Mike Dolt Enterprises
Mike Dolt Enterprises
User: BrianMuir, Location: Comox BC
Asked on: Monday, March 28, 2011 at 12:21 am EDT
Answered on: Tuesday, May 14, 2024 at 3:22 am EDT
Question: Hi, I'd just like to confirm if I have this rightI have a three phase 120/208 supply & want to connect a new load, all single phase (ie 3 wire), consisting of about 25KVA at 208 (heat pump, hot water tank), and an assortment of other loads at 120V totaling 8KVA. (basic lighting and receptacle loads)Assuming the 8KVA are evenly spread between Phase A and Phase B, then it seems to me that the worst case unbalanced load is 4KVA (when only one phase set of 120V loads are on) and this is what the neutral could be sized against.Now, I do not intend to cut back the neutral ampacity this severely, but it sure seems fiscally prudent to shave it back a couple of sizes compared to the phase lines, especially considering I am looking at long runs.Note that I am working against the CEC which requires the neutral ampacity sized to the unbalanced load. From what I can tell the NEC is similar.As always, I certainly appreciate your kind support.
Your understanding is correct.According to the CEC (and NEC), the neutral ampacity must be sized to handle the unbalanced load. In your case, the unbalanced load is 4 kVA.However, you are not required to cut back the neutral ampacity to this extent. You can shave it back a couple of sizes compared to the phase lines, especially if you are looking at long runs. This will save you money on materials and installation costs.Here is a table summarizing the ampacity requirements for your load:| Circuit | Ampacity (A)
| ---|--- |
| Phase B | 60 A |