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NEC Demand Factor Dilemma #95063
08/24/05 10:26 PM
08/24/05 10:26 PM
rwilsond  Offline OP
New Member
Joined: Aug 2005
Posts: 5
Rancho Santa Margarita, CA
For a large existing hotel two new circuits will be added to each guest room to eliminate nuisance CB trips caused by too many coffee pots and hairdryers. These new circuits will feed only one receptacle each per room (one for a hairdryer & one for a coffee pot). New panels & transformers will be added to accommodate only these new circuits. Of course, overall, only a fraction of the coffee pots & hairdryers would operate simultaneously, at any given time (short of a vast conspiracy conducted by the quests). Using connected load data to size distribution equipment and feeders results in oversizing, simply to suit the NEC.

Is there anything such as a demand factor that could be applied to the panel, transformer, and feeder sizes that would be code-compliant?

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Re: NEC Demand Factor Dilemma #95064
08/25/05 08:56 PM
08/25/05 08:56 PM
George Little  Offline
Joined: Jan 2004
Posts: 1,493
Michigan USA
I would use the sample for multi-family dwelling. Probably somewhat of an over kill but there's nothing else I can find that comes close.

George Little
Re: NEC Demand Factor Dilemma #95065
08/25/05 09:57 PM
08/25/05 09:57 PM
gfretwell  Offline

Joined: Jul 2004
Posts: 9,338
I bet the hotel could give you a good guess about their guest's habits. You have to understand the demand will be somewhat loaded up at "get up and get out" time in the morning, just like the water pressure in the shower.
Evening hair dryer use will have a lot more diversity.
They do have a pretty good idea how much coffee they use tho.

Greg Fretwell
Re: NEC Demand Factor Dilemma #95066
08/26/05 07:04 AM
08/26/05 07:04 AM
Joe Tedesco  Offline
Joined: Oct 2000
Posts: 2,749
Boston, Massachusetts USA
Look here too 2005 NEC: 220.14(J) Dwelling Occupancies. In one-family, two-family, and
multifamily dwellings and in guest rooms or guest suites of
hotels and motels, the outlets specified in (J)(1), (J)(2), and
(J)(3) are included in the general lighting load calculations
of 220.12. No additional load calculations shall be required
for such outlets.

(1) All general-use receptacle outlets of 20-ampere rating
or less, including receptacles connected to the circuits
in 210.11(C)(3)
(2) The receptacle outlets specified in 210.52(E) and (G)
(3) The lighting outlets specified in 210.70(A) and (B)

Joe Tedesco, NEC Consultant
Re: NEC Demand Factor Dilemma #95067
08/26/05 12:29 PM
08/26/05 12:29 PM
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
(Warning: talking out of my left *censored* and well outside of my experience. Take the below thoughts and evaluate them using your own experience and knowledge!)

Which suggests that no additional capacity is needed for these new receptacles, since you are not adding any square footage to the rooms. With no additional capacity required, one could get away with 0kVA transformers and panels *grins*

Actually, as the original post is worded, I don't think that the NEC provides a useful method of calculating demand for these circuits.

The rooms are already fed with circuits, and presumably these feeders have already been properly sized for the square footage of the room. These new circuits are going to be fed from a separate panel with a separate transformer. How many square feet of room area do you use to size these new feeds? The full size of the room? 1/2 of the room? A weighted average of room area based on number of receptacles?

IMHO the approach should be to feed these new circuits from the same panels that currently feed the rooms. The feeders to these panels should already be sufficient for the square footage of the rooms. Redoing the demand calculation for the entire rooms using the current NEC would clue you in should the feeders need to be increased in size, but I bet that this isn't a problem.

Presumably you would need to use additional panels to make room for the additional circuits, so the only thing left in question is the size of the feeders to these new panels. The panel rating itself is not going to be in question; it is almost certainly going to be set by the number of individual branch circuits (though I bet if you hunt, you will find someone making a 30A 42 circuit panel [Linked Image] )

Now you've reduced the unknowns to a small portion of the job: the feeders to the new panels. Remember I am making the claim that no new transformers would be needed, since the transformers and feeders for the original panels were based upon the demand calculations for the entire square footage of the rooms. At this point you could oversize these feeders at minimal cost, perhaps even saving some money. For example, if the feeders are sized to match the feeders going into the source panel, then you probably eliminate a step of OCPD.

No new calculations (just repeating the old calculations), NEC compliant, without adding lots of what is almost certainly excess capacity.


Re: NEC Demand Factor Dilemma #95068
08/26/05 12:38 PM
08/26/05 12:38 PM
George  Offline
Joined: Feb 2002
Posts: 375
I am going to agree with the previous posts. There is no need to do a calculation.

Further I suspect that 20amp breakers were tripping. By adding more circuits the problem will move to teh mains. I don't think the mains will be tripping.

Re: NEC Demand Factor Dilemma #95069
08/26/05 04:56 PM
08/26/05 04:56 PM
George Little  Offline
Joined: Jan 2004
Posts: 1,493
Michigan USA
I think we need to start thinking about all the loads that are being added to some of these services. I agree with the square foot approach if we have a barebones electrical load. But, gradually we will have to change. The IRC (International Residential Code) now has us calculating all 20a appliance circuits at 1500va. Previously we only counted 2 small appliance kitchen circuits and 1 laundry circuit. (Assume only one kitchen) Under the '03 IRC if they have 5 small appliance circuits- they count all of them at 1500va. each. I wouldn't be surprised if eventually we will be counting the 20a. bathroom circuit and not just lumping it in with the square foot approach.

George Little
Re: NEC Demand Factor Dilemma #95070
08/26/05 05:05 PM
08/26/05 05:05 PM
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
George have you seen the mains tripping on any structure built since the 70s?

I have been told by POCO engineers that the POCO usually figures the actual load as 50% of the NECs calculations. [Linked Image]

Bob Badger
Construction & Maintenance Electrician
Re: NEC Demand Factor Dilemma #95071
08/26/05 05:22 PM
08/26/05 05:22 PM
George Little  Offline
Joined: Jan 2004
Posts: 1,493
Michigan USA
Nope Bob, I'd have to say that I haven't but I was trying to use my crystal ball and based my post on what the IRC has done.

I have heard lower that 50% demand and I've witnessed much higher when I've metered some myself.

I guess it could go either way. But I doubt that it will decrease.

George Little


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