Help please! Wire sizing

[kelli]

I am finishing up a design and haven't sized disconnects and wires in a long time. If I have a 5 ton, 10 ton, 12.5 ton and 15 ton RTU, and my breakers are 3/20, 3/35, 3/45 and 3/50 respectively, what size wire and disconnect to I specify? All are 480v... I know I saw a guide somewhere before, but cannot seem to find it when I most need it. Thanks!

 

[jumper]

Your wire size would be determined by the MCA on the nameplate, breaker size would be the MOCP on the nameplate and the disconnect size would be determined by 440.12, often 115% of the compressor load and any other motors or loads.

 

[kingpb]

I think the guide may be Table 310.15 (B) in the NEC.

 

[victor.cherkashi]

Disconnect also should have H.P. rating match the equipment

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[kelli]

Thanks to all who replied. This firm does things a little differently than any I have worked for before. They don't use the MCA or MOCP ratings, they pre-calculate all 3-phase power into kilowatts and use that for all their calcs. It makes some things easier, but others more difficult for me to adjust to. I think I got it, though... TGIF!

 

[GoldDigger]

Thanks to all who replied. This firm does things a little differently than any I have worked for before. They don't use the MCA or MOCP ratings, they pre-calculate all 3-phase power into kilowatts and use that for all their calcs. It makes some things easier, but others more difficult for me to adjust to. I think I got it, though... TGIF!

One problem with that is that if you are dealing with appliances containing motors and especially if they are the only thing on the circuit, you may not know the HP or the kVA/kW of the device. And even if you do, the manufacturer's MCA and MOCP values will not necessarily correspond in a predictable way to one or the other.
That means that for NEC purposes you are required to use the MCA and MOCP rather than your own tables or calculations.

 

[kelli]

Yes, I am glad you said that. I totally agree. Thank-you, am not sure how to handle that issue, as I do not sign drawings. Am off for the week-end right now, so I am going to forget about it for 2 and a half days. :happyyes:

 

[JoeStillman]

Get the cut sheets from the mechanical dep't and size the breakers and wires per the HVAC manufacturers reqts. They all publish MCA and MOCP data. I often just Google the AC model number. If you estimate on the basis of tons, you're asking for a change order.

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[kwired]

Get the cut sheets from the mechanical dep't and size the breakers and wires per the HVAC manufacturers reqts. They all publish MCA and MOCP data. I often just Google the AC model number. If you estimate on the basis of tons, you're asking for a change order.

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Or at least make sure you use older less efficient unit values on your chart/table you do use. Then you at least don't end up with too small of a conductor. You still need to use overcurrent devices that fall within the range of what is allowed for on the nameplate of the actual installed unit though.

 

[Carultch]

Get the cut sheets from the mechanical dep't and size the breakers and wires per the HVAC manufacturers reqts. They all publish MCA and MOCP data. I often just Google the AC model number. If you estimate on the basis of tons, you're asking for a change order.

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MCA and MOCP data tells you how to size the wiring and breakers, assuming the distance is negligible.

The FLA value from the datasheet should be used for calculating voltage drop. Voltage may drop in excess of this calculation during the brief starting transient, but that is OK. You want to size wires to curtail voltage drop enough, so that the long term operating voltage is stable enough, and close enough to nominal, so that it does not cause long term damage to the equipment.