIRECTORIES
    • EBOOKS
    • PM BOOKSTORE
    • CE CENTER
    • MARKET RESEARCH
    • CLASSIFIEDS
  • EMAGAZINE
    • EMAGAZINE
    • ARCHIVE ISSUES
    • CONTACT
    • ADVERTISE
    • PME EMAGAZINE ARCHIVES
  • SIGN UP!
Columns

Parallel Vs. Series Circuits
Carol Fey

By Carol Fey
April 1, 2004
Or, the difference between an electrician and a controls tech.

I bet you already know this: there are two types of basic circuits - series and parallel.

The heating industry doesn't do much with loads in series. But switches in series are the “safety committee” that makes sure the equipment fires only when conditions are safe.

We use parallel circuits for both loads and switches. Let's compare series and parallel circuits.

All of the components in a series circuit are wired together one right after the other. The circuit might consist of power supply, switch, load, switch, load, and back to power supply.

A parallel circuit could be made up of exactly the same components, but wired together differently. And the resulting performance of the circuit would be entirely different.

Loads In Parallel Vs. Loads In Series

In a series circuit, two loads have to share the available electricity. That means that two 60W bulbs each get only half the electricity they want. They would look like 30W bulbs. It also means that if one bulb burns out, they're both out.

Furthermore, it means that if there's a higher wattage load, it gets very little electricity. It would be quite dim because electricity prefers a path of less resistance - the lower wattage bulb. The lower wattage load gets almost all the electricity it wants and is bright.

Here are a couple of ways of looking at this that might help to understand it.

One is that when loads are wired in series, they are like kids in a family all sharing the same platter of food. There's only so much food, and no one gets quite enough. However, in circuits the big guy doesn't win. The little guy (lowest resistance) is closest to getting all he wants. That's because he wants less.

Another way to look at a series circuit is that the electricity has only one path. It must go through all the loads before it can be where it wants to go - back to where it came from. The electricity just gets plain tired going through all those loads, so many of the loads look kinda tired, too.

In parallel wiring, the electricity can choose among several paths. Each path has just one load. That's a lot easier for the electricity. Each load gets all the electricity it wants, so two 60W bulbs wired in parallel are fully bright.

Here's why.

Think of a parallel circuit with two loads. Imagine it as being two simple circuits that happen to share a power supply.

First wire a simple circuit of the power supply (the electricity coming from the wall outlet), a wall switch and a 60W light bulb. Finish the circuit by bringing a wire back to the power supply. This is defined as a simple circuit.

Of course the bulb is fully bright when you close the switch.

Now add a second simple circuit, using the same power supply. Just ignore the first circuit, but use its power supply to make a second circuit of power supply, switch and load. The fact that these two circuits share a power supply changes their definition from two simple circuits to one parallel circuit. Again, a parallel circuit is two (or more) simple circuits that happen to share a power supply.

With a parallel circuit, unlike a series circuit, performance doesn't depend on how many loads there are or what the relative wattages are. Each load gets all the electricity it wants because it has a direct path to the power supply, without going through any other loads.

In low-voltage (24V) control work there is a limit to how many loads you can put in the parallel circuit. That's because the power supply is a transformer, and a transformer has a limited capacity. The capacity is expressed as a VA rating.

Think of the VA rating as a contents label. If a bottle says 12 oz., you can expect to get 12 oz. out of it. If a transformer is rated at 40 VA, you can expect to get 40 VA out of it.

You can figure how many loads you can put on a 40 VA transformer just like you'd figure how many cups you can expect to fill with a 12 oz. bottle. Divide the capacity of the cup (let's say 6 oz.) into the capacity of the bottle (12 oz.). You get 2 cups.

To figure loads on a transformer you need to take one additional step. Loads are rated in A (amps) rather than in VA (volt amps). To get VA, multiply the A rating of the load (let's say the zone valve is .32A) by the voltage of the circuit (24V) and get 7.68 VA. Now, like the bottle and cup, simply divide the 7.68 VA into the 40VA of the transformer and get 5.2. That means five zone valves of .32A each could be wired to the 40VA transformer. Fewer are fine. More is asking for trouble.

House Wiring

You can guess that house wiring is always loads in parallel. Every ceiling light is wired so that it has direct access to the power supply without sharing with any other loads. That's true even if the loads are physically very close together.

An example is four or five bulbs above a bathroom vanity. They're in a row like holiday lights, but they aren't dependent on each other. Each light is fully bright. If one burns out, the others are not affected. The loads are in parallel with each other, sharing both a power supply and a switch.

In house wiring, electrical outlets also each have their own private access to the power supply. One way to think of an outlet is that it becomes the power supply for whatever we plug into it. Another way to think of it is that it's just a connection point and, electrically, is nothing at all. It's waiting for a load and switch - think of a table lamp - to be plugged into it.

Each ceiling light may have its own switch. Wires in the wall are going from the power source coming into the house, to a wall switch, to the ceiling light, and back to the power source. Each ceiling light and switch combination has its own private connection with the power source. Each wall outlet also has its own access to the power supply. Again the outlet is just a connection waiting for a load and switch combination to be plugged into it.

Switches In Series Vs. Switches In Parallel: When we call a circuit series or parallel, the assumption is that we're talking about loads. But series and parallel also can apply to switches. Think about this. The simple circuit portion of a parallel circuit can have more than one switch. If there are two switches, they will be wired either in series with each other, or in parallel with each other.

The fact that house wiring is loads in parallel helps to explain why electricians sometimes aren't comfortable with wiring controls. Control wiring frequently involves more than one switch controlling a load. (Recall the safety committee of switches in last month's column.) Those switches may be in series or they may be in parallel.

Switches in series is the safety committee scenario. All of the switches must be closed before the load can get power. Switches in parallel means that closing either switch allows power to flow to the load. It's possible for one circuit to have some switches in series, and some switches in parallel.

Wiring Transformers - Series, Parallel Or ...?

Sometimes the job calls for more VA than one transformer delivers. For example, if you have six zone valves of 7.68 VA each, and your transformer is 40VA, you can't put all of the zone valves on one transformer. You need a larger transformer or two transformers.

If you use two 40 VA transformers, how should you wire them together? The simple answer is that you don't need to. Just put three zone valves on each. That keeps the job simpler to install and troubleshoot. Plus there's the advantage that if one of the transformers fails in the future, the other three zone valves will still have power to provide some heat to the building.

If you're set on wiring the transformers together, they must be wired in parallel. That means that a terminal on one transformer must be wired to the identically labeled terminal on the other.

We've just looked into a major difference between a competent electrician and a controls technician. The electrician is thinking about parallel circuits. A controls tech has to be able to distinguish between series and parallel.

The tech knows that putting loads in series is a bad idea. He knows that switches in series means that all the switches must be closed before the circuit works. He knows that switches in parallel means that any one of the switches make the circuit come on.

And the tech knows, if the circuit doesn't work, to check out the wiring rather than blaming the controls. That's because the wiring makes all the difference.

Share This Story

Looking for a reprint of this article?
From high-res PDFs to custom plaques, order your copy today!

Carol Fey is a degreed technical trainer, specializing in controls and electricity. She has published four books: Quick & Basic Electricity, Quick & Basic Hydronic Controls, Quick & Basic Troubleshooting, and Quick & Basic House Wiring. You can order books or get on the e-mail list for free technical information at www.carolfey.com. Or e-mail carol@carolfey.com.

Recommended Content

JOIN TODAY
to unlock your recommendations.

Already have an account? Sign In

  • Worker using the Milwaukee Tool SWITCH PACK drain cleaner

    Pipeline profits: Drain cleaning, pipe inspection create opportunities

    Drain cleaning and inspection services offer lucrative...
    Plumbing News
    By: Nicole Krawcke
  • Uponor employee, Arturo Moreno

    The reinvestment in American manufacturing and training

    Plumbing & Mechanical Chief Editor Nicole Krawcke and...
    Plumbing News
    By: Nicole Krawcke and Natalie Forster
  • March 2024 Women in Plumbing hero image of woman engineer overlayed by circle of hexagon shapes with numbers from 1 to 10

    Celebrating 10 Influential Women in the Plumbing Industry

    Celebrating Women's History Month and Women in...
    Plumbing News
    By: Nicole Krawcke
Subscribe For Free!
  • eNewsletters
  • Online Registration
  • Subscription Customer Service
  • eMagazine
  • Manage My Preferences

IPEX celebrates grand opening of new Florida distribution center

IPEX celebrates grand opening of new Florida distribution center

NIBCO Press Solutions

NIBCO Press Solutions

Bell & Gossett Illustrates Path to Net-zero at AHR Expo

Bell & Gossett Illustrates Path to Net-zero at AHR Expo

AI can boost efficiency and profitability for plumbing, HVAC contractors

AI can boost efficiency and profitability for plumbing, HVAC contractors

More Videos

Sponsored Content

Sponsored Content is a special paid section where industry companies provide high quality, objective, non-commercial content around topics of interest to the Plumbing & Mechanical audience. All Sponsored Content is supplied by the advertising company and any opinions expressed in this article are those of the author and not necessarily reflect the views of Plumbing & Mechanical or its parent company, BNP Media. Interested in participating in our Sponsored Content section? Contact your local rep!

close
  • J.J. Keller CMV vehicles on road
    Sponsored byJ. J. Keller & Associates, Inc.

    The dash cam game-changer for small business safety

Popular Stories

Underfloor heating installation with drain sewer hole in bathroom close up on water floor heating.

Using hydronics to leverage time-of-use electrical rates

Watts Nexa mobile image

Behind the Wall: Where smart plumbing gets smarter

Six tankless water heaters that feed the nutraceutical manufacturer’s operations.

How to deliver large volumes of hot water quickly and intermittently

PMCE Home-X April 29 Free Webinar: From Legacy to Leadership: Preparing Your Home Services Business for the Next Generation

Events

November 13, 2024

Future Proofing MEP: Navigating the 2026 High Efficiency Water Heating Standards

Join our deep dive into DOE’s new standards so you can future-proof your MEP practice.

EARN: 0.1 ASPE CEU; 1 AIA LU/HSW; 0.1 IACET CEU*; 1 PDH

View All Submit An Event

Poll

Will business be up or down in 2025?

Do you anticipate business in 2025 to be up or down in comparison to 2024?
View Results Poll Archive

Products

The Water Came To A Stop

The Water Came To A Stop

See More Products

Download the FREE Water Conservation, Quality & Safety eBook: Plumbing Trends Increasing Safe Water Availability

×

Keep your content unclogged with our newsletters!

Stay in the know on the latest plumbing & piping industry trends.

JOIN TODAY!
  • RESOURCES
    • Advertise
    • Contact Us
    • Directories
    • Store
    • Want More
    • Supply House Times
  • SIGN UP TODAY
    • Create Account
    • eMagazine
    • eNewsletter
    • Customer Service
    • Manage Preferences
  • SERVICES
    • Marketing Services
    • Reprints
    • Market Research
    • List Rental
    • Survey/Respondent Access
  • STAY CONNECTED
    • LinkedIn
    • Facebook
    • Instagram
    • YouTube
    • X (Twitter)
  • PRIVACY
    • PRIVACY POLICY
    • TERMS & CONDITIONS
    • DO NOT SELL MY PERSONAL INFORMATION
    • PRIVACY REQUEST
    • ACCESSIBILITY

Copyright ©2025. All Rights Reserved BNP Media.

Design, CMS, Hosting & Web Development :: ePublishing

search
cart
facebook instagram twitter linkedin youtube
  • Sign In
  • Create Account
  • Sign Out
  • My Account
  • NEWS
  • PRODUCTS
  • CONTRACTORS
    • BATH & KITCHEN PRO
    • BUSINESS MANAGEMENT
    • HIGH EFFICIENCY HOMES
    • TECHNOLOGY
    • WATER TREATMENT
    • PMC COLUMNS
      • Dave Yates: Contractor’s Corner
      • John Siegenthaler: Hydronics Workshop
      • Kenny Chapman: The Blue Collar Coach
      • Matt Michel: Service Plumbing Pros
      • Scott Secor: Heating Perceptions
  • ENGINEERS
    • CONTINUING EDUCATION
    • DECARBONIZATION | ELECTRIFICATION
    • FIRE PROTECTION
    • GEOTHERMAL | SOLAR THERMAL
    • PIPING | PLUMBING | PVF
    • PME COLUMNS
      • Christoph Lohr: Strategic Plumbing Insights
      • David Dexter: Plumbing Talking Points
      • James Dipping: Engineer Viewpoints
      • John Seigenthaler: Renewable Heating Design
      • Lowell Manalo: Plumbing Essentials
      • Misty Guard: Guard on Compliance
  • RADIANT & HYDRONICS
    • RADIANT COMFORT REPORT
    • THE GLITCH & THE FIX
  • INSIGHTS
    • CODES
    • GREEN PLUMBING & MECHANICAL
    • PROJECT PROFILES
    • COLUMNS
      • Codes Corner
      • Natalie Forster: Editorial Opinion
      • Guest Editorial
    • SPONSOR INSIGHTS
  • MEDIA
    • PODCASTS
    • VIDEOS
    • WEBINARS
  • RESOURCES
    • INDUSTRY CALENDAR
    • DIRECTORIES
    • EBOOKS
    • PM BOOKSTORE
    • CE CENTER
    • MARKET RESEARCH
    • CLASSIFIEDS
  • EMAGAZINE
    • EMAGAZINE
    • ARCHIVE ISSUES
    • CONTACT
    • ADVERTISE
    • PME EMAGAZINE ARCHIVES
  • SIGN UP!