Home Economics Classroom Range Calculation

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9-10 KW 240v ranges in a home economics classroom, fed from a 120/ 208v feeder.

What is the calculated load for the ranges on this feeder? Looking for amps?

The answer provided is 88 Amps. Referencing Table 220.56.

I took the total of all 9 ranges kw at 10 kw, added them together and got 90000.

Then factored in the derating per Table 220.56 = 90000 VA X .65 = 58500 VA

Then I take 58,600 VA. and reduce to amperage by Dividing 58500/208 X1.732 (360)= 162.5 Amps

What am I missing ... As always, any assist would be appreciated.
 
Location
OBX, NC.
I have tried several different variations of calculating this problem... Any help is appreciated.. I've been told not to even worry about this problem because it wont be on the test. regardless if I ever need to understand it it would be nice to have a clearer and accurate view. Help a brother out and share if you have know how on this..
 

GoldDigger

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Location
Placerville, CA, USA
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One thing that makes this situation interesting is the expectation that all of the ranges and ovens will be used at exactly the same time during class, negating any potential diversity factor. Although since this is not a dwelling situation, I am not sure the NEC would try to include any such factor in the first place.
 

jumper

Senior Member
One thing that makes this situation interesting is the expectation that all of the ranges and ovens will be used at exactly the same time during class, negating any potential diversity factor. Although since this is not a dwelling situation, I am not sure the NEC would try to include any such factor in the first place.

Note 5 from the table:

5. This table shall also apply to household cooking appliances rated over 13?4 kW and used in instructional programs.
 

david luchini

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Location
Connecticut
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Engineer
They used 220.55, not 220.56.

9 units, maximum connected between any two phases = 3. Twice 3 = 6.

Demand for 6 units from T220.55 = 21,000VA, or 10,500VA per phase.

Equivalent 3 phase load = 31,500VA

31,500VA @ 208V, 3ph = 87.4 Amps.
 

Smart $

Esteemed Member
Location
Ohio
They used 220.55, not 220.56.

9 units, maximum connected between any two phases = 3. Twice 3 = 6.

Demand for 6 units from T220.55 = 21,000VA, or 10,500VA per phase.

Equivalent 3 phase load = 31,500VA

31,500VA @ 208V, 3ph = 87.4 Amps.
That last part is a little tricky (author must be a bit devious... or overly meticulous).

208V nominal is actually 120V ? sqrt(3)... and 208V ? sqrt(3) is actually 120V ? 3, or 360.

31,500VA ? 360 = 87.5A
 

GoldDigger

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Retired PV System Designer
Note 5 from the table:
Which is really interesting, because section 220.55 unconditionally sets out the occasions when you can use table 220.55, while section 220.56 sets out the conditions where you MUST use table 220.56.
If there were a footnote to table 220.56 directing you back to table 220.55 for instructional programs, that would have a logical force behind it.
But trying to make that exception as a footnote to 220.55, which you are never told to look at, does not make any sense.
 

Smart $

Esteemed Member
Location
Ohio
Which is really interesting, because section 220.55 unconditionally sets out the occasions when you can use table 220.55, while section 220.56 sets out the conditions where you MUST use table 220.56.
If there were a footnote to table 220.56 directing you back to table 220.55 for instructional programs, that would have a logical force behind it.
But trying to make that exception as a footnote to 220.55, which you are never told to look at, does not make any sense.
Changed in 2014...

220.55 Electric Cooking Appliances in Dwelling Units and Household Cooking Appliances Used in Instructional Programs.
 
Location
OBX, NC.
I Need to clarify.

I Need to clarify.

They used 220.55, not 220.56.

9 units, maximum connected between any two phases = 3. Twice 3 = 6.

Demand for 6 units from T220.55 = 21,000VA, or 10,500VA per phase.

Equivalent 3 phase load = 31,500VA

31,500VA @ 208V, 3ph = 87.4 Amps.

Awesome that you have been able to share how this works out. I have a pic of 3 Phase orientation on a pane,l equally splitting up the loads. Is this what you mean by 9 units, Maximum 3 on any phase. I see the correlation but I wind up with phase A with four connections. I don't want to complicate the situation but how to view phase A and its extra phase connection. Say forget about it and I'll forget about... But if there is more to understand, then insert axe into me brain.. 20140903_182003.jpg
 

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Smart $

Esteemed Member
Location
Ohio
Awesome that you have been able to share how this works out. I have a pic of 3 Phase orientation on a pane,l equally splitting up the loads. Is this what you mean by 9 units, Maximum 3 on any phase. I see the correlation but I wind up with phase A with four connections. I don't want to complicate the situation but how to view phase A and its extra phase connection. Say forget about it and I'll forget about... But if there is more to understand, then insert axe into me brain.
Technically, a phase is line to line, not the single line connection. By IEEE convention for a 3? system, ?A is Line C to Line A, ?B is Line A to Line B, and ?C is Line B to Line C.

Confusion sets in when you have things like panel schedules that mark the Lines as ?.
 

kwired

Electron manager
Location
NE Nebraska
Awesome that you have been able to share how this works out. I have a pic of 3 Phase orientation on a pane,l equally splitting up the loads. Is this what you mean by 9 units, Maximum 3 on any phase. I see the correlation but I wind up with phase A with four connections. I don't want to complicate the situation but how to view phase A and its extra phase connection. Say forget about it and I'll forget about... But if there is more to understand, then insert axe into me brain.. View attachment 10871
Leave panel space #2 open and move everything on right side down one slot and you have equal number of connections to each line.

Bottom utilized space on left side was phase A so if you utilize phase B as the first space when you jump to the other side you continue with the connection pattern that was started on the left side, so you could also have started with space 10 and it would work out - if you didn't run out of spaces as you go down.
 

Smart $

Esteemed Member
Location
Ohio
I don't see any conditions set out in 220.56 that says you MUST use table 220.56.

220.56 says it shall be permissible to use T220.56.
Even so, the issue is being able to use Table 220.55. While the footnote may be applicable once there, in order to use it, one must be directed there through the text of a section... and I'm not aware of any section which says you can use Table 220.55 for instructional programs prior to the 2014 edition.
 
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