Voltage Drop

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When sizing motor feeders......you must size them 125% of the motor current. If this covers the voltage drop problem do you need to use a larger size wire? Or should you calculate the wire size / voltage drop based on 125%?:-?
 
Pushpin said:
When sizing motor feeders......you must size them 125% of the motor current. If this covers the voltage drop problem do you need to use a larger size wire? Or should you calculate the wire size / voltage drop based on 125%?:-?
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Determine wire size necessary for both 125% motor current and say 3% voltage drop. Use the larger of the two results.
 
125% on the conductors is required, but there is no required limit on voltage drop. "Which is correct not just ok?" doesn't really fit.
 
hmspe.......

Depends what State you are working in.....the Florida Building Code limits VD to 2% for feeders, 3% for branch circuits.
 
Dave_PE said:
hmspe.......

Depends what State you are working in.....the Florida Building Code limits VD to 2% for feeders, 3% for branch circuits.
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David I would be interested in how that requirement is worded. I think the MA Energy code has a similar requirement.

The OPs question is a perfect example of how limiting voltage drop to a percentage could get costly.

Depending on how that section is actually worded it might require that the voltage drop calculations to a motor be based on 600% or more of the motors FLA to limit the voltage drop under start up conditions.
 
Dave_PE said:
hmspe.......

Depends what State you are working in.....limits VD to 2% for feeders, 3% for branch circuits.
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Along the lines of iwire Bob's interest, I'm wondering if the Florida Building Code makes any distinction between subpanel feeders and motor feeders...?
 
From Chapter 13 of the Florida Building Code:

13-413.1ABC.1 Voltage drop.

13-413.1.ABC.1.1 Feeders.
Feeder conductors shall be sized for a maximum voltage drop of 2 percent at design load.

13-413.1.ABC.1.2 Branch circuits.
Branch circuit conductors shall be sized for a maximum voltage drop of 3 percent at design load.
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Well that answer my question, 'design' load is much better than the 'real' load in this case of a single motor.

When this same criteria is applied to an entire service it will become artificially high standard to meet.
 
hardworkingstiff said:
How many wire sizes increase would you expect for the feeders?
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All I am saying is that the NEC service calcs result in a number much larger than the actual load will be.

This is well known to the power companies and is one of the reasons they run much smaller conductors. It is all so one of the reasons for Table 310.15(B)(6)

This artificially high projected load already results in larger conductors than necessary now when used for voltage drop calculations the waste and added costs mount up.

In my opinion this is just a back door way to require wiring for future expansion.
 
Dave_PE said:
...the Florida Building Code limits VD to 2% for feeders, 3% for branch circuits.
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Good to know. I'm not registered in Florida so I haven't done work there. Out west I've never seen a restriction, but that doesn't mean there are none.

Martin
 
Do they take into consideration that the supply voltage may be something along the lines of 240 Volts (talking single phase here), and the motor nameplate rating is 230?

Seems to me you have some built-in Voltage drop allowance before you actually need to compensate for anything here.

And then there are the motors that will operate on either 240 (230) or 208 Volts. You can have gobs of voltage drop without significant loss in performance.:grin:

It's a different story if you are beginning with 208 at your supply .... :wink:
 
It does not assume anything. If the POCO only gives you 235V instead of 240V, you're almost at 2% before leaving the transformer. So right away your in trouble.

It's not really being tested in the field yet so who knows where it will go.
 
Usage of 230V nameplate motors takes into account the feeder / branch circuit run voltage drop (and was probably the only rating the distributor had - based on old voltage standards). Quite common to see it done this way.

A lot of motors designed to operate on 480V systems are name-plated at 460V. I've even seen 440V motors (though a little old school) on 480V systems. Just watch out, our utility here runs HIGH nominal voltage during the summer (just the opposite of what you'd expect). They heavily use capacitor banks on their distribution system for voltage support. Seen many a 460V motor burn-up when the utility is running +5% at 480V (504V). Add a little voltage unbalance to the mix and current unbalance goes through the roof.
 
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