Whole House Surge Protectors Are Not Enough

This misconception happens so often it's worth blogging about.  Beginning with the National Electric Code 2020 new homes are required to have whole house surge protectors.  There's a lot of life safety and equipment reasons for this, but the marketing is misleading.  Let me be clear: Whole house surge protectors are a great idea, but not a complete solution for anyone with sensitive and/or expensive gear in the home.   This is even more important if there are any medical devices. 

For this article we'll focus exclusively on short, intense power surges and compare how a whole house unit would function vs. a top rated power strip. 

At the end we'll cover important situations where a whole house protector won't do anything.

Bottom Line

For a 5kV surge a Furman PST-8 keeps the peak voltage to less than 125% of normal.  With the most optimistic case, a whole house unit will let the voltage reach at least 235% of normal, and almost 200V more than any Furman with SMP and LiFT.  

Direct Comparisons

It is true that because whole-house units sit on the service entrance they will reduce damage to your heat pump, range and fire alarms but they don't do nearly a good enough job for sensitive electronics, nor can they.  We'll go over the math and physics here.

For this comparison we'll use the current champion at Wirecutter, the Furman PST-8 with SMP and LiFT and put the numbers side by side with an average Whole House Surge Suppressor from Siemens or Square D or Eaton.  Because of their wiring (parallel) and reliability issues, the clamping voltages of the best home surge protection is still not better than 400V so we'll go with that here:

Feature	Whole House	Furman
120Vrms to Peak	170	170
Let Through Voltage	400	210
Over Voltage	230	40
% over normal	135.3%	23.5%

 

 

The problem for whole home units is that it is very hard to make a parallel mode protection with components that will activate below 400V without also activating unnecessarily. It's also incredibly expensive to make a series mode surge suppressor that will work at 200 amps.  The compromise almost all manufacturers are faced with is making a parallel mode suppressor that keeps things safe without being too sensitive, and that's how most of them get to about 400V per leg.  

 

In other words, whole house surge suppressors have some of the same challenges that a basic MOV based surge strip has, while more advanced strips with series mode protection or even just wave-shaping circuits and MOVs are going to work faster and clamp at lower voltages. 

 

Working Through the Math

First we need to understand that your normal house voltage of 120V, also known as 120Vrms has an absolute peak that is higher than that.  Peak voltages (Vpk) is 1.414x more than the RMS voltage (Vrms) so a 120Vrms waveform has a peak of  (+-) 170Vpk.  This 170Vpk is an instantaneous voltage while RMS is a summary statistic. It's that instantaneous 170Vpk that we need to talk about total let through voltages because lightning surges don't come in as pretty 60Hz sine waves, but instant, disruptive steps on top of the power companies waveform.

We borrow this image from Wikipedia for a brief summary.  The 0.707 point is the RMS voltage, which is what a good multimeter should show you. 

Let Through Voltage 

Unfortunately we are cheating here, because most whole home surge protectors do not mention the let through voltage, which is what makes it past the protector, but instead the clamping voltage, which is when the components start to work.  We're going to give them the benefit of the doubt, and assume they are the same, because it's still bad. 

A whole house unit often has a clamping voltage of no less than 400V.  400V-170V = 230V which pass through before it starts to work.  That's 235% of the normal peak voltage (170V).

Now let's compare to a Furman PST-8.  From the Wirecutter 2024 updated review: 

It turned a 5,000-volt surge into just 40 volts, thanks in part to a shutdown circuit that turns off all power when it detects a surge. The Furman PST-8 actually let less voltage through in our tests than high-end series-mode surge eliminators that can cost hundreds more.

 Doing some very simple math:

170 + 40 = 210V

This is 124% of the normal peak voltage.  I can tell you from personal experience living in a thunderstorm prone area that this is the difference between losing gear and not. 

Skipping the Whole House Protector

One famous maker of surge protectors made a statement that recommended not  using a whole house unit and only buying their power strips instead.  That's not a good idea either for one good reason:  A surge protector located immediately next to the incoming service breaker is ideally positioned to keep large current flows out of the home wiring.  The total surge current circuit could be inches instead of tens of feet and all stay inside of the electrical panel, thus reducing the possibility of in-wall fires from melted NM cables (i.e. Romex). 

Of course, that plus protecting all of the other electronics in your home which are not or cannot be put onto a power strip are also important, including the breakers in the panel itself, GFCI outlets, and everything else.

Extreme Voltage Shutdown

EVS is what Furman calls the ability of SMP to shut down in the case of low grade over voltages.  Imagine a situation when the voltage coming in is 100V or 160V, and not a sudden surge, but one that lasts more than a second.  A whole house surge protector will literally not do anything, but Furman will shut down.

This is a rare feature which is not completely exclusive to Furman but worth looking out for.