Feeder Calc

[Alwayslearningelec]

the conductors are protected by the upstream 400a breaker. So it needs 400a conductors. 500kcmil would work. The DEMAND LOAD is only 77a. So if the upstream breaker could be reduced to 100, you could use 100a conductors.

So to my prior question in my first post I guess I don't need to know the calculated load for the panel the wire is feeding? I would only size the feeder off of the breaker upstream that is protecting the conductors irregardless the panel size it's feeding? So above in first post an 800a breaker is feeding panel. I'd size wire off 800a.

 

[infinity]

So in this example the panel schedule has a total connected load of 279 amps. It being feed from a 400A breaker. I'm assuming wire to the panel should NOT be sized to the connected load because they may( and probably will) add loads. So what would you size the wire to?

No the conductors are sized according to the OCPD ahead of them. You wouldn't use 300 amp conductors on a 400 amp OCPD. But your question is about voltage drop which is entirely different.

 

[Alwayslearningelec]

No the conductors are sized according to the OCPD ahead of them. You wouldn't use 300 amp conductors on a 400 amp OCPD. But your question is about voltage drop which is entirely different.

Thanks. So if I have a 800a breaker feeder and 800a panel I don't need to know the calculated load on the panel being fed, I would just size the wire to 800a ocpd?

 

[infinity]

Thanks. So if I have a 800a breaker feeder and 800a panel I don't need to know the calculated load on the panel being fed, I would just size the wire to 800a ocpd?

Yes unless you're calculating for voltage drop.

 

[Ken_S]

How is this getting through plan review without the conductor sizes indicated?

 

[electrofelon]

You should use the calculated load of everything connected to that distribution board not a random number.

. I'd suggest providing 800 amps of conductors. And use 800 for the VD calc.

Might as well throw another opinion in here: I would use less than the calculates load, probably 66% - 80% of the calculated load.

 

[Alwayslearningelec]

Seems to be varying feedback so I'll ask again.

OCPD feeding panel. No wire sizes are given only OCPD and Panel sizes given. To size the wire feeding panel you ONLY base it off OCPD size?

 

[electrofelon]

Seems to be varying feedback so I'll ask again.

OCPD feeding panel. No wire sizes are given only OCPD and Panel sizes given. To size the wire feeding panel you ONLY base it off OCPD size?

I don't think there is one correct answer. One philosophy is that since VD is not a code requirement and if there is no mention in the plans about VD compensation, then just base wire on OCPD size. There will be other opinions.

 

[Dennis Alwon]

Seems to be varying feedback so I'll ask again.

OCPD feeding panel. No wire sizes are given only OCPD and Panel sizes given. To size the wire feeding panel you ONLY base it off OCPD size?

The simple answer you are looking for is "YES". The conductors can actually be larger but if you are feeding a sub panel and you have a 60 amp breaker then you would use a wire that is rated 60 amps or more

 

[Alwayslearningelec]

I don't think there is one correct answer. One philosophy is that since VD is not a code requirement and if there is no mention in the plans about VD compensation, then just base wire on OCPD size. There will be other opinions.

But if you need to take VD into consideration why wouldn't you still use the OCPD size?

 

[Dennis Alwon]

But if you need to take VD into consideration why wouldn't you still use the OCPD size?

Because if the panel is 200 feet away with a large load then you would need a larger conductor than than the overcurrent protective device rating. Larger wire decreases the voltage drop. So you could have a 60 amp breaker but you may need #2 wire for voltage drop

 

[electrofelon]

But if you need to take VD into consideration why wouldn't you still use the OCPD size?

I don't understand that question. If someone is telling me "I need to take VD into consideration" then that's what I do. If no one is telling me that then I probably don't or may make just some judgment based on my own experience

 

[Seven-Delta-FortyOne]

But if you need to take VD into consideration why wouldn't you still use the OCPD size?

You can. Real world, OCPDs are rarely, if ever, loaded to their max capacity.

So if you size an 800 amp feeder for xx feet to limit VD to 2%, and Calc it based on 800 amps, you’re going to have one massive feeder. When in the real world, the chances of that being necessary are infinitesimally small.

You are free to do so, however.

It’s akin to someone adding all the breakers together on their house panel and concluding they need a 1200 amp service.

 

[Seven-Delta-FortyOne]

Here you go. I just ran it in the VD Calc I have.



If you want to run 3 sets of 600’s, no one will stop you.

 

[Dennis Alwon]

But if you need to take VD into consideration why wouldn't you still use the OCPD size?

In some cases you may be able to just upsize the overcurrent protective device and the conductor, for voltage drop, and match them. But in some case the overcurrent protective device may be required to be a certain size.

I'm sorry I typed this an hour ago and didn't send it somehow...

 

[Tulsa Electrician]

One reason your getting different answers is the round up rule for 800 and less on feeders.

I'll try to explain a little. Since the conductor size is not listed. You use what you know or ask the designer.
You know the OCPD is 800, you also know the load and this is a feeder.

So in reality you have to do two things. The conductor(s) has to carry the load and be sized to the OCPD according to the NEC.

Two rules apply 240.4 and 240.6 in this case.
Other do as well however these two help in this case.

Since the load is under 700 and the next standard size for an OCPD is 800. Your conductor needs to fall with in this range unless your 800 amp is an adjustable circuit breaker and your on the 2023 NEC. Than a further reduction in conductor size can be used.

We're over 100 amps so 75 c is you choice in the conductor column for the location the conductor is installed in.

Start there and size. This is one reason a parelle set of 500 works in your case.
380 x 2 = 760. In the field we call it an over under. Over the load (760) and under the OCPD (800).

Now if you want you can do the VD if you want not mandated by code in this case only recommend.

Use your parelle 500s and the calculated load to see if these work for your VD.

A sinero:
What one can do based on 2 - 2" emt raceways and and adjustable 800 amp breaker is change the feeder to a 400 amp or 2 sets of 3/0 and dial down the breaker. Now you meet 240.4 and 240.6. Then run you VD on that if you want.

Not saying this is what I would do in your case just an example of how the rules work.
400 amp conductors on a 400 amp OCD. It carries the load.

One can submit that as an option based on what's provided. Or get a change order to size raceway and conductors to the designers choice.