blkhatsilhouette
Member
- Location
- Portland, OR
- Occupation
- Electrical Contractor
I'm looking for clarification on the proper way to size a service. If I'm supplying power to 20 - 4000VA 208v dryers, I'm trying to determine this demand for the service, as well as the neutral. My confusion comes from what I've read elsewhere, and how similar information is presented in Annex D5(a) in regard to calculating demands and then changing them to 3 phase equivalent loads...
How I feel is correct because of how 220.54 reads, but I'm unsure...
Because it's 3 phase service, max between two phases is 7 dryers, so times 2 gives me 14 (per 220.54)
14 x 4000VA = 56,000 demand on the service/feeder (155 amps)
Neutral would be:
56,000 x 70% = 39,200 demand on service/feeder neutral (109 amps)
***If done according to how they present a calc in D5(a):
Max between two phases is 7 dryers, so times 2 gives me 14
14 x 4000VA = 56,000 over 2 phases
56,000/2 = 28,000
Equivalent 3 phase load: 28,000 x 3 = 84,000 VA (233 amps)
Neutral:
84,000 x 70% = 58,800 VA (163 amps)
Any insight on this? Is this an NEC error? Example D5(a) uses ranges to demonstrate the calculation, but I can't find any difference between the general calculations on the ranges vs. dryer (other than the obvious fact that you're using nameplate for the dryer (commercial) vs in the example using the household range chart. Is there something I'm missing in terms of 3 phase multifamily making you calculate out per phase then multiply by 3?
How I feel is correct because of how 220.54 reads, but I'm unsure...
Because it's 3 phase service, max between two phases is 7 dryers, so times 2 gives me 14 (per 220.54)
14 x 4000VA = 56,000 demand on the service/feeder (155 amps)
Neutral would be:
56,000 x 70% = 39,200 demand on service/feeder neutral (109 amps)
***If done according to how they present a calc in D5(a):
Max between two phases is 7 dryers, so times 2 gives me 14
14 x 4000VA = 56,000 over 2 phases
56,000/2 = 28,000
Equivalent 3 phase load: 28,000 x 3 = 84,000 VA (233 amps)
Neutral:
84,000 x 70% = 58,800 VA (163 amps)
Any insight on this? Is this an NEC error? Example D5(a) uses ranges to demonstrate the calculation, but I can't find any difference between the general calculations on the ranges vs. dryer (other than the obvious fact that you're using nameplate for the dryer (commercial) vs in the example using the household range chart. Is there something I'm missing in terms of 3 phase multifamily making you calculate out per phase then multiply by 3?
