Multi-wire Circuits the Movie

Multi-wire circuits can be confusing to people. Breaker for multiwire circuits

How multi-wire circuits function, and problems related to them, are important to understand for electrical safety.  In a multi-wire circuit, the neutral (grounded conductor) is shared between two circuits.  Most circuits have their own neutral.  Generally speaking, multi-wire circuits perform just fine as long as the rules of installation are not violated.

Each leg of the multi-wire circuit must terminate on a different bus bar. 

Because bus bars are 180 degrees out of sync with each other, the neutral current can travel on the neutral wire safely.  If they were to terminate on the same bus bar, the current from the two circuits gets added together.  This is because the circuits are no longer 180 degrees out of sync with each other.  Wiring the circuits this way can result in overloading the neutral.  The amount of current leaving the breaker however, would not be more than normal and the circuit breaker would not trip even with the overheating wire.

The following read-along, video-blog attempts to discuss some of these issues.  Please pause or rewind the video where necessary to suit your own level of understanding and learning curve.  The demonstration board is set up to do many more experiments than what this post is about, so pay more attention to the overlays in the presentation and ignore all the rest.  Perhaps some of the other components will show up in a future post.

Charles Buell, Real Estate Inspections in Seattle

 

Bonding grounds and neutrals together in sub-panels

Parallel Paths, be careful

Parallel Paths

One of the most common defects I find related to remote distribution panels (sub-panels) is ground wires and neutral wires bonded together. 

This is especially true if the work has been done by homeowners or handy persons. 

In simple terms, the only place we want to bond the grounds and neutrals together is in the service equipment. Many people refer to it as the “main panel” or a variety of other terms.  

Regardless of what you may improperly call it, the point where you can disconnect all power to the building is the service equipment.  At this point, the ground and neutral are connected to the earth through a system of pipes, rebar, rods, and or wires.  The purpose of connecting the system to earth has little to do with the function of the electrical system.  This provides a layer of protection against lightning surges or static charges that would otherwise build up on the electrical system.

It is a bit like the spark you get from nose to nose when static charges build up on you and the person with the other nose.  This happens because you have no means of sending that excess energy to the earth.

The second important function of all those ground wires running in all the circuits throughout the home is to provide an emergency path back to where they are connected together in the service equipment.  In this way, if there is a short between the energized conductors and some metal component that is grounded, there will be a path back to the point of connection to trip the breaker associated with that circuit.

Circuit breakers trip on heat curves and amperage curves and a short circuit represents many times the amperage rating of the breaker tripping it instantly.  Likewise if there is a problem with the circuit that is resulting in over-amperage, the breaker will trip within the time curve of the breaker–not necessarily exactly the rating of the breaker.  A 20 amp breaker could actually not trip for a few amps above 20 amps for X amount of time without tripping.  Depending on the appliance, the appliance might finish its job before the breaker trips and we would never know it is misbehaving.

But lets get back to not connecting grounds and neutrals together in sub-panels.  Installing the green screw in this sub-panel has resulted in connecting the grounds and neutrals together.  It needs to be removed.

Improper bonding

Green screw bonds the neutral bar to the grounded metal box

When we do bond them together we create two paths back to the connection at the service equipment.  The amount of current that will flow on the two paths will be proportional to the resistance of those paths.  For example if metal conduit or a very large wire is used as the equipment grounding conductor from the sub-panel to the service equipment a large percentage of the neutral current could flow on the bare conduit or bare ground wire (or coated ground wire as the case may be) back to the service equipment.  In some cases the metal conduit might be a proportionally better path than the neutral wire feeding the sub-panel and the majority of the neutral current could then flow on the bare conduit.

I consider it best practice to always provide  a ground wire inside metal conduit but there are probably millions of installations that rely on the metal conduit as the path back to the service equipment.  As long as neutrals and grounds are not bonded together in the sub-panel this is rarely an issue. 

Now if grounds and neutrals are joined together in the sub-panel, the current of all the 120 volt circuits that are operating will travel on the metal conduit, and the neutral wire, as well as the ground wire if present.  This is multiple paths.

So in the following picture where there is no ground wire inside the conduit, but instead the only path back to the service equipment, is the metal conduit, its being disconnected is a serious problem for fire safety and ability of the breakers to trip if there is a fault to ground.  The receptacles of the circuits in this sub-panel tested as ungrounded,.  Fortunately, in this case, the neutrals and grounds were properly isolated, so there was little risk of neutral current running on the bare conduit.

Disconnected electrical conduit

Disconnected conduit feeding condo sub-panel

If they are bonded together in the sub-panel, who is going to be brave enough to grab the two ends of the pipe and stick them back together?

A competent electrician will know enough to test the metal components and/or make sure electrical circuits are turned off, but what about the handyman?  What about your Honey that works on your Honey Do list?  Most people would be unaware of the dangers present and working with the exposed metal components with bare hands could be deadly.

Here is a video demonstration done with students at Bellingham Technical College to show the effect on different size “paths” in a simulation of grounds and neutrals connected together at a sub-panel.  The “light” is the load symbolizing the sub-panel.

A big thanks to Gary Smith for his improvements to this video.

Here is a picture of the wiring diagram for the demonstration in the video:

Charles Buell Real Estate Inspections in Seattle

Handrails—what you need to know

Understanding the requirements for proper handrails can be complicated.  The brand new deck in the picture to the right, with no proper handrails, demonstrates the issue.

Handrails missing

The hazards associated with stairs is well documented. As a result, the requirements for proper handrails are very specific in the building codes.  In this article I am not going to attempt to cover all aspects of handrail requirements, but will instead focus on “graspability” requirements.

It would be easier if there was only minimum and maximum widths and thickness requirements, but it is not so simple.  There are those requirements for sure, and there are different requirements based on shapes and types as well.  There are basically three types of handrails, although the codes group them into two types.

A Type I handrail covers the round/oval shapes, as well as square/rectangular shapes, where the total perimeter measures less than 6-1/4.”  (In the 2015 IRC and earlier, this measurement was 6″–I suspect it was changed to align with the requirements of Type II handrails).  

A Type II handrail is for handrails where the total perimeter measures more than 6-1/4.”

Type I, Handrails with Circular Cross Section: 

Handrails with a circular cross section shall have an outside diameter of not less than 1-1/4 inches and not greater than 2 inches.

Type I, Handrails with Non-Circular Cross Section: 

If the handrail is not circular, it shall have a perimeter of not less than 4 inches and not greater than 6-1/4 inches and a cross section of not more than 2-1/4 inches. Edges shall have a radius of not less than 1/64.”

Type II, Handrails with Irregular Cross Section:

Handrails with a perimeter greater than 6-1/4 inches shall have a graspable finger recess area on both sides of the profile. The finger recess shall begin within 3/4 inch measured vertically from the tallest portion of the profile and have a depth of not less than 5/16 inch within 7/8 inch below the widest portion of the profile. This required depth shall continue for not less than 3/8 inch (10 mm) to a level that is not less than 1-3/4 inches below the tallest portion of the profile. The width of the handrail above the recess shall be not less than 1-1/4 inches (32 mm) and not more than 2-3/4 inches. Edges shall have a radius of not
less than 1/64.”

Type II handrails seem an exception to allow for older handrail installations that could not meet the requirements of Type I handrails.  2-3/4″ is exceptionally wide, but being that wide is not inconsistent with many older buildings.  I personally think, in new construction, most Type II handrails should be avoided when the total width is more than 2.” 

Keep in mind, the code is a minimum standard.

Here is a picture of the different types and some guidelines as to the requirements.

handrail requirements

Various shapes of handrails per 2018 IRC, R311.7.8.5

Charles Buell, Real estate Inspections in Seattle