David-Engineer
Member
- Location
- Portland, Oregon, U.S.
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.
"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.
