3 phase load calculation

[kwired]

I think you missed something. Your balanced load is three (3) 4992VA loads and three (3) 1498VA loads. That gives a balanced load of 19470VA.

19470VA balanced load at 208V, 3ph give a current of 54.04 Amps. So A, B and C would see 54.04 Amps. And adding the last unbalanced load (of 1622VA) between A and B would add 7.8 Amps to A and B.

So the total unbalanced load would be 61.84 Amps on A and B, and 54.04 Amps on C.


(As a side note, you probably can't multiply the MCA of the heat pump by 0.8 to get the load current. This would only work for a single motor load.)

That is same result I come up with (with the provided load information). I also agree that the actual current rating would be best to use for the calculation and not the MCA. Using the MCA will not be a problem other than you will likely be able to use smaller feeder than what using MCA will give as a result.

I think it is easier to come up with total VA that is balanced then divide by voltage and divide again by 1.73 to come up with total amps then add any unbalanced amps to which ever phases it is connected to. Do not try to add amps per phase per load - you will get thrown off easily. This one was not too hard with all 208 volt loads. Throw in some 120 volt loads and it will get even more complicated.

 

[KHall]

three questions about panel schedule

three questions about panel schedule

Calc at full MCA and largest motor as energy recovery vent

1- There seems to have been conflicting ideas (or I providing incomplete info) about the individaul circuit loads. Does the VA total for each piece of 208V,1ph equipment get divided in half between the two phases of the panel that it is connected to?

2- Does the spreadsheet calculate the balanced and unbalanced portions of the total load separately, or does it just total the VA load of all phases and view the sum as all 3 phase? If so, does that matter?

3- Why select the ERV as the largest motor? Wouldn't one of the HPs be larger?

Thanks to everyone again. I appreciate the help!

 

[tryinghard]

1- There seems to have been conflicting ideas (or I providing incomplete info) about the individaul circuit loads. Does the VA total for each piece of 208V,1ph equipment get divided in half between the two phases of the panel that it is connected to?

Yes

2- Does the spreadsheet calculate the balanced and unbalanced portions of the total load separately, or does it just total the VA load of all phases and view the sum as all 3 phase? If so, does that matter?

Yes it totals all phases the ballance is proved by the VA bottom of each phase.

3- Why select the ERV as the largest motor? Wouldn't one of the HPs be larger?

HP's are only required to calc at 100% the ERV is a largest motor

 

[iwire]

Does the VA total for each piece of 208V,1ph equipment get divided in half between the two phases of the panel that it is connected to?

Yes

Huh?

 

[kwired]

1- There seems to have been conflicting ideas (or I providing incomplete info) about the individaul circuit loads. Does the VA total for each piece of 208V,1ph equipment get divided in half between the two phases of the panel that it is connected to?

Find total VA first. If single phase source divide by voltage to get amps.

If three phase divide balanced portion of load by voltage and divide that result by 1.73 to get amps. Add remaining unbalanced single phase amps to the appropriate phases.

3- Why select the ERV as the largest motor? Wouldn't one of the HPs be larger?

Thanks to everyone again. I appreciate the help!

I disagree that the ERV is lagest motor - the largest heat pump compressor is.

440.3 and 440.7 are used to come up with this conclusion.

 

[tryinghard]

Huh?

A single phase 2 pole circuit supplying a 2KVA load should have 1KVA on each phase of the circuit - have I completely lost it?

 

[Smart $]

Hmmm.... didn't read all the posts since my earlier one to this thread in their entirety, but from what I glean through moderate skimming is that the unbalanced condition is being overly scrupled... and errantly I might add. Before any accurate current determination can be made, power factor must be included in the calculations. Throw in one low pf 1? line-to-line motor with a bunch of resistive loads on a 3? system and your VA-calculated current levels get all wacky compared to true current levels.

 

[Smart $]

A single phase 2 pole circuit supplying a 2KVA load should have 1KVA on each phase of the circuit - have I completely lost it?

If you have a 1?, 2P load, it is only one phase. You are intermixing "line" and "phase" terms. A "line" has no VA on it other than that of voltage drop (I?R losses).

Splitting the VA to two lines is a means of approximating line current when a system is balanced. The higher the degree of unbalanced the system is, the more inaccurate this method gets... perhaps exponentially so. Even if your VA per line add up as completely balanced, the probability of it being true in use is close to infinitely not.

 

[tryinghard]

I disagree that the ERV is lagest motor - the largest heat pump compressor is.

440.3 and 440.7 are used to come up with this conclusion.

I use the minimum circuit ampacity for a calc and leave it at that. Might be wrong but seems to line up with 440.6(A) & 440.7 exceptions, at least the way I understand it.

Multimotor does not include this exception so I suppose the unit should be calced accordingly looks like 440.12(B), the compressor RLA would usually be used as the largest motor.

 

[tryinghard]

Splitting the VA to two lines is a means of approximating line current when a system is balanced

Wouldn't a heat pump at a particular load, maybe minimum circuit ampacity, be balanced?

 

[Smart $]

Wouldn't a heat pump at a particular load, maybe minimum circuit ampacity, be balanced?

In and of itself yes. But when you start combining this one load with other loads of differing power factors, the current balance can easily be skewed when determine by VA alone. A "best" balancing method must take power factors into consideration, especially when you have varying types and quantities of 2-pole loads on 3? systems.

A "minimal" unbalance can be tolerated when sizing is done through Article 220 service and feeder calculations... but I've never put a number to how much unbalance can be tolerated (and neither has the NEC :roll. The goal is to balance the system as best you can. Your method is pretty much the conventional approach, so don't think I'm saying you are out in left field somewhere on the issue.

 

[tryinghard]

...A "minimal" unbalance can be tolerated when sizing is done through Article 220 service and feeder calculations... but I've never put a number to how much unbalance can be tolerated (and neither has the NEC :roll. The goal is to balance the system as best you can. Your method is pretty much the conventional approach, so don't think I'm saying you are out in left field somewhere on the issue.

Smart $, I've been a left-fielder before but I do prefer the infield . It seems to me given this single phase equipment the load balance is within 12% +/- and there is limited chance at rearranging. I mean there are only so many places these loads can connect to 2 pole breakers. I guess a single phase load center could be considered and it would result in a particular load upstream if upstream is 3 phase.

 

[kwired]

I use the minimum circuit ampacity for a calc and leave it at that. Might be wrong but seems to line up with 440.6(A) & 440.7 exceptions, at least the way I understand it.

Multimotor does not include this exception so I suppose the unit should be calced accordingly looks like 440.12(B), the compressor RLA would usually be used as the largest motor.

Using MCA is what you are supposed to do for the branch circuit. For a feeder serving multiple units you will have a feeder larger than minimum that is needed - not a problem from NEC but if you are using this design as a bid against someone that designed it as light as possible you may have a higher bid. If you use MCA of all units to design a feeder for multiple units you have 125% of all units in your design. You only need 125% of largest motor plus 100% of all others.

... I guess a single phase load center could be considered and it would result in a particular load upstream if upstream is 3 phase.

If the HVAC is a major load of the facility and there is no other three phase loads then this takes away from any advantages of having three phase service - you have unbalanced the service with some of the major load of the facility.

 

[tryinghard]

...For a feeder serving multiple units you will have a feeder larger than minimum that is needed - not a problem from NEC but if you are using this design as a bid against someone that designed it as light as possible you may have a higher bid. If you use MCA of all units to design a feeder for multiple units you have 125% of all units in your design. You only need 125% of largest motor plus 100% of all others.

I understand this equipment as multimotor and if so the unit amps are determined by either the MCA or 440.33, if one calcs out the unit the factoring will not be 125% of the whole load it will only be 125% of "the" one largest motor (usually a compressor) then the other motors added accordingly. My design includes the MCA and calcs them at 100% for the continuous plus another 25% of the fan motor thereafter. I believe breaking these down beyond the MCA doesn't change the result enough to create different feeder size or main circuit breaker.

If the HVAC is a major load of the facility and there is no other three phase loads then this takes away from any advantages of having three phase service - you have unbalanced the service with some of the major load of the facility.

Now the thoughts have gone full circle . This equipment can be included in 3 phase or single phase supply and if 3 phase its load balance is what it is again rearranging is limited (the equipment can be 3 phase but this sounds like its past the design stage).

 

[tryinghard]

I have a 3 phase panel with 7 different 208V/1 phase loads. I calculated the load for each 2 pole circuit by multiplying the load amps by 208. The VA totals for each phase are as follows ...

A = 18,242 VA
B = 18,242 VA
C = 16,224 VA

How do I convert this into a total connected load figure for this panel and would that (along with 25% of my largest motor) be the basis for sizing my feeder and main breaker?

The system is 120/208, 3 phase, 4 wire.

KHall, do you have the nameplate info? If so calc it using this info compared to MCA and see what the end result is.

 

[kwired]

.. I believe breaking these down beyond the MCA doesn't change the result enough to create different feeder size or main circuit breaker.

For the six units mentioned in the OP probably not. For 100 units in a hotel it will make a difference in load calculations.

 

[Smart $]

Smart $, I've been a left-fielder before but I do prefer the infield . It seems to me given this single phase equipment the load balance is within 12% +/- and there is limited chance at rearranging. I mean there are only so many places these loads can connect to 2 pole breakers. I guess a single phase load center could be considered and it would result in a particular load upstream if upstream is 3 phase.

I would only consider a 1? panel for L-N loads of a 4-wire delta service with a stinger for 3? equipment.

In cases where the balance is off by VA determination, the practical approach is to upsize transformer(s) if determined necessary and conductors accordingly... approaching the issue from the perspective of conventional knowledge and wisdom, since the NEC has no prescribed method for ratings adjustments of unbalanced systems.

 

[tryinghard]

Three actual unit label examples:
Unit 1
19.6 MCA (30A max fuse)
15 compressor RLA
.8 Fan 1/8hp
19.6A = 15x1.25+.8

Unit 2
29 MCA (50A max fuse)
22 compressor RLA
1.4 Fan 1/4hp
28.9A = 22x1.25+1.4

Unit 3
25.4 MCA (40A max fuse)
19.2 compressor RLA
1.4 Fan 1/4hp
25.4A = 19.2x1.25+1.4

The MCA already includes the largest motor at 125% plus remaining motor, with multimotor units there?s no need to calc the units when the label includes the minimum circuit ampacity.

The use of MCA for load calculations at 100% is complete and code compliant.

 

[LarryFine]

The use of MCA for load calculations at 100% is complete and code compliant.

Agreed, and it's okay and a good idea to use the MOC as well. After all, this isn't rocket surgery.

 

[kwired]

Three actual unit label examples:
Unit 1
19.6 MCA (30A max fuse)
15 compressor RLA
.8 Fan 1/8hp
19.6A = 15x1.25+.8

Unit 2
29 MCA (50A max fuse)
22 compressor RLA
1.4 Fan 1/4hp
28.9A = 22x1.25+1.4

Unit 3
25.4 MCA (40A max fuse)
19.2 compressor RLA
1.4 Fan 1/4hp
25.4A = 19.2x1.25+1.4

The MCA already includes the largest motor at 125% plus remaining motor, with multimotor units there?s no need to calc the units when the label includes the minimum circuit ampacity.

The use of MCA for load calculations at 100% is complete and code compliant.

Nothing wrong with using 100% of nameplate MCA if you wish - you will not be too small with your feeder size. Like I said before this is for sizing the branch circuit (an individual load). 440.3 refers you back to 430 for anything not modified by 440. Sizing feeders is not covered in 440 so you have to go back to 430 rules for sizing feeders.

Lets say we have nine of the mentioned unit 1 connected to a 208 volt 3 phase source and was balanced across phases.

If we size a feeder to supply these nine at 100% of nameplate MCA we will need a feeder capable of carrying 102 amps. 2 AWG copper at 75 degree.

If we size a feeder at 100% of all the motors plus 25% of the largest one we will need a feeder capable of carrying 84 amps. 4 AWG copper at 75 degree.

The more units you have the more that extra 25% of each compressor will add up if using the MCA for feeder and service calculations.