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#89883 - 10/22/04 11:50 AM Water Heater Load Calc
mustangelectric Offline

Registered: 02/08/04
Posts: 496
Loc: Bentonville, AR
Do I just count the first water heater or the first 1500W of a system with 2 water heaters on line?

Load analysis on plans shows 1500 W and DF of 1.0 which gives a load of 1500W but the load shown on the panel schedule is 2 water heaters using 1750 each leg of 208 single phase service..

i think the plans are wrong..

thanks for any replies.



[This message has been edited by mustangelectric (edited 10-22-2004).]
Electricity has no respect for ignorance!

2014 / 2011 NEC & Related Books and Study Guides
#89884 - 10/22/04 05:39 PM Re: Water Heater Load Calc
Scott35 Offline

Broom Pusher and

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Hi Mustangelectric;

From what is described, it appears the Load calcs for the 2 Water Heaters were figured as Non-Coincidental Loads (is this possible?), and the Nameplate rating of the higher load (in this case both loads are equally listed as 1500 Watts @ 208 VAC), is figured at 100%, where the usual load is expected to be continuous at full Nameplate rating
(I think this would be found in Table 220-30 of the '99 NEC, or possibly the '96 NEC... not sure, but I do remember it under Optional Calcs section of Article 220, and pertains to Electric Thermal Storage Equipment).

If this is the case, then it would be permissible to do the Load Calculation for the Water Heaters this way, and compile the data to the complete Service / Feeder Load Summary.

The Panel Schedule will reflect the actual Loads of each Circuit, normally in Volt-Amp values, and will include an LCL adder for Continuous Loads - whereas the LCL may not be included within the Load Summary, as described above.

A debate could be made regarding if a Water Heater - under normal operating circumstances - would be a Continuous Load or not.
Would one (or could one) draw the rated 1500 VA for 180 minutes and longer?
I used to view them as Non-LCL, until reality clicked in; whereas a Storage Tank Water Heater may easily - either by intentional use or mistake / accident - be drawing Nameplate rating for 3 Hours and above.
One scenario would be high capacity Tanks, and the usage was intentional, such as:

    [*]20 minutes of Hot Water use to wash dishes,

    [*] Immediately afterwards, someone takes a 20 minute shower,

    [*] Another person fills up a Bath Tub after person finishes shower,

    [*] Washing Machine is kicked in, doing extra large load and warm water use,

    [*] etc....

All these instances occuring one after another, barely giving the storage tank enough time to actually store hot water.

As for the mistake / accidental scenario, someone leaves the hot water running and just takes off (short term memory problem, I would guess!), leaving the water flowing until someone notices it and puts a stop to the rampant waste.
This could also be from an accidential situation, where hot water pipes failed and began leaking massive amounts of hot water.
Mr. Gopher and Mr. Rat may have chewed through the pipe, causing a leak, or they may have knocked over something (coupled with a "Domino-Effect" like falling of objects), which caused considerable damage to the hot water pipe(s), therefore allowing near-full volume output until someone comes along to end the chaos.

Now looking at the Panel Schedule's listed Load of 1750 VA, this falls short of what the rated 1500 VA Load + LCL should actually be!
It would be 1875 VA listed in the Cells of the Panel Schedule for the Water Heater Circuits.
Water Heater #1... let's say Circuits 18/20;
ØA Cell = 1875 VA for Circuit 18,
ØB Cell = 1875 VA for Circuit 20.

Water Heater #2... let's say Circuits 25/27;
ØA Cell = 1875 VA for Circuit 25,
ØB Cell = 1875 VA for Circuit 27.

Not sure where the 1750 came from, as it would be 116.667% of the rated load VA. Anyone know whazzup with this?
Kind of looks like a "Guesstamation" thing to me...

BTW, feel free to substitute Watts for Volt-Amps in this case - since the Load is very much pure Resistance. I am "Programmed" to view Loads as Volt-Amps, especially when Panel Schedules are described!
Not that it is correct, only that it's default to me. Trying to go against this train of thought results in "Mental Blue Screens" and Logical Paradox problems, which requires someone to manually restart me. ....

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!


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