Wire size to breaker size

[RBBELEC.]

Hello everyone, I apologize if this topic has came up before, but to be honest I hope it has alittle bit so someone may have an answer to this problem.

What I have is an apartment building and when I do the voltage drop calculations the wire size becomes so large by half way through the building the it wont fit in the meterstack breaker or the panel. I ended up putting in 225 amp loadcenters in the units due to the wire size because they can handle up to 300mcm SER, but some of the runs are even longer which in turn makes my wire size larger. So if I have a 150 tenant breaker the smallest size SER I can run is a 4/0 and the breaker can also handle up to 300mcm, what does a guy do after you break the 300mcm mark.

We have tried the crimp Pin Adapters with not much luck (to close together). There just isn't enough room in the can for that size of wire and the bending radius is completely over done this way. None of this would really be a problem is the design team of the project would have put the electrical room in the middle, but instead its on the very end of building.

Some mentioned to me the other day that in the code, there is a section saying somewhere that if the breaker is only good for 300mcm(or whatever size) that is all the bigger wire one would ever need for that breaker. if this is true (which I cant see that being the case due to the invention of Pin Adaptors) does anyone know where it is in the code and or does anyone have any other great solutions that may help.

Any info is greatly appreciated. Thanks in advance everyone.

 

[iwire]

Just curious when you did your voltage drop calculations what did you use as the current?

Did you use the breaker size or did you use the calculated load?

If there are multiple loads at different spots on a single feeder did you use the point to point method?

Finally does your area have voltage drop requirements as the NEC does not.

 

[kwired]

I second some of what Bob brings up, and have to ask if you really need to figure as much drop as you did, but if you do, you may just need a splice box to reduce conductors in. Short run to a splice box will not introduce any significant voltage drop and can practically be ignored in your VD calculations. Not saying how easy that may be to incorporate into your project. May be possible to splice in existing equipment if you have sufficient space to do so. You may also find using panels with 225 amp mains is still less cost then all the extras necessary to put in a splice box.

 

[MasterTheNEC]

I will agree with iwire (bob) on this. You have have not given enough information to really steer you one way or the other. Do you have an actual calculated load at the end of these feeds?

My suggestion is to provide us your longest run, end calculated load and I am more than sure someone where can do the math for you. In regards to voltage drop, since this was not posted in the NEC forum everything is fair game and the International Green Code does have a voltage drop provision that some localities are pushing. Also places like California (Title 24) also has a voltage drop provision that is being enforced as well but as iwire said it is purely a recommendation in the NEC and not enforceable.

Since the values you are talking about are rather high based on the panels you are referencing, post again the longest run, and engineers selected OCPD and Panels determined prior to your making the changes and someone will be able to help I am sure.

 

[mwm1752]

I'll assume your feeders are AL. Change to CU & MC cable, I know the CU is more expensive - the MC does not use the same temp column as SER cable for current. Where do you find 300MCM AL I've only seen it once. and ditto for the previous post (need to know)

 

[RBBELEC.]

added info.

added info.

Hello again everyone and thank you for the reply's. We did use calculated load for each run/apartment unit. They are AL. The largest calculated load we have is 35,923VA it is on a 120/208v system. Are longest run at that VA is 376 feet. North Dakota does have its own VD Calc. and that goes as follows= ( C.M.A= KxLxI divided by 3%drop x voltage) K=42.4 for AL and I= calculated load. We get are 300mcm through Graybar Distribution.(I wanna say its made by Kingwire. we have to special request it and we don't use it often). I hope this helps.

 

[winnie]

I know that this would be odd for residential, but would it even be reasonable to run three phase to the individual apartments?

Given that you have 120/208V, I pr(ass)ume that you have 208/120 three phase to the building.

If you go to 3 phase with the same load, then the current per phase drops, you have less VD on the neutral (none if balanced) and the required wire size goes down. Using 100A at 208V, 375 feet and aluminium, the southwire calculator suggests #300 al conductors.

Also, what are the exact voltage drop requirements that you need to meet? Could you split the feeders so that loads more sensitive to voltage drop are separate from loads that _cause_ problematic voltage drop.

-Jon (Note: I am making an oddball suggestion well outside of my field of expertise!)