Over-current rating Heating load

Status
Not open for further replies.
B

Bea

Senior Member
Resistive Boiler supplies in floor heating for dwelling.
Boiler is rate 240 volt 1 phase 16KW.
No minimum branch circuit or maximum over-current.
Calculate load as continues as required in 424.3 125% 83 amps.
Load is subdivided and protect be 50 amp over-current devices.
Question is can provide single circuit # 2 CU THHN protect 100 amp breaker.
Or must OPC device be limited to the 125% calculation 90 amp
 
mdshunk said:
These two 50 amp breakers... are they factory installed? Seems like they should have been 45's.
Click to expand...

They are

Marc
If you refer to 422.11 this is compliant
 
Bea said:
Question is can provide single circuit # 2 CU THHN protect 100 amp breaker.
Or must OPC device be limited to the 125% calculation 90 amp
Click to expand...

I have always used the 125% as my basis for the OCP. I am not sure if you can go higher but I have never had a problem with my install. I vote 90 amp---
 
Just curious: if the load has its own integrated panel, and is divided into two 50a protected circuits, why must the single feeder's OCPD need to further protect the pair of breakers? I would think that the only concern would be the usual protection of the feeder.
 
LarryFine said:
Just curious: if the load has its own integrated panel, and is divided into two 50a protected circuits, why must the single feeder's OCPD need to further protect the pair of breakers? I would think that the only concern would be the usual protection of the feeder.
Click to expand...

You are correct and I agree Thank You
 
LarryFine said:
Just curious: if the load has its own integrated panel, and is divided into two 50a protected circuits, why must the single feeder's OCPD need to further protect the pair of breakers? I would think that the only concern would be the usual protection of the feeder.
Click to expand...
That's what I was starting to think, until I realized that each factory breaker is a wee bit oversized to suit the NEC, which is why I voted for a 90.
 
mdshunk said:
That's what I was starting to think, until I realized that each factory breaker is a wee bit oversized to suit the NEC, which is why I voted for a 90.
Click to expand...
But, as long as each 50a breaker is compliant in its sizing, and since they are the last OCPD's in the supply, the feeder's OCPD should not be required to act as additional protection for the entire load; it just needs to protect the feeder.

Let's say we're supplying a 20Kw residential electric furnace (such as back-up for a heat pump), and the furnace contains a pair of 60a breakers (or let's even say it doesn't, and we mount a sub-panel with a pair of 60a breakers nearby).

Are we also required to size the feeder conductors to the furnace (or to the sub-panel) to redundantly protect the entire furnace as a single load? I'd say the two 60a breakers could be supplied from any compliantly-protected feeder.
 
Ahh Excuse moi for a min but pretty close to this topic.


I know i did read somewhere in the NEC that the heating equment must have max of 42 or 45 amp and any heating load must be subdivied to mutli breaker per factory set up ??

Like this example

45 amp @ 480 volts 3 ? supply 37.4KW but when you get larger unit like this example

90 amp @ 480 volts 3? supply 74.8 KW

the ditto with 208 volt system
45 amp @208 volts 3? supply 16.2 KW
90 amp @208 volts 3? supply 32.4 KW

so if the code say each heating section can not have more than 45 amp per section ??

i know i did see the single phase verison example on this thread so i just want to understand this question little more clearer

Thanks

Marc
 
frenchelectrican said:
Ahh Excuse moi for a min but pretty close to this topic.


I know i did read somewhere in the NEC that the heating equment must have max of 42 or 45 amp and any heating load must be subdivied to mutli breaker per factory set up ??

Like this example

45 amp @ 480 volts 3 ? supply 37.4KW but when you get larger unit like this example

90 amp @ 480 volts 3? supply 74.8 KW

the ditto with 208 volt system
45 amp @208 volts 3? supply 16.2 KW
90 amp @208 volts 3? supply 32.4 KW

so if the code say each heating section can not have more than 45 amp per section ??

i know i did see the single phase verison example on this thread so i just want to understand this question little more clearer

Thanks

Marc
Click to expand...
I have see PLENTY of heating units with 60's in them.
 
frenchelectrican said:
that what i thought so i did read that somewhere in the NEC but not sure where it was stated
Click to expand...

From the '02 NEC:

424.22 Overcurrent Protection.
(A) Branch-Circuit Devices. Electric space-heating equipment, other than such motor-operated equipment as required by Articles 430 and 440 to have additional overcurrent protection, shall be permitted to be protected against overcurrent where supplied by one of the branch circuits in Article 210.
(B) Resistance Elements. Resistance-type heating elements in electric space-heating equipment shall be protected at not more than 60 amperes. Equipment rated more than 48 amperes and employing such elements shall have the heating elements subdivided, and each subdivided load shall not exceed 48 amperes. Where a subdivided load is less than 48 amperes, the rating of the supplementary overcurrent protective device shall comply with 424.3(B). A boiler employing resistance-type immersion heating elements contained in an ASME rated and stamped vessel shall be permitted to comply with 424.72(A).
(C) Overcurrent Protective Devices. The supplementary overcurrent protective devices for the subdivided loads specified in 424.22(B) shall be (1) factory-installed within or on the heater enclosure or supplied for use with the heater as a separate assembly by the heater manufacturer; (2) accessible, but shall not be required to be readily accessible; and (3) suitable for branch-circuit protection.
 
Status
Not open for further replies.
Top