240V Delta High Leg Panel Amperage Sizing

Status
Not open for further replies.

theWilly

Member
Location
Dallas, TX USA
With a 240V Delta High Leg system how do I go about sizing the panel main breaker?

My loads will all be either 120V single phase or 240V 3-phase. Do I basically add up all my VA's (A*1.732 for the 3-phase loads) and then divide the total VA by (240 * 1.732) ?

I'm aware of the 125% on 3-hour loads and diversity on multiple peices of kitchen equipment, etc....I just need to double check that my thinking is correct on converting my final VA to Amps.

Thank you for the help!
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
IMO, you would have to look at the load per phase as it will not be evenly distributed.
 

theWilly

Member
Location
Dallas, TX USA
Ok, the 120V single phase loads will be the bulk of it (loading up the A & C legs). The B leg will be nearly empty compared to A & C.

So....do I take the higher VA of A and C then divide by....?
 

theWilly

Member
Location
Dallas, TX USA
Let me rephrase that....

Say I have this:

Phase A: 50A of single phase @ 120V = 6000VA
Phase B: 0 VA
Phase C: 40A of single phase @ 120V = 4800VA

Then in this example I would examine each phase to find that A is the highest at 50A, so size the main breaker for this.

Is this correct?

Now to further complicate it, what happens when I add a 20A 3-phase load @ 240V ?

Is it as simple as just taking the 20A added to each phase, then size off the highest, so... phase A (50A) add the 20A = 70A
 
Last edited:

theWilly

Member
Location
Dallas, TX USA
Here is my concern with the above mentioned solution.

On a panel schedule where 3-phase loads divided amongst the phase this would happen with the A leg:

6000VA in single phase + 2771VA from the 3-phase (20A*240V*1.732 / 3)

So I get 8771VA / 120V = 73A .... but we're not really providing 120V to that 3-phase either...right?

Its not a large difference here, but on a panel heavily loaded with 3-phase loads that could make a big difference.

I've read conflicting opinions on dividing up the VA among the legs in that manner and I'm not entirely sure what the accurate way to go about this is.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Don't make it so complicated.

You can have (1) three-phase load grouping, from which you can determine a per leg loading in amps
You can have (2) single-phase 120V load groupings, from which you can determine a per leg loading in amps
You can have (3) single-phase 240V load groupings, from which you can determine a per leg loading in amps.

Now just add together the the per leg loading from each group. The largest value will provide you with the size of your main breaker.
 

Smart $

Esteemed Member
Location
Ohio
Ok, I follow you. But I'm not clear on the correct way to get the per leg amp loading of the 3-phase load.
If you have the 3? load (total or individual) in amperes, that is the per leg loading. If you have 3? VA (total or individual) divide by 240, divide by sqrt(3), result is per leg amperes.
 

theWilly

Member
Location
Dallas, TX USA
If you have the 3? load (total or individual) in amperes, that is the per leg loading. If you have 3? VA (total or individual) divide by 240, divide by sqrt(3), result is per leg amperes.

Ok...so then with the example above, if I have a 240V 3-phase motor at 20A then I just add 20A on each leg.

These panels are designed before the building is built, so all I have to work with is specs and cut sheets. Lighting and small appliances typically give me wattages, larger motor loads provide amps.

So assuming the panel is loaded with either 120V single phase or 240V 3-phase I will:

1 - take the 120V Total VA per leg / 120V = Amps
2 - take the 240V total amperage and apply that on each leg (no conversion to VA and dividing it between legs or splitting the amps between the 3 legs)
3 - Total the amps up at each leg and size the breaker for the maximum value.

So from the example, taking the A-leg: 6000 VA of 120V and a 20A 3-phase motor results in 70A on that leg. Am I following correctly?

Please excuse me, I'm a stickler for detail and would like to make sure I've got this straight.
 
Last edited:

Smart $

Esteemed Member
Location
Ohio
...

1 - take the 120V Total VA per leg / 120V = Amps
2 - take the 240V total amperage and apply that on each leg (no conversion to VA and dividing it between legs or splitting the amps between the 3 legs)
3 - Total the amps up at each leg and size the breaker for the maximum value.

So from the example, taking the A-leg: 6000 VA of 120V and a 20A 3-phase motor results in 70A on that leg. Am I following correctly?

Please excuse me, I'm a stickler for detail and would like to make sure I've got this straight.
That's [conventionally] correct.
 
Status
Not open for further replies.
Top