generator main disconnect

uemjg · August 29, 2013, 11:24:58 PM

I'm needing a recomendation on brand/model of main ac disconnect for a 240v/80amp generator...40 amps per leg. This would be connected in between my generator and ac breaker panel.

Not looking for a transfer switch.

wg4t50 · August 30, 2013, 12:24:44 AM

Finding a 40 or 50 amp 2 pole breaker in a metal box is not a rare item, check with any local electrical supply even WW Grainger would have it.

bobofthenorth · August 30, 2013, 05:58:42 AM

I bought the smallest breaker box that Home Depot had - slots for 4 breakers. Mounted it on the genset and put a 30 amp 2-pole breaker in it.

uemjg · August 30, 2013, 06:48:01 AM

So a 40 amp 2 pole disconnect refers to each pole capable of handling each hot line up to 40 amps?

belfert · August 30, 2013, 06:53:31 AM

What about an air conditioner disconnect sold at any home improvement center store?

Yes, a 40 amp disconnect would be 40 amps per leg. You almost never add the amperage of each leg together when describing 240 volt.

BJ · August 30, 2013, 08:05:28 AM

50 amp service 15 kw generator... learning from older band bus drivers that have had many problems with auto switches I went to a good electrical supploy and paid big bucks for a two way manual switch on for the 50 amp, off for everything else and again on for the generator only. this way there are no foul ups. when one is on the other is off. It is backed up with a 75 amp fuse.. why mess around with gagdets

TomC · August 30, 2013, 08:14:36 AM

I have a 4 breaker box-two for the generator and two for the land line. I made a heavy sheet metal slider that only allows either at a time but not both. Has been 100% reliable since 1994. Good luck, TomC

bevans6 · August 30, 2013, 09:20:35 AM

You probably need a three pole switch because you need to switch two hots and a neutral. You don't need to switch ground. Your generator, as the source, needs to bond neutral to ground when it is supplying power, as we all know. If you disconnect the generator you need to remove the neutral bond to the load center so whatever else you connect the load center to can supply the neutral/ground bond.

Brian

uemjg · August 30, 2013, 10:23:34 AM

as I mentioned at the start, I do not need a way to transfer my source from generator to shore power...I will always be on generator or batteries.

bevans6 · August 30, 2013, 10:30:51 AM

If you only have one supply source to the AC load center, why not just open the main breakers to disconnect the generator? Or do you also have an inverter? You mention batteries but not an inverter. BTW all you said was you weren't looking for a transfer switch, no mention at all of not needing to accomodate a shore source or that you have batteries.

Brian

uemjg · August 30, 2013, 10:38:39 AM

Quote from: bevans6 on August 30, 2013, 10:30:51 AM
If you only have one supply source to the AC load center, why not just open the main breakers to disconnect the generator? Or do you also have an inverter? You mention batteries but not an inverter. BTW all you said was you weren't looking for a transfer switch, no mention at all of not needing to accomodate a shore source or that you have batteries.

Brian

My generator vendor says I must install a "main disconnect" before connecting to the load distribution panel (breaker box).

bevans6 · August 30, 2013, 10:56:40 AM

Ah! In that case the typical load center (breaker panel or distribution box) that is the same as that used in home construction will be fine. Since a house panel by code must have a separate main disconnect, the panel has two separate sections, one is the main disconnect section with a bridged breaker at whatever load size you need (in your case a bridged 40 amp breaker) and the load section has all the distribution breakers. You will need to get one that either does not have neutral bonded to ground or one that has a way to disconnect that bond, since your generator should be providing that connection. In typical home construction the neutral is bonded to ground by the load center, except in some locales where they like to bond neutral to ground in the meter base. In your case, the generator normally would do it, although there are reasons to have the generator float neutral with respect to ground, and have the bond in the panel. That is not normal on a coach, though.

As long as you never have a second source of AC power - external or an inverter - this will work. If you ever go to add a second source of power, then you need to have a main disconnect that also switches neutral. At that point, a transfer switch is what is normally used, and most decent sized inverters designed for permanent install in mobile applications include such a transfer switch.

Brian

wg4t50 · August 31, 2013, 03:27:31 AM

from bevens6,
"If you ever go to add a second source of power, then you need to have a main disconnect that also switches neutral. At that point, a transfer switch is what is normally used, and most decent sized inverters designed for permanent install in mobile applications include such a transfer switch."

Not real sure where this comment comes from, or I am miss reading it. While disconnecting the neutral is not a bad idea, it is not required switching between commercial power source and the generator source, have never seen a manual overlapping switch. Yes there are automatic transfer switches that have the overlapping neutral option, usually seen on high dollar commercial buildings (usually govt bldgs). Having been in the commercail generator business 40+ years, have never heard of this requirement by any inspector or the NEC.
Not an expert, just lots of sperience

Sure am open to learning.

Thanks

Len Silva · August 31, 2013, 06:45:18 AM

In an RV, the neutral must be disconnected when switching from the generator because the neutral is bonded to ground at the generator. It is not so much disconnecting the neutral as it is removing the neutral-ground bond. It is just easier to switch the neutral.

This is another vote for having the generator connected for 120 vs 240. You only need a two pole switch rather than a much more expensive three pole switch with 240.

Emcemv · August 31, 2013, 04:21:24 PM

I put a 6 breaker sub-panel right at the output of the generator before the transfer switch. It has a double pole 60A breaker for the generator disconnect, a 30A twist lock for hooking the house up to the generator and a quad 120v outlet box for whatever. My bus came with a 60A breaker as the disconnect with everything jammed into a tiny breaker box, a disaster waiting to happen.