panel size

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
KVA is KVA.
If your loads are unbalanced, then balance them. Engineers try to do it on drawings.
Electricians should try to do in the field as well. This thing will run all day on a 150 amp panel. But I would still install a 200 amp.
If you want to spend some extra money on a 250 amp breaker, then you need a 400 amp bus.
150 amp, 200 amp or 250 amp will all work.
How do you balance (4) 120 volt loads in a 3 phase panel?
 

powerpete69

Senior Member
Location
Northeast, Ohio
Occupation
XXX
How do you balance (4) 120 volt loads in a 3 phase panel?
Put a single phase breaker on phase A, B and C. Then put the last single phase breaker on phase A or B or C. Obviously, the best it can be balanced.
The 200 amp panel is up and running in theory. At most it has a 41 KVA load on it with 30 KVA to spare.
Seems pretty "cut and dry".
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
It is upon us in the field to distribute loading when it's not done by a designer. Grouping loads by current levels helps. For example, when laying out a residential panel, I'll try pair up the 120v circuits something like this:

A - Kitchen recs A
B - Kitchen recs B
A - Refrigerator
B - Microwave oven
A - Garb. disposer
B - Dishwasher
A - Laundry rec(s)
B - Garage recs
A - Fam. rm. recs
B - Liv. rm. recs
A - Bedrm recs A
B - Bedrm recs B
A - Main fl. ltg. A
B - Main fl. ltg. B
A - Bedrm ltg. A
B - Bedrm ltg. B
A - Ext. ltg. front
B - Ext. ltg. rear


Etc.
 

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
Put a single phase breaker on phase A, B and C. Then put the last single phase breaker on phase A or B or C. Obviously, the best it can be balanced.
The 200 amp panel is up and running in theory. At most it has a 41 KVA load on it with 30 KVA to spare.
Seems pretty "cut and dry".
You stated if the load is not balanced then balance it. Obviously you can't balance the load. At best you are going to have 2 60 amp loads and 2 20 amp loads on one phase. Not sure how you figure a 150 amp panel is adequate is one phase has a 160 amp load.
 

powerpete69

Senior Member
Location
Northeast, Ohio
Occupation
XXX
You stated if the load is not balanced then balance it. Obviously you can't balance the load. At best you are going to have 2 60 amp loads and 2 20 amp loads on one phase. Not sure how you figure a 150 amp panel is adequate is one phase has a 160 amp load.
Ahhh....I see your confusion. 2-20 amp single phase loads (2 X 120 X 20 = 4.8 KVA) are not the same as 2-20 amp three phase loads. (2 X 208 X 20 X 1.73 = 14.39 KVA. You can't count the 20 amps at 120 VOLTS the same as you can the 208 VOLTS since they are slit thru the neutral of transformer in the single phase situation.
Do not separate out loads....this confuses things.
You start with 41 KVA like I calculated. Three phase power = Volts X amps X 1.73
Then divide this out like I did on an earlier post and you get 117 amps at 208V three phase!!! Don't confuse with single phase.
Trust the math...and again do not separate out single phase loads!! Work with KVA only!!
 

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
So you are telling me if I were to install a 3 phase panel with 3 20 amp 120 volt loads all connected to a single phase (lets say A) then I measure the current on all 3 phases of the feeder they would be exactly the same even though there is nothing connected to 2 of them?
WOW. That's amazing!
 

powerpete69

Senior Member
Location
Northeast, Ohio
Occupation
XXX
So you are telling me if I were to install a 3 phase panel with 3 20 amp 120 volt loads all connected to a single phase (lets say A) then I measure the current on all 3 phases of the feeder they would be exactly the same even though there is nothing connected to 2 of them?
WOW. That's amazing!
Perhaps you have a point there! Go with 200 amp panel.
 
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