Box fill calculations

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chocho77

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I have a 3 phase heater(Delta connected). The leads from the heater are gong to a round box and are connected to a 4 prong terminal block( 3 phases+ground). These wires are #10AWG.
The feeder wires, coming from the disconnect, are #8AWG. I'm checking the box fill:
--3 feeder wires #8 x 3 cu.in.=9cu.in.
--3 heater wires #10x2.5 cu.in.=7.5cu.in.
--1 ground wire#10x2.5 cu.in.=2.5cu.in.
--terminal block - DOUBLE the largest conductor(#8AWG)=2x3cu.in.= 6cu.in.
The minimum size of the box would be 25cu.in.? Am I right?
I would appreciate your feedback!:)
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
While there is common sense that the terminal block is using volume, it is not included in the list of items to be calculated by 314.16.

From the OP description, this " round box and . . . 4 prong terminal block" sounds like it is part of the Utilization Equipment, that is, it is not part of the Premises Wiring (System). As part of the heater manufacturer's equipment, the box volume falls outside of the National Electrical Code.
 

chocho77

Member
While there is common sense that the terminal block is using volume, it is not included in the list of items to be calculated by 314.16.

From the OP description, this " round box and . . . 4 prong terminal block" sounds like it is part of the Utilization Equipment, that is, it is not part of the Premises Wiring (System). As part of the heater manufacturer's equipment, the box volume falls outside of the National Electrical Code.

Let me clarify : the round box is explosion proof and the terminal block is mounted inside.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
Let me clarify : the round box is explosion proof and the terminal block is mounted inside.

:? Is it supplied by the installer of the supply wiring to the heater, or is the box part of the manufactured assembly that is the heater (that is, did the manufacturer attach the box in the manufacturer's factory)?
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
. . . how to include it in the calculations because is using volume?
My point is that I don't think the NEC will help here, other than to point to 90.1(B) and say, on the strength of your character, this is one of the "convenience and adequacy" pinch points and that one can always do more than the minimum required by the Code.

However, if you do replace part of the manufactured equipment in order to have an explosion proof j-box with more volume, aren't you now altering a listed assembly, a manufacturer's listed equipment?
 

kwired

Electron manager
Location
NE Nebraska
I agree - by the definition the terminal block is not a device, but how to include it in the calculations because is using volume?

Why is it not a device?

I still agree with Al H. If this is supplied with the equipment, then it is more likely covered by listing of the equipment more so than by NEC.
 

chocho77

Member
My point is that I don't think the NEC will help here, other than to point to 90.1(B) and say, on the strength of your character, this is one of the "convenience and adequacy" pinch points and that one can always do more than the minimum required by the Code.

However, if you do replace part of the manufactured equipment in order to have an explosion proof j-box with more volume, aren't you now altering a listed assembly, a manufacturer's listed equipment?

The heater manufacture provided 19cu.in. round explosion proof box with the heater; the installer(another company - UL certified) added controls, disconnect, etc. and made up an heater unit explosion proof; and we,the buyer of the unit, are trying to figure out if 19cu.in. are enough?:?
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
The heater manufacture provided 19cu.in. round explosion proof box with the heater; the installer(another company - UL certified) added controls, disconnect, etc. and made up an heater unit explosion proof; and we,the buyer of the unit, are trying to figure out if 19cu.in. are enough?:?
The answer doesn't lie within the National Electrical Code (NEC), rather the Nationally Recognized Testing Laboratory testing requirements and other manufacturer and engineering standards that relate to this specific heater / box.

The box is outside of the purview of the NEC, as it is not part of the Premises Wiring (System), which is defined in Article 100 of the NEC.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
IMO, if the ground bar fits the description of 314.16(B)(4) then the calc is correct. We can argue it all day but if I were doing it I would have that extra room. Seems logical to deduct for the bar if it is wider than a device box (2 in)....
 
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