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?!
Re: When is a system considered overloaded?#78576 10/05/0105:55 PM10/05/0105:55 PM
The unit values herein are based on minimum load conditions and 100 percent power factor, and may not provide sufficient capacity for the installation contemplated
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.
[This message has been edited by Bill Addiss (edited 10-05-2001).]
Re: When is a system considered overloaded?#78578 10/06/0101:28 AM10/06/0101:28 AM
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 " 35 " Thompson Just Say NO To Green Eggs And Ham!
Re: When is a system considered overloaded?#78579 10/06/0102:23 AM10/06/0102:23 AM
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?
Re: When is a system considered overloaded?#78580 10/06/0104:06 PM10/06/0104:06 PM
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).