How to calculates total service demand load for a restaurant when I have high leg service and a mix of 3phase and 1phase loads?

V

Volt-Amps

Member
Location
Indianapolis
Do I calculate 3 phase loads separate from 1 phase loads then sum up the two on the 3 phase high leg service?
I'm thinking:
  • For 3 phase 120/240V panels having mostly 3 phase and 1 phase 240V loads: ( total KVA on panel x 1000) ÷ (240 x √3)
  • For 1 phase 120/240V panels having 1 phase 120V loads: ( total KVA on panel x 1000) ÷ (240)
Then sum up the two and that would be the total demand on the building service.

Could you gurus chime in and help stop me if I'm doing it wrong?

Thanks much
 
Volt-Amps said:
the 3 phase high leg service
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I treat a 'high leg service' by that I mean utility supplied 240/120V open delta with two pole mounted transformers different than a full delta with a common core or a padmount with triplex cores or three pole mounted transformers in a delta (3 equal kVA ratings).
The open delta has a split phase 120/240 transformer (lighter) and a single phase 240 transformer (stinger) and they may be of unequal rating.
The simplest method with open delta is to tabulate the pure 3-phase 240 loads and the 120/240 or 240 single phase loads separately.
If you can know for sure which set of wires is on the '240' stinger transformer (not the open jaw) you can 'top it off', with single phase 240 loads, if you put any 240 single phase loads on the 'open jaw' (or the wild leg to neutral) it would pull from both transformers.
 
With high-leg (open delta) service, it helps me to break the loads down. Keep 3-phase 240V, single-phase 240V, and 120V loads separate. The 120V stuff only uses the 120/240V transformer, while 240V loads might pull from both, depending on which legs they're connected to. Just be careful not to overload one side, especially if the transformers aren’t evenly rated.
 
baserunner4 said:
With high-leg (open delta) service, it helps me to break the loads down. Keep 3-phase 240V, single-phase 240V, and 120V loads separate. The 120V stuff only uses the 120/240V transformer, while 240V loads might pull from both, depending on which legs they're connected to. Just be careful not to overload one side, especially if the transformers aren’t evenly rated.
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Not all high leg delta systems are open delta. For retail, offices and such where the three phase load may be limited to just HVAC the chances are higher it will be open delta. Restaurant could have more three phase loads and may or may not be high leg delta.

Light industrial type places may very well have a lot of three phase load and maybe are supplied by full delta. One reason not to be full delta would be because it is in remote location and they decided to go open delta and not run a third primary phase conductor, but they also may have needed to use larger transformers than they would have with a full delta.
 
kwired said:
Not all high leg delta systems are open delta. For retail, offices and such where the three phase load may be limited to just HVAC the chances are higher it will be open delta. Restaurant could have more three phase loads and may or may not be high leg delta.

Light industrial type places may very well have a lot of three phase load and maybe are supplied by full delta. One reason not to be full delta would be because it is in remote location and they decided to go open delta and not run a third primary phase conductor, but they also may have needed to use larger transformers than they would have with a full delta.
Click to expand...
Yeah excellent point, I think would improve clarity in communication between the electrician, electrical engineer and utility worlds if the terms we used for these were 'open delta' or 'closed delta', and add a suffix like corner grounded or ungrounded if does not have the mid point of one winding grounded.
 
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