220.54 & 220.55: Single Phase Panels & Three Phase Service
Article 220.54 in the 2008 NEC states the following:
"Where two or more single-phase dryers 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."
Article 220.55 has a very similar 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."
My question is: What are you supposed to do if you have single-phase dryers and single-phase ranges on single-phase panels fed by a three-phase service? None of the examples I have found in the NEC cover this situation. Such a situation could occur for a multifamly dwelling.
Do you calculate as though the dryers and ranges were on three-phase panels (even though they are not) and take twice the maximum number that would be connected between any two phases if they were on a three-phase panel? (Case#1.)
Or do you take twice the maximum number that would be connected between any two phases on the single-phase panel that they are each on? (Case #2.)
Or does the requirement above not apply because the dryers and ranges are not directly supplied by a 3-phase feeder or service? (Case #3.)
For example, say you had a multifamily dwelling fed by a three-phase service. There are 10 dwellings and each is served by a single-phase panel. Each dwelling has a single-phase dryer and single-phase range.
Using the cases defined above, you would come up with the following number of dryers and ranges as the basis of your calculation:
Case #1: 4 x 2 = 8
Case #2: 10 x 2 = 20
Case #3: 10
As you can see, how the code is interpreted makes a big difference.
I appreciate any insight into this topic. Thanks.
See the Example D5(a) in Annex D. It is a situation like you describe. Essentially, it would be based on Case #1 (4x2=8) but it is a little more complicated that that.
Thanks David! I didn't notice that example D5(a) was using single-phase panels to feed the individual dwelling units. I didn't recognize that the phrases "Service to each dwelling unit would be two phase legs and neutral" and "120/208-V, 3-wire system" implied that the dwelling units were using single-phase panels. I appreciate the help.
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