Voltage Drop

[Alwayslearningelec]

So the engineer designed (2) sets of 1/0 from 300A breaker to the wiring trough but also added a note that VD must be kept below 2%. The norm is 3%. The run is 300' to the trough. At the 120/208 voltage I see it seems the 1/0's won't work at all. The Southwire VD calculator I'm using is asking current at end of run. Not exactly sure what to put at the (2) sets go to the trough then splice and go to a 60A panel and panel MDP which is 225A. For now I'm entering 300A current at end of run. It is giving me (2) sets of 500. Far off from the 1/0 the engineer is specifying. Would also have to upsize the conduit if my calculation is correct.

Ridiculous the engineer did not specify proper size.

 

[electrofelon]

When people specify a voltage drop they need to tell you what current to use. Just very generally, I would use 50- 66% of the OCPD rating. Think about it, other a few rare circumstances such as maxing out a 100% rated breaker for example, why would you ever need greater than 80%?

 

[LarryFine]

Voltage drop is dependent on only two things: actual load current and total circuit impedance.

 

[Alwayslearningelec]

Ok, kind of need to know how I should approach this. I already bid the job and don't want to be stuck with having to increase the feeder size while I priced the 1/0's with 1-1/2" conduit.

I thought I would automatically use the 300A breaker rating. I can't determine what current should be used by the panel/breaker ratings?

 

[infinity]

Ok, kind of need to know how I should approach this. I already bid the job and don't want to be stuck with having to increase the feeder size while I priced the 1/0's with 1-1/2" conduit.

I thought I would automatically use the 300A breaker rating. I can't determine what current should be used by the panel/breaker ratings?

The panel cut sheet or schedule from the engineer should have the total load. Use that to determine the VD.

 

[Alwayslearningelec]

The panel cut sheet or schedule from the engineer should have the total load. Use that to determine the VD.

So if I have two sets from 300A breaker in main board going to the trough( being spliced) then a feeder to panel MDP and feeder to the 60A panel. I'd use the total loads of each panel and add them together?

 

[ptonsparky]

So if I have two sets from 300A breaker in main board going to the trough( being spliced) then a feeder to panel MDP and feeder to the 60A panel. I'd use the total loads of each panel and add them together?

Yes. That would get you the VD to the trough. Then do the calculations to the 60A using it's loads.

That would be worst case. Someone is trying very hard when that occurs.

 

[LarryFine]

I can't determine what current should be used by the panel/breaker ratings?

No, that would only tell you what you already know: how much current can pass through each point.

You need to find the calculated load, which someone had to do to determine the equipment sizing.

Find out who the engineer was for the electrical planning, or the permitted plans should have them.

 

[electrofelon]

Although many people would default to using the calculated load, in reality that will be unrealistic and excessive. I would apply some sort of diversity Factor to it. For bid documents, this needs to be specified so everyone is on the same page. If it is not specified then I really don't see any way that someone can complain about using a lower than calculated load figure.

 

[Alwayslearningelec]

So I'm still not clear.

Once again, I have 300A breaker( 2 sets 1/0 feeding a trough where those sets are spliced as shown in my first pic. Then a 2-1/2" w/ 3 #250's feeds a 225A MDP and then 1" w/ 3#6's feeds a 60A panel.

This is a little dicey as the two sets are not feeding ONE load. They are splice, then change sizes and feed TWO different loads.

1. The current of WHICH equipment would I use to determine the voltage drop of the two sets of 1/0?
2. For that equipment I would use the calculated load OR LESS?

 

[infinity]

1. The current of WHICH equipment would I use to determine the voltage drop of the two sets of 1/0?

You would use both.

 

[Alwayslearningelec]

You would use both.

So I got that now. Which current would I use for each panel? The calculated load, the panel current rating or some variation of either? Thanks

 

[Dsg319]

So I got that now. Which current would I use for each panel? The calculated load, the panel current rating or some variation of either? Thanks

Calculated load.

 

[Dsg319]

Am I reading it wrong that they are taking one set of the parallel conductors and tapping it off to a 60amp panel?

Are you allowed to tap one set of parallels?

 

[ptonsparky]

Am I reading it wrong that they are taking one set of the parallel conductors and tapping it off to a 60amp panel?

Are you allowed to tap one set of parallels?

After the J box, they could be taps but that wouldn't explain the 250 amp fuses in a 400 amp switch. My thought, the taps weren't made until the gutter.

 

[Alwayslearningelec]

Am I reading it wrong that they are taking one set of the parallel conductors and tapping it off to a 60amp panel?

Are you allowed to tap one set of parallels?

Look at riser in my picture. They are still two sets coming out of trough( one to MDP and one to the 60A panel) but diff size conduit/wire.

 

[Alwayslearningelec]

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

 

[infinity]

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

Says right at the bottom 227.98 amps.

 

[Alwayslearningelec]

Ok so take 227A +50A for the other panel. That's 277A. The run to the trough is 300'.

Engineer has note that VD is to be no more than 2%. So the 2 sets of 1/0 WILL NOT WORK.

I would need two sets of 300MCM. Unreal

 

[infinity]

Ok so take 227A +50A for the other panel. That's 277A. The run to the trough is 300'.

Engineer has note that VD is to be no more than 2%. So the 2 sets of 1/0 WILL NOT WORK.

I would need two sets of 300MCM. Unreal

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