Wire Size conflict

[TechSer]

I have recieved two conflicting bids for the installation of a 30hp rotery (air) compressor.

The install is 100' from our breaker box
unit is 3ph 240v 76 FLA

One bid #2 thhn w/ a 125 amp breaker and a 100 amp disconnect.

The other - #6 thhn w/ a 100 amp breaker and 100 amp disconnect.

Thanks in advance...

TS

 

[bob]

I have recieved two conflicting bids for the installation of a 30hp rotery (air) compressor.

The install is 100' from our breaker box
unit is 3ph 240v 76 FLA

One bid #2 thhn w/ a 125 amp breaker and a 100 amp disconnect.

The other - #6 thhn w/ a 100 amp breaker and 100 amp disconnect.

Thanks in advance...

TS

I would think that the mfg. would have provided a minimum wire size and max breaker size in the instructions. That said, the #6 is not larger enough for this load.

 

[charlie b]

I agree that a #6 is not large enough. Neither would a #4 be large enough. You could get by with a #3, as it has enough ampacity and would give you a voltage drop of just over 1% (an acceptable value). But I don't know if #3 is commonly available. It might actually be cheaper for the contractor to get the slightly larger #2, if only on the basis of supply and demand.

 

[TechSer]

Thanks Bob,
The compressor is a rebuild off brand so they have no info on it...
I am rebuilding a bridge saw for them and stopped them from going with the cheap bid before checking it out...
Turns out this guy also under sized the wire for the saw (hence the rebuild) and a transformer!
This will be a lawsuit, and I'm willing to bet he does not have a license.
A good example of spending a dollar to save a dime.

TS

 

[kingpb]

If the Compressor motor is rated as intermittent duty (5 or 15 min), the 85% of 76 FLA is 64.5 A, and therefore #6AWG would be acceptable.

Don't know if this is the case, but certainly something that may have been factored into the bid, rightly or wrongly.

 

[Rockyd]

Any track record of contractor A vs B? Sometimes the cheapest on the front side, can be ugly in the regard to permitting, changes and henpecking, on the backside.

Check the licenses, "Our insurance only allows for licensed contractors", is all that need be said. Surely either bidder can provide a binder?

Just a thought.

Editted to insure thread stays alive...curiosity...

 

[charlie b]

This will be a lawsuit. . . .

We are treading thin ice here. Forum members are permitted to assist a non-electrician with understanding and evaluating the information that came with a bid from an electrical contractor. So long as the discussion remains on that topic, this thread will be allowed to remain open.

But we are not allowed to give any information that might be used to support one side or the other in a legal dispute. If this thread appears to head in that direction, it will be removed.

 

[infinity]

I agree that a #6 is not large enough. Neither would a #4 be large enough. You could get by with a #3, as it has enough ampacity and would give you a voltage drop of just over 1% (an acceptable value). But I don't know if #3 is commonly available. It might actually be cheaper for the contractor to get the slightly larger #2, if only on the basis of supply and demand.

#3 is commonly available around here. I have a question, when performing a calculation for this compressor would you use the nameplate FLA or the ampacity listed in 430.250?

 

[kingpb]

Article 430.6 explains when you can and cannot use nameplate information. In general, although there are excpetions, you use the tables for overcurrent, cable etc, and use namplate for overloads.

 

[iwire]

My understanding is that you always use the nameplate if available, and if it is not known then you use the tables. Perhaps someone can provide the actual NEC article reference.

I agree, the references are spread out among 422, 430 and 440.

Basically if it is a factory assembled compressor / motor unit it will have a name plate.

If someone went to MacMaster Carr and bought a compressor, a motor, a controller, a receiver etc and cobbled them together into a air supply than we would use the motor plate.

 

[kingpb]

Bob,

I had to edit my post after looking in 430. It is a little confusing as to why I would be required to use the tables, if in fact I know for sure what is on the nameplate.

I have never had a proplem with going by the nameplate first, but edited my reply becasue I didn't want to mislead somebody as to what the NEC actually says.

 

[jim dungar]

Branch circuit wiring must always be done using NEC values, 430.6(A)(1).
Running current protection is done using nameplate data, 430.6(A)(2).

edit: The table values allow the motor to be changed some day without having to re-wire. All general duty NEMA motors are designed to have actual currents below that of the NEC table values.

 

[iwire]

My answer was not that clear now that I look back at it.

What I was trying to say is if it is a factory assembly we will find a tag on the assembly indicating Maximum OCP and MCA.

If it is just a motor we use the motor plate for the overloads and the motor plate HP to find the current for the circuit in the tables.

Boy it is less clear the more I think about it.

BTW thank you for pointing out that you edited.

 

[iwire]

Branch circuit wiring must always be done using NEC values, 430.6(A)(1).
Running current protection is done using nameplate data, 430.6(A)(2).

Jim, would you say the same if the unit is marked with the minimum circuit ampacity and maximum breaker or fuse size as most compressors I see are?

 

[infinity]

Jim, would you say the same if the unit is marked with the minimum circuit ampacity and maximum breaker or fuse size as most compressors I see are?

This is where I see some confusion. Without the min and max data could we still use the 76 A FLA for the calculation or would we have to go to the table and use the 80 amps listed there?

 

[jim dungar]

I see no exception to 430.6(A) except for:
Exception No. 1 - multi-speed motors (basically two motors in one)
Exception No. 2 - equipment containing specific types of fan or blower motors like furnaces
Exception No. 3 - multi-motor appliances like HVAC equipment


440.21 or 422.11(A) both have Article 240 as their starting point. To me that means the circuit conductor protection comes before that of the equipment.

 

[LarryFine]

Turns out this guy also under sized the wire for the saw (hence the rebuild) and a transformer!

I . . . stopped them from going with the cheap bid before checking it out...

Wow, they don't learn! (I reversed the snippets on purpose.)