NEC question re: resistance heating elements 1

[warrenelec]

good afternoon,

I have an electric resistance water boiler heater rated at 420 KW inside of an ASME vessel. Power is divided into 12 circuits drawing 44 amps each which is acceptable according to NEC 422.11(F) (circuits subdivided into < 48 amps/circuit - code listed below for reference). The catch is that within the same terminal housing the lead wires for the 12 circuits are wired into 3 power distribution blocks - approx. 175 amps/block. According to NEC 422.11 (F)(3):

"(3) Water Heaters and Steam Boilers. Water heaters and
steam boilers employing resistance-type immersion electric
heating elements contained in an ASME-rated and stamped
vessel or listed instantaneous water heaters shall be permitted
to be subdivided into circuits not exceeding 120 amperes
and protected at not more than 150 amperes."

My question: if the circuit breakers are only protecting the power input to the distribution blocks, is this a code violation? in other words, would this heater be considered a "3 circuit heater" that exceeds the maximum of 120 amps/circuit? all help is appreciated. Thanks!

NEC 422.11
(F) Electric Heating Appliances Employing Resistance-
Type Heating Elements Rated More Than 48 Amperes.
(1) Electric Heating Appliances. Electric heating appliances
employing resistance-type heating elements rated
more than 48 amperes, other than household appliances
with surface heating elements covered by 422.11(B), and
commercial-type heating appliances covered by 422.11(D),
shall have the heating elements subdivided. Each subdivided
load shall not exceed 48 amperes and shall be protected
at not more than 60 amperes.
These supplementary overcurrent protective devices
shall be (1) factory-installed within or on the heater enclosure
or provided as a separate assembly by the heater
manufacturer; (2) accessible; and (3) suitable for branchcircuit
protection.
The main conductors supplying these overcurrent protective
devices shall be considered branch-circuit conductors.

 

[alehman]

So it appears to be set up for three incoming 460V/3ph circuits?

I see your concern. Paragraph (F)(3) should apply and it seems like 120A is the maximum allowed. Have you talked to the manufacturer?



Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[warrenelec]

Yes, thats correct - 460/3 phase power. The specs for the unit were given to the manufacturer by our engineer who no longer works here. the unit was purchased 3 years ago and is to be installed now; we are unsure if this design is acceptable and need clarification to determine if this will require some sort of redesign

 

[TurbineGen]

I would not think that it would be a code violation if each of your individual heaters is 44 Amps and each individually protected as such from your distribution block. However if the elements are not individually protected from the distribution block (ie only the supplimentary breakers are used) then I would consider this a code violation. I agree with alehman that you should consult the manufacturer. 3 years ago this would fall under the 2005 NEC code, which still reads the same as your post.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[warrenelec]

TurbineGen,

Thanks for the input - thats what I was afraid of!