Voltage Drop

[augie47]

I get parallel 250s ...in any event 1/0s won't cut it at 2% (they will work at 5%) but as infinity notes his requirements are unrealistic.

 

[Alwayslearningelec]

Modern over engineering. The panel calculations are typically bloated and not highly accurate as far as the real world usage.

Can you explain a little more about panel calcs being bloated? you mean the calculated load or panel rating or both? Shouldn't the calc load be fairly close to reality?

 

[infinity]

Can you explain a little more about panel calcs being bloated? you mean the calculated load or panel rating or both? Shouldn't the calc load be fairly close to reality?

Most panel loads have some built in demand factors meaning that not every load in a panel will be running at the same time.

 

[electrofelon]

Can you explain a little more about panel calcs being bloated? you mean the calculated load or panel rating or both? Shouldn't the calc load be fairly close to reality?

3 main things:

1. actual draw of equipment is almost always less than nameplate. Just one example, meter an HVAC unit and you will find it typically draws 66% if the MCA (MCA does have a 25% of largest load fudge, but the draw is still less than that even taking that into account).

2. NEC calcs are overly conservative

3. NEC calcs generally don't take diversity into account and it is calculated like everything will be on at the same time.

Based on years of experience and frequently referencing utility demand data for existing buildings, I find a building service load is typically 40% of the service equipment rating. Hence my comments in post number two

 

[Alwayslearningelec]

Ok thanks. Curious as to how others think I should approach this. I already priced the job with the 1/0's shown on the engineers drawings.

I did not take the drawing note about 2% into considerations. My bad. I did make them aware of this and will follow up with client tomorrow.

Is it as straight forward as saying the 1/0's( and conduit size) won't work because of the calculated load and having to keep VD to 2% or could I spin it a little and state that the calculated load is probably overblown and would really be a particular % less.....although we'd never really be able to determine that based on a calculation at this point?

 

[augie47]

Seemingly you need a clarification from the designer.... the design calls for 1/0 and 2%. I would ask which he wants.

 

[electrofelon]

Ok thanks. Curious as to how others think I should approach this. I already priced the job with the 1/0's shown on the engineers drawings.

I did not take the drawing note about 2% into considerations. My bad. I did make them aware of this and will follow up with client tomorrow.

Is it as straight forward as saying the 1/0's( and conduit size) won't work because of the calculated load and having to keep VD to 2% or could I spin it a little and state that the calculated load is probably overblown and would really be a particular % less.....although we'd never really be able to determine that based on a calculation at this point?

Not sure if you are seeing any of my posts, but as I said in post number two I would often assume 50 to 66% of NEC calc for the voltage drop calc.

 

[infinity]

Ok thanks. Curious as to how others think I should approach this. I already priced the job with the 1/0's shown on the engineers drawings.

Are you a gambler? If so install the #1/0's as per the drawing and keep you fingers crossed. The fact that the engineer came up with the #1/0's initially seems like this isn't your problem. It should have been the designers job to do the calculation not yours.

 

[kwired]

What I don't get is they selected conductor size, raceway size, did load calculations yet placed a note essentially telling you check the voltage drop and probably have to ignore all the other specifications if voltage drop isn't met? Why aren't they calculating voltage drop and making any conductor or raceway adjustments if necessary on specifications? What if your raceway isn't large enough once you figure out what size conductors are needed for VD?

 

[Tulsa Electrician]

Still curious about a neutral.

 

[Alwayslearningelec]

Not sure if you are seeing any of my posts, but as I said in post number two I would often assume 50 to 66% of NEC calc for the voltage drop calc.

Explain further. You mean 50-66% of which number?

 

[wwhitney]

Would the calculated load of this panel be the total VA? If so I'm getting 227A.

The panel schedule in post #17 seems off.

First, the most heavily loaded conductor is C at 32450 VA, which is 270A @ 120V. So a 250A panel is too small if the loads are arranged as shown. The loads could be better balanced to make a 250A panel work, though.

Second, all the loads are shown as single pole. What kind of load is 11582 VA @ 120V? That's 96.5A.

Cheers, Wayne

 

[infinity]

Second, all the loads are shown as single pole. What kind of load is 11582 VA @ 120V? That's 96.5A.

Since this is a distribution panel I would guess that those 1Ø loads (1D, 1G, 1J, etc.) are sub-panels.

 

[Alwayslearningelec]

Since this is a distribution panel I would guess that those 1Ø loads (1D, 1G, 1J, etc.) are sub-panels.

Correct

 

[wwhitney]

Since this is a distribution panel I would guess that those 1Ø loads (1D, 1G, 1J, etc.) are sub-panels.

OK, good point, so a bunch of 120/208 3-wire subpanels supplied by this 208Y/120V 3-phase panel.

Still, the panel at location 1G is supplied by a 60A breaker, but the load on leg C is shown as 11582 VA, which is 97A @ 120V. That's a problem.

Cheers, Wayne

 

[infinity]

Still, the panel at location 1G is supplied by a 60A breaker, but the load on leg C is shown as 11582 VA, which is 97A @ 120V. That's a problem.

I agree likely another error.

 

[hmspe]

I think the designer should have provided a conductor size which would comply with their voltage drop requirement, and if anything other than the calculated load for the panels was used for the voltage drop calculation then the basis of design should have been stated.

I can't speak for all engineers, but I sometimes have similar notes in my drawings. That's mostly because actual conduit routing and the number of conductors in each conduit is determined by the EC where I design projects. It is not unusual that I call out PVC underground but what is installed is EMT overhead. That changes the length, which changes the voltage drop calculation.

When I do voltage drop calculations I use the calculated load. I don't see anything in the NEC that allows a diversity factor for voltage drop calcs. I agree that actual load in commercial is typically 50% of the NEC calculation and actual load for residential is even less, but just because I know that does not mean I can use those factors.

 

[electrofelon]

.

When I do voltage drop calculations I use the calculated load. I don't see anything in the NEC that allows a diversity factor for voltage drop calcs. I agree that actual load in commercial is typically 50% of the NEC calculation and actual load for residential is even less, but just because I know that does not mean I can use those factors.

Since voltage drop is not an NEC requirement, I don't see that any justification is needed for whatever method or diversity Factor is used.

 

[Elect117]

This looks like the same plans from an earlier post with the tap. The EGCs are still sized incorrectly. #6 is 55A without a note for everything to be 75°C. Which would violate the tap rule. Breaker of 60A compared to the 300A is 20%, so you need to make sure you are within 10ft of conductor and meet the other related tap rules.

The note is almost impossible to follow. The service is existing and not being touched. Why would you size the service conductors to 2%. Unless they mean the feeder conductors. Then you can attempt to do that by using the load, distance etc. on a online calculator.

Don't forget to increase the EGC sizes proportionally as well. 250.122(B)

 

[infinity]

This looks like the same plans from an earlier post with the tap. The EGCs are still sized incorrectly. #6 is 55A without a note for everything to be 75°C. Which would violate the tap rule.

No need to worry about the 75° C. Almost no equipment has 60° C terminations therefore the #6 tap conductor is good for 65 amps. You're correct that the 10' tap rule would apply to the #6's.