conductor ampacity - to battery bank

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coulter

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This is an existing installation of a rectifier and 120V, 400AH battery bank. The rectifier output has a 100A CB feeding the battery bank. The conductors are routed about 3 feet inside of the rectifier cabinet, 4 feet of condut from the rectifier to the battery bank, then about 4 feet from the end of the conduit to the battery bank connections (free air). Conductors are #4 Cu, marked DLO or RHH or RHW. It's good, flexoble wire. CB is marked 75C.

The installation matches the prints. The engineering firm that did the prints is generally pretty good. The workmanship is good, and the EC that did the work generally does good. Nothing is getting hot, no VD issues. I don't have any issues on this part.

The issue: #4 from a 100A CB just look small.

The way I am reading 310.16, the install would have to have #3 Cu on the 100A CB, or a 90A CB with the #4 Cu. However, if one were to use 310.17, then #4 is fine on a 100A CB.

Question is: Is there any justification for using 310.17?

Another way to ask: Is there enough of the conductors in free air to justify using 310.17?

If it matters, we are under the 2002 NEC.

carl
 
Ditto, by your explanation there is no way Table 310.17 applies, and you can't get to 100A in Table 310.16 with #4 AWG even if you say the ambient temp is 20 deg C.
 
I wonder if these wires would be covered by the manufacturer's instructions instead of the NEC. Its almost like internal wiring of a UL listed appliance.

Also, I know RMS = DC, but were the tables in 310 ever meant to cover DC applications? I wonder if the breaker is a standard AC breaker, or if it is a special DC breaker?
 
steve66 said:
I wonder if these wires would be covered by the manufacturer's instructions instead of the NEC. Its almost like internal wiring of a UL listed appliance.

Also, I know RMS = DC, but were the tables in 310 ever meant to cover DC applications? I wonder if the breaker is a standard AC breaker, or if it is a special DC breaker?
Click to expand...

It is not part of the premises wiring. Just for information, UL508a allows #4 to carry 105A.

As for the breaker, many regular breakers have DC ratings at fairly low voltages (like under 30VDC).
 
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I design DC power plants for a living and have a couple of thoughts here.

Typically we never use the tables to size the wire. We size the cable to the designed voltage drop. The only time we ever get in trouble is when the runs calculated is on very short runs. In that case NEC 310.16 is the minimum, otherwise the VD calculation is always larger than 310.16.

Second I have never seen any of the primary distribution cables ran in conduit, it is always open-air or on cable rack.

So my gues here is the calculated the size on VD and made a mistake by not double check 310.16 when they installed it in conduit.

Now with this being said the rectifiers will never supply there full current under normal conditions. They only time they do is under re-charge, which at the point the rectifiers go into current limit. Depending on the rectifier type and manufacture current limit is 105 to 110% of rated current.
 
Petersonna very good. Now you know why battery rectifier plants do not fall under NEC rules. It is not premise wiring, therefore NEC can?t say squat.
 
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