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#86069 - 09/05/03 10:35 AM Distance question
sambroak Offline
Junior Member

Registered: 09/05/03
Posts: 5
Loc: Charleston, SC
I'm kind of new to this stuff. I'll try to explain the whole thing in as few words as possible:

We have had some questions arise here at work about if there is a distance that you have to have a main power cutoff from a distribution box? We are not talking about residential work, what we do is build mobile communications systems. So we run tactical generators to an automatic transfer switch, then off to distribution boxes, to outlet boxes etc.

We are in the process of adding another distribution box that will supply power to up to 6 - 10 ton HVAC systems, and it also has a feed thru to go to the rest of the system. We are trying to figure out if we need to have a circuit breaker in the new distro box or not (is almost $2000 cheaper if not installed).

The company LEX POWER is building this junction box for us and they had a main 400amp circuit breaker, but it only cut power off to the 6 outlets for the generator and not to the feed thru. (my opinion put the 400 amp breaker in the distro box, but make it kill power to the whole system and not just to the HVACS)

any info would be greatly appreciated.

The generator would be no more than 100 foot away from the new distro box.

Thanks in advance

2014 / 2011 NEC & Related Books and Study Guides
#86070 - 09/05/03 10:49 AM Re: Distance question
rmiell Offline

Registered: 11/09/00
Posts: 261
Loc: La Junta, Co. USA
It's a little hard to tell from your post, but I will give it a stab.

If the dist. boxes are downstream from the main service disconnect, there is no limitation on distance that distribution box can be, or needs to be. If the new distribution box is the actual service disconnect, then it needs to be at a location as close to where the utility power enter the building.

The idea is to keep the unfused leads to a minimum. If there are fuses or breakers ahead of these leads, then there is overload and short circuit protection for them, so there is no length maximum.

The main disconnect must shut off all power. If the main disconnect just shuts off the 6 HVAC units, but not the feed thru part, then you have a code violation, unless the next box is right next to the main disconnect. The violations would be, disconnects not grouped, unprotected conductors, and could be # of disconnects (which can only be 6 maximum), among others.

Have the 400amp disconnect everything, and put you dist. panels wherever (working clearences code acceptable) you want to.

Rick Miell

#86071 - 09/05/03 11:12 AM Re: Distance question
sambroak Offline
Junior Member

Registered: 09/05/03
Posts: 5
Loc: Charleston, SC
thanks for the post rmiell.

I did leave out that each of the outputs to the HVAC units were individually fused, so in my opinion the 400 amp breaker was a little overkill/useless. Here is a quick layout of our system:

1. Generator (when the contactor is opened all power is killed to the whole system)
2. ABT (transfer switch incase primary generator fails will switch to back up)
3. NEW junction box (splits to 6 outputs to HVACS and feedthru for rest of system)
4. Distribution box (splits the 3 phase into 3 seperata phases to supply power to 3 different communications tents)
after the last distro box it goes to some 15, 20, & 30 amp outlet boxes. Every one of the outlets have their own individual breaker, so I think it would just be overkill to have the 400 amp breaker in the NEW junction box, unless it killed all power, and just not to the HVACS.

Once again I appreciate your reply, and have a great weekend.



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