Highest-Rated Motor, 1? vs. 3?

[Smart $]

I'm aware the highest-rated motor is determined solely by rated-load current, regardless of being 1? L-N, 1? L-L, or 3?. The requirement is written regarding determination of disconnect, ocp, controller, ampacity, etc. ratings... in the circuit pathway. However, how should this be applied when doing 220.14(C) load calculations?

For example, say we have the following motors on a service:

HP	Volts & ?	FLA
2	120V 1?	24
5	208V 3?	16.7

Because we calculate in VA, should we include only 120V ? 24A = 2,880 VA ...or... 120V ? 24A ? √3 = 4,988 VA (...? 125% for highest-rated motor) in the calculation?

Now before answering that question, consider that if these were the only two loads on the service, there would be no way to balance the system out. Additionally, Article 430 makes no provision for balancing out the conductor current with other non-motor loads. Furthermore, when determining service and feeder ocpd and conductor size, a typical 3? load calculation simply divides the total VA equally on each conductor.

Opinions?

 

[david luchini]

1) Wouldn't the 5 HP motor be "higher rated" than the 2 HP motor?

2) NEC tells you to calculate motor loads in terms of Amps. You generally convert that to VA to add it with other loads that are in VA, and then it is converted back to Amps. If motors are the only load, I'd just leave them in Amps.

3) If you do convert to VA, wouldn't you want to times your 120V motor by 3, rather by √3 when applying it to the 3 phase system, in order to get an adequate amperage? 4988*1.25 + 6016 = 12,251VA = 34A @ 208, 3ph. But you know your highest leg will see 16.7 + 24 = 40.7A at full load.

 

[Smart $]

1) Wouldn't the 5 HP motor be "higher rated" than the 2 HP motor?

In HP rating, yes... but as to applying the 125% of highest rated motor, 430.17 says "shall be based on the rated full-load current as selected from Table 430.247, Table 430.248, Table 430.249, and Table 430.250." I also referred to several respectable web pages, and they agree....

http://ecmweb.com/nec/motor-calculations

2) NEC tells you to calculate motor loads in terms of Amps. You generally convert that to VA to add it with other loads that are in VA, and then it is converted back to Amps. If motors are the only load, I'd just leave them in Amps.

I know that... and I only used the motor-load-only scenario as food for thought. But the problem being discussed is when motor loads are mixed in with other non-motor loads calculated in VA. When you get to service or feeder ocp and conductor sizing, have you ever seen anyone take into consideration motor load imbalance such posed by the motor-load-only scenario. All the motor references I've researched never even get into how to apply the motor load calculations under Article 220.

3) If you do convert to VA, wouldn't you want to times your 120V motor by 3, rather by √3 when applying it to the 3 phase system, in order to get an adequate amperage? 4988*1.25 + 6016 = 12,251VA = 34A @ 208, 3ph. But you know your highest leg will see 16.7 + 24 = 40.7A at full load.

Uhh... yeah. Was thinking 208V 1? as I was typing, even though I correctly used 120V for the voltage part of the calculation. :slaphead:

But then it'd be 8640*1.25 + 6,016 = 16,816VA = 47A @ 208 3?


Then how does this correlate to other non-motor loads balancing out the the system?

From the other extreme, 2880*1.25 + 6,016 = 9,616VA = 27A @ 208V 3?.

Doing it the former way, the total VA would include 7,200 extra VA compared to the latter, for a 2,880VA 120V motor!!! Doing it the latter way could result in undersized ocp and conductors!!!

 

[david luchini]

In HP rating, yes... but as to applying the 125% of highest rated motor, 430.17 says "shall be based on the rated full-load current as selected from Table 430.247, Table 430.248, Table 430.249, and Table 430.250." I also referred to several respectable web pages, and they agree....

Thanks, I overlooked that section. :slaphead:

Uhh... yeah. Was thinking 208V 1? as I was typing, even though I correctly used 120V for the voltage part of the calculation. :slaphead:

But then it'd be 8640*1.25 + 6,016 = 16,816VA = 47A @ 208 3?

Which works out perfectly... 24*1.25 + 16.7 = 46.7A.

Then how does this correlate to other non-motor loads balancing out the the system?

From the other extreme, 2880*1.25 + 6,016 = 9,616VA = 27A @ 208V 3?.

Doing it the former way, the total VA would include 7,200 extra VA compared to the latter, for a 2,880VA 120V motor!!! Doing it the latter way could result in undersized ocp and conductors!!!

The extra 7,200VA is irrelevant, it doesn't create a larger service than you need for your load. The point of the load calculation is to properly size the conductors for the load. If your load cant be balanced across three phases, you need to address that.

 

[Smart $]

...

The extra 7,200VA is irrelevant, it doesn't create a larger service than you need for your load. The point of the load calculation is to properly size the conductors for the load. If your load cant be balanced across three phases, you need to address that.

Well I see the extra 7,200VA as relevant if I can offset it with a non-motor load... and that is the real issue behind my OP: Can we use non-motor loads to offset [load calculation] multi-motor current imbalance? If we have no loads with which to offset, then obviously we'd have to calculate with the extra 7,200VA, i.e. if we're making all the conductors the same size and we're dividing the total VA by 3. But if we have enough non-motor load to offset the imbalance, we can figure at the actual VA ? 125%. Do you agree?

 

[david luchini]

Well I see the extra 7,200VA as relevant if I can offset it with a non-motor load... and that is the real issue behind my OP: Can we use non-motor loads to offset [load calculation] multi-motor current imbalance? If we have no loads with which to offset, then obviously we'd have to calculate with the extra 7,200VA, i.e. if we're making all the conductors the same size and we're dividing the total VA by 3. But if we have enough non-motor load to offset the imbalance, we can figure at the actual VA ? 125%. Do you agree?

You can offset the extra 7200 VA. It doesn't really exist, its just added as a method for getting the proper conductor size for the unbalanced load.

As you noted, per Art 220, your load would be 9616 VA, but that can't be balanced over three phases. It's incumbent on the designer to make sure they have adequately sized feeders or services for the calculated load.

Annex D1(b) shows an example. Just adding the loads without considering imbalance would give you 100A @240V, but figuring the imbalance requires a 100A service.

 

[Smart $]

You can offset the extra 7200 VA. It doesn't really exist, its just added as a method for getting the proper conductor size for the unbalanced load.

As you noted, per Art 220, your load would be 9616 VA, but that can't be balanced over three phases. It's incumbent on the designer to make sure they have adequately sized feeders or services for the calculated load.

Annex D1(b) shows an example. Just adding the loads without considering imbalance would give you 100A @240V, but figuring the imbalance requires a 100A service.

Agree.

Also note that it's not just an extra 25% of the highest rated motor on the service overall, but an extra 25% of the highest rated motor per conductor. However, in the example it should actually be 3A under Line B... 25% of 10A is 2.5A, and 220.5(B) only allows fractions smaller than 0.5 to be dropped. So in my 2-motor-only-load example, rather than using an extra 7,200VA as calculated, the example suggests I should use an extra 25% of the 3? motor for the other two conductors...???

And... ummm... that last bit in your post should be a 110A service.

 

[jumper]

And... ummm... that last bit in your post should be a 110A service.

Hey, he was distracted by a helicopter/Klinger/Col. Potter.....

 

[david luchini]

Hey, he was distracted by a helicopter/Klinger/Col. Potter

:lol: