220.55 Calulating ranges

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Brice Pesterfield

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Wichiat Falls,Texas, USA
I am studying for my masters test and really cant figue out how to find the average number of range units between 2 phases when converting from single phase to three phase.

Question: Whats the demand load for twenty single phase 14 KW 208V ranges on a three phase service?
Step1: Determine the maximum number of ranges between any two phases....... 7 Units. Where did 7 come from. Its been a cram couple of weeks and maybe my brain is fried???
 
Brice Pesterfield said:
I am studying for my masters test and really cant figue out how to find the average number of range units between 2 phases when converting from single phase to three phase.

Question: Whats the demand load for twenty single phase 14 KW 208V ranges on a three phase service?
Step1: Determine the maximum number of ranges between any two phases....... 7 Units. Where did 7 come from. Its been a cram couple of weeks and maybe my brain is fried???
Click to expand...
You have 20 ranges to spread over three phases.
Naive arithmetic tells you that is only 6.66 ranges per phase.
Common sense tells you to split them up 7,7,6.
If you put all 20 on one phase you have zero on the other two, but that has a maximum number pet phase of 20.
 
Brice Pesterfield said:
I am studying for my masters test and really cant figue out how to find the average number of range units between 2 phases when converting from single phase to three phase.

Question: Whats the demand load for twenty single phase 14 KW 208V ranges on a three phase service?
Step1: Determine the maximum number of ranges between any two phases....... 7 Units. Where did 7 come from. Its been a cram couple of weeks and maybe my brain is fried???
Click to expand...
While the code uses the phrase "between any two phases" it really means between any two lines (i.e. across a phase). You have 20 ranges and 3 three phases: 20 ? 3 = 6.67... but you cannot have portions of a 1? range across more than one phase, so you have to round up to 7. Distribution would be 7, 7, and 6 ranges across the 3 different phases.
 
As stated in the info notes there is an example in annex D

220.55 Electric Cooking Appliances in Dwelling Units
and Household Cooking Appliances Used in Instructional
Programs. The load for household electric ranges,
wall-mounted ovens, counter-mounted cooking units, and
other household cooking appliances individually rated in
excess of 13?4 kW shall be permitted to be calculated in
accordance with Table 220.55. Kilovolt-amperes (kVA)
shall be considered equivalent to kilowatts (kW) for loads
calculated under this section.
Where two or more single-phase ranges are supplied by
a 3-phase, 4-wire feeder or service, the total load shall be
calculated on the basis of twice the maximum number connected
between any two phases.
Informational Note No. 1: See the examples in Informative
Annex D.
Informational Note No. 2: See Table 220.56 for commercial
cooking equipment.
Click to expand...
 
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