Power Factor Correction Add-On Products Viability

[Sahib]

At the installation stage of a piece of electrical kit, an economic benefit is that it may reduce the size the size, and thus cost, of the cable feeding it. And the labour to install that cable.
70mm² XPLE is cheaper than 95mm² XPLE for example. Without PFC, the 70mm² XPLE might be just too small so you'd have to use the next standard size up, the 95mm² .

With the PFC you may well get by with the smaller cable. It's just an example. You'd have to factor in the cost and installation of the PFC. Each case needs to be taken on its own merits.

It should not be against the spirit of NEC recommending spare capacity.

 

[Sahib]

My limited experience has shown that an "energy efficient motor" is optimized for (near) full load and the efficiency at say 25% or 50% load is worse than that of a "standard" design.

No. The Efficiency of Energy Efficient motor is higher than that of standard motor at all operational points.

 

[Besoeker3]

It should not be against the spirit of NEC recommending spare capacity.

It's quite simply a commercial matter to minimise the cost of installation by using a smaller cable.
And, in my case, not relevant to the NEC regulations. I thought the conductor sizes I gave might have clued you in.

 

[kwired]

Is it against the spirit of NEC recommending spare capacity?
On what sound basis can you make that assertion?

No it is not. 90.8 even mentions it, but also does not require nor does it prohibit allowing for spare capacity.

[h=2]90.8Wiring Planning.[/h] [h=3](A) Future Expansion and Convenience.[/h]
Plans and specifications that provide ample space in raceways, spare raceways, and additional spaces allow for future increases in electric power and communications circuits. Distribution centers located in readily accessible locations provide convenience and safety of operation.

 

[Besoeker3]

No it is not. 90.8 even mentions it, but also does not require nor does it prohibit allowing for spare capacity.

And, even if it did, if my 70mm² was marginal on, say a 0.7pf motor, using PFC to get it up 0.95, typically what gets specified here, the current would drop to about 74% of its previous value. So, plenty of margin.

 

[Sahib]

If cable size for reduced current due to PFC only is used without any increase for spare capacity, it is not recommended by NEC. Prudent design takes into account spare capacity. And that is my point.

 

[Besoeker3]

If cable size for reduced current due to PFC only is used without any increase for spare capacity, it is not recommended by NEC. Prudent design takes into account spare capacity. And that is my point.

Do you think reducing the current requirement to 74% does not yield sufficient spare capacity???

 

[Sahib]

NEC doesn't recognize power factor correction as a reason to allow smaller conductors to a motor, or even higher efficiency motor to have smaller conductors than lesser efficiency of same output rating.

May be the wiring regulations of B's country also does not recognize as NEC.

 

[kwired]

May be the wiring regulations of B's country also recognises it.

Maybe, question was about NEC.

 

[Sahib]

Maybe, question was about NEC.

You caught me before my editing.

 

[Sahib]

NEC doesn't recognize power factor correction as a reason to allow smaller conductors to a motor.

However, higher size cable to reduce voltage drop may be replaced with lower size cable due to PFC equipment at the load end, thereby effecting considerable saving.

 

[kwired]

However, higher size cable to reduce voltage drop may be replaced with lower size cable due to PFC equipment at the load end, thereby effecting considerable saving.

That would not be smaller conductors allowed to the motor under general NEC rules though, that is just a situation where the higher PF would yield less voltage drop. Per NEC the minimum size conductor you can run to said motor is calculated before considering any VD or PFC and must still remain that minimum size regardless what the VD and PFC end up being.

 

[kwired]

I have presented figures. Do they contravene the NEC?

per what you said in post 52?

As I said in my previous reply to Sahib, NEC will have a minimum conductor size you must run to the motor that can not be lessened because of VD or PFC. But also sort of never requires us to increase that size as a general rule either. Those that increase because of VD do it for efficiency or other design reasons, maybe even an energy code reason.VD thresholds mentioned in NEC are not requirements (other than a few specific instances on specific applications) but rather a suggestion inserted into the text via what NEC calls "informational notes".

NEC requires us to use a table within to determine full load current rating of nearly all standard sized motors you are likely to encounter and is categorized into different tables for single phase, three phase, DC motors, etc, but then next major classification is horsepower then voltage - your motor full load amps you will use is taken from the tables and used for any calculations you will need for that particular load. Those tables factor in worst known efficiency or PF you would ever find on a motor of same output rating and are almost always higher than the nameplate on the motor you will be using.

I think at least part of the idea is that if you must replace the motor it is likely to still be same hp rating but may not have same efficiency or PF as the original - but the conductors will still be sufficient because they were originally selected for worst case motor of same output rating.

 

[Sahib]

Per NEC the minimum size conductor you can run to said motor is calculated before considering any VD or PFC and must still remain that minimum size regardless what the VD and PFC end up being.

In other words, the minimum cable size is chosen based on tabulated motor current per NEC. But sound design includes voltage drop also for final cable sizing per NEC recommendation.

 

[Open Neutral]

One note for the OP.

I avoid "High PF" vs "Low PF" especially when dealing with Suits.
I use "good" vs "poor" as it saves explanations as to the goal.

 

[kwired]

In other words, the minimum cable size is chosen based on tabulated motor current per NEC. But sound design includes voltage drop also for final cable sizing per NEC recommendation.

Pretty much so. Again VD mentioned in NEC is merely a suggestion and not a requirement so you can have pretty severe VD and still be NEC compliant. Some places do enforce energy codes and that is where there can be rules that say you must meet a minimum VD requirement because the loss from VD will be resistance heating in the conductors other than that is it more of a performance/design issue.

 

[Sahib]

VD mentioned in NEC is merely a suggestion and not a requirement so you can have pretty severe VD and still be NEC compliant.

But the OP's case is not about NEC compliance but about the viability of the PFC equipment. Accordingly, cable size reduction with PFC consistent with NEC requirement may also be considered by OP for any justification of using PFC equipment.

 

[kwired]

But the OP's case is not about NEC compliance but about the viability of the PFC equipment. Accordingly, cable size reduction with PFC consistent with NEC requirement may also be considered by OP for any justification of using PFC equipment.

You can't reduce a conductor to less than NEC minimum required for the application.

If you have a situation where you would have otherwise increased the conductor size because of VD, but then reduced that VD by correcting PF, then that could possibly mean you don't increase conductor size for overall VD but if you would have a situation where say a 14 AWG conductor gives you an acceptable level of VD you still must run a 12 AWG conductor if NEC requires a minimum of 12 AWG for the particular application - can be for various for other reasons that the 12 AWG is required.

 

[K8MHZ]

As an engineer for the government, we frequently are required to review products from vendors. The latest product to cross my desk is an add on panel-enclosed capacitor bank to be installed at the main distribution panel of a commercial building with a substantial HVAC load. The bank is always hooked up and is never switched out of line.

The distribution system at our installation is owned by the local power utility. The utility already performs PF correction at the substations here using capacitor banks (switched as needed).

Our initial appraisal was that any benefit would be minimal. Can anyone refer me to proper studies on this topic? Is it all hype at the usage level? If there is any payback, are we talking a few years or decades?

Thanks.

Hello Philleepe!!

Are we having fun yet??

 

[Besoeker3]

90.8 is more of a general thing and applies to more than just motors but with NEC as the rules, most motors will already be required to have a conductor ampacity of 125% the full load current rating, some situations that is overkill some it may not be.

NEC doesn't recognize power factor correction as a reason to allow smaller conductors to a motor, or even higher efficiency motor to have smaller conductors than lesser efficiency of same output rating.

Yes, I know about the NEC specifying conductor ratings for motors and the 125% rule. I don't wish to upset or alienate anyone but those regs seem a bit antiquated to me. We designed high speed motors for various applications that had efficiencies in the high nineties. Using conductors for say, a 55kW (75hp nearest?) per NEC would be significant overkill.

Anyway, as you can see, for much of what we did the NEC wouldn't apply. Ships, 132kV fed arc furnaces.......