Panel Demand

[brother]

Im just curious and you can remain anonomous , But when you are doing a commercial remodle job or just adding more circuits do you actually do a panel calculation/demand load?? the reason I asked is because Ive seen too many times where alot of 'electricians' are not doing that. They see some spares and just add on. and if there arent any spares in the panel then they just add another panel and put another breaker in that old panel and move some of the circuits to that one. I know in reality in MOST cases this shouldnt be a problem (ie..tripping breakers) even when you dont do the calc and you see spares. However, on at least 1 rare occasion Ive heard of it tripping the main. Basically a 225 panel with 32 circuits and a guy just added 2 more, it was a 42 circuit panel. Most were supposely receptacles etc. but after about a day or so it tripped the main. Come to find out just too much load.

Just recently a job at a hospital a contractor was asking about the 'demand factor' (do they(hosptial staff) keep track as we SHOULD) on the job he was doing a remodle for one of the offices. I just referred him to the 'people in charge' and the panel schedules/prints SHOULD be accurate.

So do you even bother with it (demand)?? What about residential, Ive never heard of it happening in houses, but im sure it is possible.

 

[Doug S.]

Funny you ask.
The last time I dropped some circuits in a panel I did took a pretty good look...

I was adding a Fridge, Disposal, DW, Insta-Hot (water) and Microwave outlets to a PB that fed the conference room for the bosses bosses offices. =)
I don't want to be called because the lights went off mid-power point presentation...

Funny what motivates us eh?

Regards,
Doug S.

 

[brother]

Funny you ask.
The last time I dropped some circuits in a panel I did took a pretty good look...

I was adding a Fridge, Disposal, DW, Insta-Hot (water) and Microwave outlets to a PB that fed the conference room for the bosses bosses offices. =)
I don't want to be called because the lights went off mid-power point presentation...

Funny what motivates us eh?

Regards,
Doug S.

Yes i see what motivates. would have been interesting to see what wouldve happen if that main had tripped. lol I was just doing a review and in the commercial its clear that the receptacles cannot be added in the general lighting load calc.

 

[kbsparky]

On residential applications if we are adding substantial loads, then yes, we will perform a new load calculation on the entire building/service.

Loads that we consider substantial can include, but are not limited to:

Heat pumps or A/C units
Hot Tubs
Gas to electric water heater
"on demand" water heater
Supplemental electric heaters
Gas to electric range
Pools
Large additions, including separate or additional kitchens
2nd clothes dryer
etc.


I usually won't do a load calc for a couple of new receptacle circuits for home theater or office, unless it looks like they are already loaded to max ... :roll:

 

[Lcdrwalker]

Rather than doing a load calc, I will contact the POCO for a billing history. From there I can get peak demand (15 min window) and average usage. For resi work, aquick look for heaters, AC, range etc., will give a good guestimate.

 

[iwire]

The NEC allows a few methods to figure it out.

• Do the calculations
  
• Put a recording meter on the supply
  
• Use past billing history

See 220.87 in the 2005 NEC.

 

[Jim W in Tampa]

On commercial work in old buildings your very unlikely to find the real load. You might even find circuits that are not being used anymore and caped off. At best your guessing for anything other than lighting circuits. The time of year could play big factor. Here in Tampa its cooling off so RTU is not running as often but in July they might never shut off. Best advice i can give is take a reading with all lights on and all offices with computers turned on. Then simply warn customer of what could happen. With an office that can't afford the main breaker to trip suggest a larger service and then your a-- is covered. Even if you install larger service the poco might not increase there side untill its too late. The past demand is just that. With things changing and being added there simply is no perfect way. Recently did an icecream distributing company. Take that reading at 9 am to 2 pm and it be very low. Now come back at 4pm and 20 trucks plug in for overnight and you have a huge load.

 

[Karl H]

Before adding a new panel or "Heavy Loads" to an existing system.
We use a "Power-Sight" meter to monitor the existing load for
a few days. That tells us if the system can handle X amount
of new load.

 

[sparkchaser1961]

On commercial work in old buildings your very unlikely to find the real load. You might even find circuits that are not being used anymore and caped off. At best your guessing for anything other than lighting circuits. The time of year could play big factor. Here in Tampa its cooling off so RTU is not running as often but in July they might never shut off. Best advice i can give is take a reading with all lights on and all offices with computers turned on. Then simply warn customer of what could happen. With an office that can't afford the main breaker to trip suggest a larger service and then your a-- is covered. Even if you install larger service the poco might not increase there side untill its too late. The past demand is just that. With things changing and being added there simply is no perfect way. Recently did an icecream distributing company. Take that reading at 9 am to 2 pm and it be very low. Now come back at 4pm and 20 trucks plug in for overnight and you have a huge load.

Thanks. good advise

 

[acrwc10]

I have only once seen a house loaded to the point it tripped the 100 amp main. I had all electric heat and cooking appliances, this was a very large property with a pool, hot tub, and they had a ship load of out side lighting also. I was told it tripped when they had a party and everything was on. If a load calc had been done it must have been before the outside things were added.

 

[charlie]

. . . I will contact the POCO for a billing history. . .

That will only work if they have a demand and reactive meter. The newer electronic meters will read demand and reactive but we have no use for that data for smaller commercial services and do not record the data. :smile: