Calculate the electrical dryers load in 208v three phase

[wwhitney]

That column should really be titled "number of dryers connected to the same phase conductor". As that's the effect of the red sentence. Which makes sense, as the benefit of load diversity is due to loads sharing a conductor.

Cheers, Wayne

 

[Do You Like Ham]

@binwork91 220.54 states: "where two ore 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".

Thus, if you have 40 dryers total, and they are evenly distributed across phases, you'll have 13 driers on AB, 13 driers on CA, and 14 dryers on BC...

28 = 2x maximum (14).

 

[Do You Like Ham]

@wwhitney not sure I agree with "number of dryers connected to the same phase conductor" - e.g., in the example of 40 driers with 13 driers on AB, 13 driers on CA, and 14 dryers on BC... there'd be 26 driers connected to the A phase conductor, and 27 driers connected to the B and C phase conductors... you'd end up slightly oversizing in that case. If you used the same logic when interpreting 220.55 using column C, you'd end up slightly undersizing.

 

[Do You Like Ham]

@binwork91 to further clarify... one still accounts for the load of all 40 driers... e.g., the Annex D5 shows that the per phase demand from the "maximum number connected between any two phases" x 2 is divided by 2 then multiplied by 3 (which ends up being the load of 42 driers in this case.. e.g., 28 ÷ 2 x 3 = 42... so it's a bit conservative when your quantity is not a factor of 3.

 

[WA_Sparky]

If I'm not mistaken this code is specifically meant for multiple equipment powered by the same service. The only time I used this was for dorm style living where there are groups clustered in a common space. If it's connected like the typical 1 dryer per metered tenant panel, you want to use the number of dwelling unit table to derate

 

[Psychlo]

I agree that the OP's #2 answer is correct, according to Annex D. But I question the reasoning behind it.

Here's the rub for me....

Why do we end up with a higher 3-phase kW value than 1-phase (68,250 vs 58,000)? Splitting the same load across 3 phases versus 2 phases shouldn't increase the total kW demand, should it? Are they trying to account for the inevitable imbalance across phases? Seems 3-phase would be more efficient, not less. I must be missing something.