Today’s typical home is wired for at least 100-amp service, and many are wired for twice that number. This makes sense given the multiplicity of modern appliances and electronics in a home. The demand is obviously higher if you have an electric stove/range, or an electric vehicle using a basic in-house Level 1 charger.

The need for power—and more of it—became clear when a neighbor who was having a modest addition put on the house asked me for some advice. The situation was this: the contractor told him that for various reasons, their 100+ A service would be cut to 50 A for a month or two during the construction. The reasons for this cutback were not clear, but it had something to do with cable capacity.

The question I was asked was simple enough: could they—a couple plus two children approaching teen years—manage on just 50 A and, if not, what steps could the take to minimize disruption? (Actually, the word my neighbor used was “survive” rather than “manage” but I feel that’s overly dramatic.)

The semi-quantitative assessment

My answer was also simple, as I gave the prudent engineering response “it depends” followed by “I’ll think about it and get back to you.” Then I set out to develop a firmer answer by doing some semi-quantitative assessment.

My first impulse was to check the web and, sure enough, there were plenty of apps for assessing house power needs. However, these required a detailed inventory of the loads which was more than I was ready to do. Then I thought I would create an Excel spreadsheet but soon realized that sort of analysis could easily become more precise than the problem merited. After all, my neighbor wanted a simple answer:

1. It’s no problem,
2. it’s definitely a problem, or
3. it’s a manageable “maybe” problem.

Instead, I took out my “back of the envelope” pad and decided to do some rough assessments, Figure 1.

Figure 1 This “back of the envelope” pad serves as a visible reminder that rough and imprecise input numbers should get appropriate analysis and not impute undeserved precision to the results. Source: Bill Schweber

I didn’t actually use this custom-made pad, but instead I kept it in front of me as a constant reminder that I should stick to estimates that were rough enough that they could be added up “in my head” on that pad. The reason for this simplicity is there are a lot of fuzzy numbers in the assessment.

For example, without knowing the make and model of various higher-current appliances such as the electric stove/range, any number I did use would likely have a ±10 to ±20% error band. Further, while the individual errors might cancel each other out to some extent they could also accumulate, resulting in a fairly large error band. In other words, random errors can aggregate either way.

The danger when using a spreadsheet is that soon you fall into a mental false-accuracy trap, since its available precision of more digits soon leads to the sense that there is corresponding accuracy as well, which is clearly not the case here (yes, I could restrict the cells to a few digits, but that’s another thing to do). It’s been my experience that it is very easy make the leap from rough estimate to a firm “you can bet on it” number, even if there is no basis for doing so; I’ve seen that happen in preliminary design review meetings many times when the project manager asks for some numbers.

Complicating the assessment, some of the larger loads such as the stove/range or microwave oven are under the direct control of the house occupants, while others such as heating system, refrigerator, and separate freezer control their own on/off cycles.

Numbers guide but don’t prove

I asked some questions about what was in the house, made a list, and went online to get a sense of how much current each uses. These rough estimates are for current consumption from a 120 VAC line; for those with 230 VAC lines, the current numbers should be cut in half, so that 50-A maximum would be 25 A:

1) Big loads you can’t control (these intermittent, asynchronous loads cycle on and off with unknown duty cycle; they may add up all at once, or hardly at all)

• Refrigerator/freezer: 6 A (will be higher for a few seconds, as the compressor kicks in)
• Separate outside freezer: 3 to 5 A, depending on outside temperature (same note as above)
• Oil-fired heating system: 5 A, temperature-dependent (same note as above)
• Electric water heater: 5 to 8 A

Total: around 20 A

2) Small loads (some you can control, some not; not an issue unless you are close to maximum limit

• Large TV: 1 to 2 A
• smaller screens: 0.5 A
• Various chargers: 0.5 A or less
• House lights: 0.5 A each
• House network boxes: 1 A

Total: 5 to 10 A

3) Bigger loads that you can control

• Clothes washer: 4 to 6 A
• Clothes dryer: 15 to 20 A
• Air conditioning: 8 to 10 A (but not a factor as this is a winter situation)
• Kitchen range top: 5 to 10 A, depending on model and temperature setting
• Kitchen oven: 8 to 12 A, depending on model and temperature setting
• Toaster Oven: 8 A
• Dishwasher: 9 to 12 A
• Microwave oven: 8 A
• Hair dryer: 10 to 12 A

Total: it depends on what you are using and when, but it adds up very quickly!

Conclusion: The loads you can’t control add up to around 25 A (all of these won’t be on 100% of the time) plus small loads of 5 to 10 A bring the total to 30 to 35 A, so the family will have about 15 to 20 amps of headroom on the loads that can be controlled. That’s doable but also cutting it close; you could have a case when just one additional modest load causes a droop and a brown-out of the supply voltage. That, in turn, brings on other operational problems in both motorized and all-electronic products.

Electrical service to older homes

As a curiosity, I checked out some older houses (1930 vintage) in the area, many of which are still occupied by descendants of the original families. Some of the present occupants said when the houses were built, they were outfitted with 30-A service and used knob-and-tube wiring rather than metal conduit or Romex (NM, or non-metallic sheathed) cable, Figure 2. While they have upgraded to 100+ A overs the years, some still have the knob-and-tube in the attic (not even close to code-approved now).

Figure 2 Early wiring used knob and tube insulation (a) which was replaced by (b) metal conduit (still in wide use) and (c) PVC-coated non-metallic sheathed cable, usually referred to as Romex. Sources: Arc Angel Electric Co., Meteor Electrical, D&F Liquidators

What’s your sense of the home-AC service situation? Have you ever been on a temporary or permanent limited-power budget at home? Have you ever had the corresponding “average load” versus “peak load” power-supply rating dilemma, either for line AC or with the DC supply in a product?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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Reference

• Arc Angel Electric, “Understanding Knob and Tube Wiring: A Comprehensive Guide”

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