When is a system considered 'overloaded'? Aside from tripping the main breaker on a daily basis what practical criteria should we go by?

Bill

I guess the most basic answer would be something along the lines of "When the rating of all or part of the system is likely to be exceeded under normal operating cnditions."

I know, I know. That's a bit of a vague "cop-out" answer, and the definitions of "likely" and "normal" are open to argument.

Don't know if you've found it yet, but go here
http://www.tlc-direct.co.uk/Book/6.2.3.htm

for the standard diversity allowances in the U.K. As the regs. point out though, these are just starting suggestions and it's really up to the designer to estimate, ask about, or anticipate the likely loading. Maybe clairvoyance would be a handy attribute?!

I would think that Article 220 would apply here...

If the computed load exceeds the service size, then it isn't big enough. Is it really that simple?

'66

If you look at 220-3(a)FPN it says:
Table 220-3(a) under Dwelling units lists 3VA per sq. ft. (If I'm reading it right that also covers the 20A Bathroom circuit) This Table is the same as it was in 1984! (oldest Book i've got) The FPN 'disclaimer' was added sometime after.

In short, It doesn't appear to be much help for computing loads in the average house that I've been in.

Bill

[This message has been edited by Bill Addiss (edited 10-05-2001).]

Well, in my opinion there's quite a few different angles to this question.

1: Is this a system which has been designed to minimum requirements, but not to actual loads.
2: Is the overload affected on only a few branch circuits, or a certain subfeed.
3: Does this effectively overload the Transformer[s] feeding the occupancy [>130% FLKVA].
4: Does this overload any SDS [Separately Derived Systems] in the occupancy.
5: Was this something that the designing Engineer was aware of prior to the project's completion.

Virgil's message with the Minimum service size per NEC Art 220 [Computed Loads] I think covers it very well.
Is it just as simple as figuring the service size per a calculated load? In most 1 family dwellings, it may. In a Commercial Warehouse, it easilly could fall too low!
We built many Tilt-Up Shells with 400 amp 208Y120 VAC services and I could see this being exceeded very easilly for T.I.'s if the Tenant has high machine usage.

Even in "Strip Malls", the NEC Minimum load calcs can be exceeded. Case - In - Point:

A particular Client of mine who owned and operated Tanning Salons.
At one location, the Salon occupied two separate units, divided by the usual Dimising Wall. Each unit had an existing 100 amp 208Y120 VAC 4 wire service, feeding into 125 amp 24 space panels.
After figuring all the proper calcs for the Tanning Beds [multiple types], the HVAC, Lighting and all that jazz, Both services and panels were upgraded to 200 amp. This just barely made the calcs!
Although there was never any overloading of the main OCPDs, the calcs just made it!
I would imagine that if any more loads were added, plus all were active coincidentally [at the same time], the main, or mains would become excessively heated - very close to tripping the OCPD!
If the systems were loaded up high, plus the other occupancies were at high loads, the Transformer would easilly fail! [75 KVA for 10+ services].

For at least 3 years after I did that project [and before the location was sold to someone with very low maintenance skills], I never heard of any overload problems.
This approach in my words was pure "Crapshooting", since the potential for overloading several key items was present, but never occurred.


Simply to sum it up and answer the question, IMHO if the system has been designed with the installed / design loads in mind [as per outlet locations and branch circuit considerations], but the entire COMPUTED LOAD per service[s] / subfeed[s] / Transformer[s] are allowed to remain inadequate, that's an overloaded system to me.

The last Bank Branch I Engineered required no less than 200 amp 208Y120 VAC service, just to conform to extreme basic NEC calculations. If I had used those values when designing the project, there would have been a serious overloading of the main OCPD!
I opted for a new 400 amp service, which cost the Client $20,000 more for the total Electrical Installation [on a $1.5 Million project]. There was absolutely no reluctance from the Client! I gave them all the pros and cons to consider, and they still had no problem with the extra upfront money.
It would have costed upto 20x the amount if done at a later time.

This is the approach taken when I do any system's design - for any EE project.

Sometimes you just can't allow for every situation [such as the Tanning Salon], other times you can [such as the new Bank Branches].

Just my opinion.

Scott SET

Scott,

Thanks, My question is more general in nature, and nothing specific in mind. It may be one of those intangibles.


As an example, a house built 30 years ago with Electric appliances has a 100A service. Individual appliances get upgraded over the years by small steps, self cleaning oven, more gadgets and portable loads, new hairdriers, etc. Recessed lighting is added all over the place, jacuzzi tub, window airconditioners, Swimming pool, Dehumidifiers, Tv's in every room etc etc.
The original 3va/sq.ft. doesn't cut it anymore as a benchmark for general use receptacles and lighting. Much of the new load can be portable in nature and used or not used at any time. Are there any set guidelines or recomendations to go by?

If we are going to go by computed loads, how does one figure all these portables as they pertain to individual branch circuit loads and large, sometimes used items as they pertain to service capacity?

Bill

Go with a figure of 180W for each light and "convenience" receptacle outlet?

I should add, other than appliance, small appliance and bath circuits, etc. as per 220?

[This message has been edited by sparky66wv (edited 10-06-2001).]

Realisticly, to go by absolute minimum NEC would be inadeqaute somewhere or somehow. The FPN's that elude to this also include 90-1(b) FPN.

How did the NEC come up with the 3W figure? Could we use similar data/equations with updated variables?

Bill:

The 3VA per sq. ft. figure in your 1984 NEC is the same in the book I have here which has copies of some tables from the 1975 edition.

Overloaded circuits has become something of a problem in more recent years here. I already mentioned the 30A ring with 3kW washer, 3kW dryer, and 3kW dishwasher, so I won't go into that again.

Many older properties were wired with only one 5A branch for lighting. It was always bad from the point of view of a blown fuse plunging the whole house into darkness, but with 1200 watts available it was quite sufficient in load terms when houses were built with a single luminaire or pendant light in each room.

In recent years though, many people have added patio lights, extra wall lights, kitchen worktop lights, etc. and just tacked them onto the existing 5A circuit with no consideration for the possibility of an overload.

An overload on the service entrance here is more than just the inconvenience of resetting a main breaker. Where a main C/B is used, it's for ground-fault protection only. The service cable protection against overcurrent is providred solely by the PoCo's main fuse. As this is sealed, it means a call-out to replace it.

This is a particular problem with the growing popularity of instant electric showers during renovations of an older property. Some of the latest models are rated as high as 9.5kW, and just recently I had someone want one of these installed. Unfortunately, the house had only an older 40A service, so I had to explain that they'd need an upgrade from Eastern Energy - Cost over £300 ($450).

When I calculate the load for a new custom home I use 5va per sqft. It has been my experience with custom homes that "designer" people the homeowners hire like to add lots of different, elaborate "mood" lighting. Not to mention over-the-top accent lighting outside on the house and landscape. I had one house a few years ago that I cut too close for my liking at 5va. Then 2 years after they were in the house they wanted to pitch the gas furnace and convert to geo-thermal w/electric heat bank back-up. Service upgrade on a 2 year-old custom home!

>Many older properties were wired with only one 5A branch for lighting.
Dare I ask what wire size was used?

Originally, it was almost always 1/.044 size (i.e. a single-strand 0.044 in. diameter). From 1970 the metric equivalent is 1 sq. mm. Both are just a fraction larger than #18 AWG.

hmmm,
i'd probably have thought it to be doorbell wire

Hehe...

The metric size cables are standardized right across Europe now, but in many other countries they specify 1.5 sq. mm as the smallest to be used. That's about 15% larger than #16.

Haven't the slightest idea of your smallest standard Over-Current-Protection Device (OCPD), but if 15 Amps, then at 220 volts it would not likely be overloaded, just mechanical protection is needed ????

I can empathise with your concerns and frustrations to the Residential load / service calcs.
Looking at the complete picture, who could have imagined in 1970 that the average house would have so much stuff!
No one would have even expected so many Houses to have 3+ Televisions, Hot Tubs / Spas, Tons of Deco and Task lighting, Increased variety of Small Appliances, Computers in a Home [ENIAC at a House!!! joke], window AC units, and so on.

The real kicker is the concept of "The All-Electric House" in the 1950s, along with increased NEC minimum circuits and grounding in the 1950 - 1960 time period. This must have been a drastic change to Residential load and service calcs than the preceeding time periods [1930 - 1940].
Someone must have been aware that the Residential loads have and will change over time!

Time periods:

1930 - 1940: Simple Incandescent lighting, minimal kitchen appliances, one Radio, very few Television sets owned, very few general purpose receptacles are needed.

1950 - 1960: All-Electric cooking appliances, Central Air, Forced Air heating, Televisions becoming more common, more general purpose receptacle are needed, grounded receptacles in Kitchens, Bathrooms and exterior.

1970 - 1979: More specific circuitry requirements, GFCI's for Bathrooms, etc.

1980 - 1989: Increase in Kitchen loads, more TVs and electronic equipment, people begin to buy Spas and Hot Tubs, more swimming pools, home workshops become more common, garage door openers more common, GFCI's in Garages and outdoor locations, etc.

1990 - 1999: Interior deco and task lighting accelerates, more types of Kitchen appliances and Kitchen layout styles, more home workshops, more Spas and Hot Tubs, Home Computers becoming very common, increasing HVAC installs, Central Vacuum systems becoming more common, exterior spec / deco lighting becomes more common, every room has a TV set plus an Audio system [almost every room!], high power blow dryers are used by almost every household, increasing "DIY'ers" and Contractor based room additions / remodels / other alterations, more swimming pools, GFCI's required in Kitchens or other locations within 6' of sinks [I think that's the distance.?], Dedicated circuit[s] for Bathrooms, etc.

2000: >75% of Homes in North America have at least one PC, Advanced ["High Tech"] systems - such as Audio / Video, Lighting control and other equipment control,- become more widely used, spec type lighting increases, Kitchen appliances continue to become larger variety and more affordable, pending the 2002 NEC's requirements for AFCI dedicated circuit[s], Spas and Hot Tubs holding steady, Home improvement / Remodel / Addition / Alteration projects continue, Home Workshops / Home Based Businesses continue, HVAC systems, swimming pools, increased requests for ample or additional general purpose receptacles, and so on.

2002 - 2010: Who knows???

A few factors have been responsible for a decrease in consumed Electrical Power [KWHs], such as the Deregulation situations here in California, plus the Mandatory Energy Conservation Standards [AKA Title 24, part 6, Chapter 1 - Energy Efficiency Standards] which are the State wide default code[s] requirements throughout California. These factors could influence the future load demands in the typical Residential occupancy.

FYI: A little history on California's Energy Conservation standards [CBC, CMC, CPC, CEC]:

The OPEC oil embargo of 1973 brought the U.S. into the infamous "Energy Crisis". Most everyone on this board remembers that situation. Also, most everyone here has heard of the Smog problem in Southern California.
These two factors were the primary influence for the California State Legislature to create the California Energy Commission in 1974, to deal with energy-related issues and adopt condervation standards for new buildings.

The Subchapters involved are:

Subchapter 1: General Provisions - All Occupancies.
Subchapter 2:Mandatory - Equipment Manufacture and installation of systems and equipment.
Subchapter 3: Mandatory - Mechanical systems and equipment.
Subchapter 4: Mandatory - Lighting systems and equipment.
Subchapter 5: Performance and prescriptive approaches [the calcs and compliance sheets themselves].
Subchapter 6: Special topics and repairs.

These standards apply to the "Envelope" [Framing, Glazing, Insulation and Foundation], Mechanical, Plumbing and Electrical Lighting for both Residential and Non-Residential occupancies.

The first generation of Title 24, part 6 was 1978 [2/78], which involved Residential occupancies (Including Hotels and High-Rise Apartments). The first generation Non-Residential followed soon after in 1978.
After this, newer compliance standards were introduced in 1983, 1984, 1987, 1988, 1992, 1995 and 1999 - with the latest revision this year [2001].

The goal in 1978 was to decrease the level of Electrical Power Consumption [KWHs] so that by 2009, the standards would save more energy than seven [7] average fossil fuel burning power plants could produce.

The last Energy Calc I did was under the 1999 standards. At that time, the amount of power saved, plus the reduction in consumed energy eliminated the need of around 6 average power plants! Smog levels and Acid rain was dramatically reduced.
All this got completely screwed from Deregulation!

Current Title 24 requirements for Residential Lighting calls for Energy Efficient lighting [40 lumens per watt minimum] used in Bathrooms and Kitchens [as "Primary" lighting source], plus for exterior lighting at exterior doors.
Insulation, Glazing and similar requirements increase the building's thermal conductivity resistance, which results in lower HVAC consumption.

Electric Resistance type heating used with HVAC systems is not allowed. Heating is either fuel [gas, etc.], or heat pumps.

Water heating for over 25 gallons may not be Resistance Electric. If Recirculating pumps are used, they must be controlled so they are "off" during non-use periods [not running while people are sleeping or away!].

For Non-Residential occupancies, there's a vast amount required for HVAC, Plumbing and Envelope.

For Lighting, there's 4 methods of calculation - but the maximum lighting power levels fall less than what you would see listed in the NEC article 220.
However, the NEC value is always used for calculations of service sizes, as covered in article 220 - unless the designed loads are higher than NEC minimums.

Example:
Commercial Bank Branch:
CEC [Title 24] maximum lighting power= 1.6 watts per square foot.
NEC load calcs= 3.5 VA per square foot [general lighting load].

The installed lighting equipment cannot exceed the maximum lighting power of 1.6 watts per square foot [actual and total consumed watts of lighting fixtures is figured instead of VA], but the service and feeders are calced using 3.5 VA per square foot.

Lighting equipment is influenced by these standards, so are lighting controls.

These standards will continue to increase in the future, so this will have resulting effects on new / remodel / addition projects and corresponding circuitry.


Well, I hope this message was enlightening to everyone.

Scott SET

P.S. edits to as many typo's and spelling errors I could find.
Iee R knot sutch ae gud spelor, axtuly mie spelyng sux!
[translation available upon request ]
S.E.T.

[This message has been edited by Scott35 (edited 10-08-2001).]

Glenn:

The smallest OCPD generally used in domestic panels is 5A for lighting circuits, the next fuse size being 15A, but C/Bs are also available in 10A. Many new ranges of breakers are actually 6A instead of 5A due to the adoption of European stadards.

Twin-&-earth (like Romex) 1 sq. mm can be run the same way as the larger sizes, with no special requirements.

All this got completely screwed from Deregulation!

It did???

Steve,

I say this because now California is racing to construct 15 new power plants - 4 or 5 of them already in operation, all due to the rolling blackouts and high prices per KWH / MWH to power generating companies.
These are the results we have been facing in California from Deregulation.

In 1999, the Energy Standards reduced KWH consumption equal to the output of 6 average fossil fuel burning power plants.
This considerably reduced the output power demands on power plants, loss of power [blackouts] to customers, pollution and acid rain, reliance on more fossil fuels, allowed more conservation of fossil fuels, even reduced U-239 waste disposal levels! [highly radioactive U-238 spent, or used fuel after reactions].
Now California is building more power plants which will no doubt be running at least 50% output load continuously.

That's what I mean by everything has been screwed by Deregulation.

Scott SET

Put an amp clamp on it for starters, and if the feeder wire isn't getting warm, thats a good sign. Its a place to start anyway.