Calculating Water Heaters

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patsfan51

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Hi,

I was wondering if someone could help me with clearing up some confusion on water heaters.
Art. 422.11 (E)(3) says to size overcurrent protection at 150%.
Art. 422.13 FPN (120gal or more) and 422.10 says to size branch circuits at 125% based on cont. load.

What are the guide lines here?
I'm Studying for my jmans exam using mike holts book. In this book there are two example ?'s.
1-- What is the max size overcurrent protection for a 45kw 240v water heater--->4500/240=18.75x1.50=28a next size up 30a

2-- What is the calculated load for the conductor sizing and overcurrent protection for a 45kw, 230v water heater?-->4500/230=20ax1.25=25a

When do I use 150% and when do I use 125%? Is it in the question "what is the max..." and "what is the load or min. size..."?

thanks
 
422.11(E)(3) does not say to size the ocpd at 150%. It says if the circuit supplies a single non-motor-operated appliance, the ocpd cannot exceed 1) what is marked on the appliance, 2) not exceed 20A if the appliance max ocpd is not marked but rated 13.3A or less, and 3) not exceed 150% when appliance max ocpd is not marked and rated over 13.3... allowing next size up if 150% does not correspomd to standard rating.

In other words, this puts a cap on the ocpd and branch circuit ampacity.

The other sections and notes refer to minimum ocpd and branch circuit ampacities.

Altogether they mean it has to be at least sized to the minimum yet cannot exceed the maximum.

As for what is the load, it is always the rated current times the applied nominal voltage expressed as VA, kVA, MVA, and so forth. In cases where there is no significant power factor involved, such as resistance-element type water heaters, the load can be referred to interchangeably with wattage of an equal value. In instances where the code has you multiply by 125% for continuous load, this is always a calculation for the determination of ocpd and ampacity, but does not change the actual load value.
 
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