220.55 question

[haitham]

regarding household cooking appliances section 220.55 :
wanna understand this sentence :
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.

and i have this question too about the correct method of the following two ,to get max demand according to column c ,table 220.55 :
it it is required to calculate the max.demand of 23-single phase ranges each unit rating 15kw ,supplied by 3 ph system .

1 - the max demand will be = 39.5 kw at 23 unit .
this mean every phase will supply around 39.5/3= 13 kw .

2- but if the calculation use the units per single phase 23/3=8 unit
the max demand=24.5 per phase at 8 unit.
and max demand for all phases will be 24.5*3= 73.5 kw .


so which method is the correct one ?

 

[Dennis Alwon]

From Annex D ex 5 (D)(a)-- does this help

Minimum Size Feeders Required from Service Equipment to Meter
Bank (For 20 Dwelling Units — 10 with Ranges)
For 208Y/120-V, 3-phase, 4-wire system,
Ranges: Maximum number between any two phase legs = 4
2 × 4 = 8.
Table 220.55 demand = 23,000 VA
Per phase demand = 23,000 VA ÷ 2 = 11,500 VA
Equivalent 3-phase load = 34,500 VA
Net Calculated Load (total):
40,590 VA + 34,500 VA = 75,090 VA
75,090 VA ÷ (208 V)(1.732) = 208 A

 

[kwired]

I looked at this for a while not making any sense whatsoever out of it at first. But then if you think about it table 220.55 demand factors were designed with single phase supply in mind. The doubling of the maximum number of ranges connected between any two phases is a method of adjusting the demand factor for three phase supplies. It really would be correct ratio if the factor were 1.732 instead of 2 but they must want to be a little conservative for some reason and went with 2, I think.

If you had 10 ranges balances as best as possible across three phases like in the example Dennis posted, you would have four connections to A, three to B, three to C, (or at least one of the lines would have 4 connections, if it wasn't A). What is different here is the load diversity used to come up with the demand factor is no longer 10 ranges on a single phase line with all 10 lines connecting to A as well as B. Instead there are 7 ranges connecting to A, 7 to B, and 6 to C, but there is not 20 ranges though there is a third phase sharing some of the load but at a different load diversity level then if all were on a single phase line. I don't know exactly how that load diversity level is determined but can see that it would likely be different.

That is my analysis anyway of why it is what it is, feel free to shoot that idea down or fill in what I may be missing though.

 

[haitham]

ok answering this question can help,


according to the rule mentioned above and your comments Dennis Alwon,kwired

the ranges connection to the supply have to be like 1 or 2 in the attached pic ?

 

[kwired]

ok answering this question can help,


according to the rule mentioned above and your comments Dennis Alwon,kwired

the ranges connection to the supply have to be like 1 or 2 in the attached pic ?

Though I now see it could possibly be written a little clearer as to what the source/load relationship may be I think the intention was for something more like this:



Each unit may possibly have a neutral as well and wouldn't really change anything.

Add, I guess your connection might still apply, but really isn't a good design decision to connect them all to one phase in most instances.

 

[iwire]

Though I now see it could possibly be written a little clearer as to what the source/load relationship may be I think the intention was for something more like this:



Each unit may possibly have a neutral as well and wouldn't really change anything.

Add, I guess your connection might still apply, but really isn't a good design decision to connect them all to one phase in most instances.

:thumbsup:

 

[iwire]

ok answering this question can help,


according to the rule mentioned above and your comments Dennis Alwon,kwired

the ranges connection to the supply have to be like 1 or 2 in the attached pic ?

That is no good as the code and the power company require a reasonably balanced load across all phases.

 

[kwired]

That is no good as the code and the power company require a reasonably balanced load across all phases.

And this leads partly to why I said it could be written a little clearer as to the source/load relationship.

If one had a three phase high leg delta supply you wouldn't use the high leg to supply any of those ranges, and the situation is primarily the same as if it were fed from a single phase source. Exception may be if some of them have no neutral connection. Sort of a rare situation but does happen.

 

[haitham]

I dont mean by the pic. all ranges would supply by one single phase ,it is according to the example above 10 ranges shall distribute on all phases as 4/3/3
so i just draw the units of one phase (4 ranges ) ,and the question is the meaning of two phases in the nec rule means " ranges supply by line and neutral ,or by two lines

 

[Smart $]

I dont mean by the pic. all ranges would supply by one single phase ,it is according to the example above 10 ranges shall distribute on all phases as 4/3/3
so i just draw the units of one phase (4 ranges ) ,and the question is the meaning of two phases in the nec rule means " ranges supply by line and neutral ,or by two lines

AFAIK, Code is referring to electric ranges intended to be connected 3-wire: two line conductors and a neutral.

 

[haitham]

Thank you Smart $, iwire, kwired , Dennis Alwon

 

[haitham]

AFAIK, Code is referring to electric ranges intended to be connected 3-wire: two line conductors and a neutral.

the neutral is PEN ..common wire work as grounding and neutral connecting to the body of machines ?

 

[Smart $]

Not in the US. Used to about a half century ago. But not anymore. Now a separate EGC (PE) wire is required.

BTW, we do not count the EGC wire when we refer to the circuit as 2-, 3-, or 4-wire.

 

[haitham]

Not in the US. Used to about a half century ago. But not anymore. Now a separate EGC (PE) wire is required.

BTW, we do not count the EGC wire when we refer to the circuit as 2-, 3-, or 4-wire.

ok Smart I got it