A little help on tapping a disconnect please

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E.C.

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I have a problem: The facility where I work hired an A.C. guy who has been contracting residential for 20 years. I always bump heads with him on things and he says 'Show me in the code book where I can't do that'. The boss likes it because he is a good bullshooter and gets things done on the cheap. For example our latest project is to install a 3ph Air Tank filling machine (scba) for our swat team. The have a storage building that is 200 ft. from our main building and the only 3ph power going out there is feeding a 10 ton a.c. unit.The new machine calls for a 40amp max breaker. The a.c. disconnect is protectected by a 60a breaker with #4 wire. Instead of running a trench and coduit for the air machine he wants to tap that disconnect and use an auxillary contact on the air machines starter to ensure the a.c. unit doesn't start, thereby over amping the circuit. This all seems like bad wiring practice to me and something I wouldn't do. As we are a government agency there will be no permits or inspection. The A.C. guy is like 'show me in the code where I can't do that!'. The boss is like 'if it works do it'. I don't want to wire it that way and have the thing burn down cause some $5 contact stuck on the air machine. Can you help out with some code references? Thanks ...E.C.>
 
Code aside for the moment, what size are the 200' feeder conductors? I'm thinking of treating the load end of this feeder as a sub-panel that feeds two separate loads.

Taps should be okay as long as the rules are followed. If you need to, you could interrupt the 24v that energizes the AC unit whenever the SCUBA compressor is running.
 
Since when did the goverment get thrifty ? I think this at best is very cheap. I would view it as a sub panel that likely is not sized for the load. If no permit or inspections probably no code book either.
 
Digging a new trench and running an additional feeder would likely violate 225.30 since you would have more than one supply to the building.
 
E.C. said:
I have a problem: The facility where I work hired an A.C. guy who has been contracting residential for 20 years. I always bump heads with him on things and he says 'Show me in the code book where I can't do that'. The boss likes it because he is a good bullshooter and gets things done on the cheap. For example our latest project is to install a 3ph Air Tank filling machine (scba) for our swat team. The have a storage building that is 200 ft. from our main building and the only 3ph power going out there is feeding a 10 ton a.c. unit.The new machine calls for a 40amp max breaker. The a.c. disconnect is protectected by a 60a breaker with #4 wire. Instead of running a trench and coduit for the air machine he wants to tap that disconnect and use an auxillary contact on the air machines starter to ensure the a.c. unit doesn't start, thereby over amping the circuit. This all seems like bad wiring practice to me and something I wouldn't do. As we are a government agency there will be no permits or inspection. The A.C. guy is like 'show me in the code where I can't do that!'. The boss is like 'if it works do it'. I don't want to wire it that way and have the thing burn down cause some $5 contact stuck on the air machine. Can you help out with some code references? Thanks ...E.C.>
Click to expand...

If the $5 contact sticks in the CLOSED position on the air machine, then the A.C. unit wont come on. Inconvienent, yes, but hazzard no.

If the $5 contact sticks in the OPEN position on the air machine, then the A.C. unit will come on while the SCBA compressor is running, and the overall circuite will be in an overload condition. The OCPD at the source will open, and both machines will shut off, and the conductors will not overheat. Inconvienent, yes, but hazzard no.

As electricians, we want it it ALL. But sometimes, the customer is satisfied with what works, at the right price, as long as no codes are being violated.
 
I'm suprised he's going to the trouble of installing the interlock.:D
Meets the NEC in theory.
 
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How often is this being used ? If every day i would say it be a problem but once every other week for 30 minutes might work out. Good no but agree no safety issue
 
Do any of you do this for load sheding while runing on an emergency generator, the proposed install sounds just like a load shed setup.

It's done every day and it works.
 
E.C. said:
I have a problem: Can you help out with some code references? Thanks ...E.C.>
Click to expand...

um, if you tap the line side of the disconnect feeding the AC, and set a
second fused disconnect to supply the SCBA, and interlock the ac and scba
so they both can't run at the same time, there's nothing wrong with that.
nor even very difficult or expensive to accomplish. a control relay, a 6x6 box,
and a bit of wire.

as long as you can accept the interruption of your AC.....

sorry.... don't worry... you can hammer him twice on the next one.... :wink::wink:
 
Fulthrotl said:
um, if you tap the line side of the disconnect feeding the AC, and set a
second fused disconnect to supply the SCBA, and interlock the ac and scba
so they both can't run at the same time, there's nothing wrong with that.
nor even very difficult or expensive to accomplish. a control relay, a 6x6 box,
and a bit of wire.

as long as you can accept the interruption of your AC.....

sorry.... don't worry... you can hammer him twice on the next one.... :wink::wink:
Click to expand...

The OP might be interested in the method of taping the disconnect.
I would either use a junction box ahead of the disconnect or like what was mentioned before, install a small panel.
Post a sign that says "Turn off air conditioner when filling tanks"
The only bad thing that could happen is that you might raise a whole generation of divers that think that you shouldn't fill tanks while an air conditioner is running:grin:
Really good suggestion I saw here, but not cheap, was to use an MTS.
 
E.C. said:
The have a storage building that is 200 ft. from our main building and the only 3ph power going out there is feeding a 10 ton a.c. unit.The new machine calls for a 40amp max breaker. The a.c. disconnect is protected by a 60a breaker with #4 wire.
Click to expand...

Don't really see the problem. The breaker sizes aren't really relevant since you're dealing with HVAC and motor loads. What are the nameplate ampacities for the AC unit and SCBA compressor? Do they sum to less than 70A after adding the 125% factor to the SCBA unit and using MCA for the HVAC unit? Is that #4 wire copper? If so, you could put a 70A breaker on it (or even 80A if necessary). I think you may need a multi-breaker panel instead of the AC disconnect, with a 60A breaker for the AC unit and a 40A for the compressor. But this doesn't require a 100A feeder, 70A may be enough.
 
jrannis said:
The OP might be interested in the method of taping the disconnect.
I would either use a junction box ahead of the disconnect or like what was mentioned before, install a small panel.
Post a sign that says "Turn off air conditioner when filling tanks"
The only bad thing that could happen is that you might raise a whole generation of divers that think that you shouldn't fill tanks while an air conditioner is running:grin:
Really good suggestion I saw here, but not cheap, was to use an MTS.
Click to expand...

well, if ya gotta do it, i'd rip the existing disconnect off the wall, put a
12x12x6 box in it's place, put the disconnect back up, nippled to the box,
nipple the new disconnect to the other side of the box, use split bolt connectors,
and a single piece of wire for each leg, and take each end of that short
piece to each end of the disconnects.
 
suemarkp said:
Don't really see the problem. The breaker sizes aren't really relevant since you're dealing with HVAC and motor loads. What are the nameplate ampacities for the AC unit and SCBA compressor? Do they sum to less than 70A after adding the 125% factor to the SCBA unit and using MCA for the HVAC unit? Is that #4 wire copper? If so, you could put a 70A breaker on it (or even 80A if necessary). I think you may need a multi-breaker panel instead of the AC disconnect, with a 60A breaker for the AC unit and a 40A for the compressor. But this doesn't require a 100A feeder, 70A may be enough.
Click to expand...

You may be really surprised if you read the specs that come with most AC units. They will tell you the conductor size per foot of the run. Usually its more than we realize
 
suemarkp said:
Don't really see the problem. The breaker sizes aren't really relevant since you're dealing with HVAC and motor loads. What are the nameplate ampacities for the AC unit and SCBA compressor? Do they sum to less than 70A after adding the 125% factor to the SCBA unit and using MCA for the HVAC unit? Is that #4 wire copper? If so, you could put a 70A breaker on it (or even 80A if necessary). I think you may need a multi-breaker panel instead of the AC disconnect, with a 60A breaker for the AC unit and a 40A for the compressor. But this doesn't require a 100A feeder, 70A may be enough.
Click to expand...

At 200' out there I would be doing a voltage drop calc. on the wire size ( unless you like replacing motors ) as far as the interlock idea it is actually a good idea, but I do agree "why is the A/C guy doing this? is he licensed as an electrical contractor also ?
 
So many HVAC guys, and even electricians install wire to the breaker rating and not the MCA rating. That will give you plenty of margin for distance, but until we know the nameplate amps involved, we're just guessing as to what a good solution would be. A 200' foot run isn't horrible at 208V, and I'd expect to be running wire one size larger than a normal short run.
 
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