Branch circuit sizing

Grouch

Grouch

Senior Member
Location
New York, NY
When running a load calculation to calculate a service or feeder size, I use the nameplate values whether I'm following the standard or optional calculations of Article 220.

For example, I'll use the nameplate wattage of a hot water heater, listed at 240 volts.

If I have, which I always do, a 208 volt service, where in the code does it state that I can downsize the branch breaker for the water heater due to connecting to a 208 volt supply? At 208 volts, the wattage of the water heater (rated at 240 volts) will be derated to 75%. And the current will be derated to about 86%.
 
With the standard method some loads, such as dryers, require you use a specific rating even if the nameplate is lower.
 
augie47 said:
With the standard method some loads, such as dryers, require you use a specific rating even if the nameplate is lower.
Click to expand...
Understood, for sizing the service or a feeder.

But when sizing the branch breaker, from a 208 volt supply, feeding a 240 volt rated hot water heater, where in the NEC does it state I'm allowed to downsize the breaker based on the supply voltage? The wattage and current to the water heater will be decreased.
 
Grouch said:
But when sizing the branch breaker, from a 208 volt supply, feeding a 240 volt rated hot water heater, where in the NEC does it state I'm allowed to downsize the breaker based on the supply voltage? The wattage and current to the water heater will be decreased.
Click to expand...
220.14(A) (in Part II for Branch Circuits) says "An outlet for a specific appliance or other load not covered in 220.14(B) through (L) shall be calculated based on the ampere rating of the appliance or load served."

So Article 220 just tells you to determine the ampere rating; it doesn't specify a "nameplate" rating. Surely the water heater documentation will include an ampere rating at 208V, even if the nameplate does not.

Cheers, Wayne
 
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