Switch HP rating

[mudpuppy]

The bearings on my furnace blower motor are shot, so I'm replacing the motor. I found a replacement motor from the company that made the original motor (Emerson). The old motor is a 1/3 hp and they cross-referenced to a 1/2 hp replacement (due to "OEM torque requirement"). I think I'm fine on wire size (the furnace is wired with 16 AWG, which the new motor instructions say should be ok), but here is my problem. There is an interlock switch in the motor circuit that shuts everything down if the blower compartment is opened. This switch is rated at 10 A, 1/3 hp. I'm wondering if this switch will be adequate for the new 1/2 hp motor. I assume this rating is based on the starting current of the motor. The interesting thing is the old motor nameplate is 9 A (and I measured it running at about 8.5 A), while the new, bigger motor's is nameplate is 7.1 A (must be a lot more efficient?). I don't have a copy of the NEC handy, so I can't look anything up on the subject, and I can't find a 1/2 hp switch that would easily fit (it's a Cherry E69 switch, and they only have 1/3 hp switches in that size).

Anyone have any thoughts? I'm seriously considering just bypassing the interlock altogether to avoid all doubt of switch failure.

 

[chaosiscash]

120 or 240? The table value for 1/2 hp @ 115 VAC is 9.8 A.

I'd bypass the interlock switch completely and save myself a few bucks, assuming this is at home, of course.

 

[Jahrtrix]

Mudpuppy,

The interlock switch protects you from breathing the exhaust gases being burned in the furnace.

If this is a gas or oil furnace the return air can pull these burned gases back into the living area.

The door seals the return air from pulling the burned gases back into your home. If all you are

doing is changing motors then the blower load is the same no matter how large a motor you

install. I would keep the switch in its place and not worry about changing out the motor as the

amperage draw will not change. the only change would be a stalled motor condition or short.

I might mention that the also isolates the return air from any bad odors or contaminates the

furnace may be installed near (Restrooms?)

jahrtrix

 

[mudpuppy]

It is a 115 V motor, and yes it is in my home.

Jahrtrix, you make a good point that the load isn't going to change, so the current draw shouldn't change much (other than possibly higher losses in the new motor)--I hadn't thought of that. You also make a good point about drawing the air from the basement into the system--my house is 100+ years old and the basement is dirty and musty and I wouldn't want that. Plus I rarely go into the basement so it would take a long time for me to notice if the door fell off or if I forgot to put it back on after a filter change.

I'm not too worried about the continuous current through the switch--what I am worried about is the starting current. However, the switch is normally closed so it is not like the contacts have to make this current. Plus the motor is a permanent split capacitor type and I just read that these types of motors only draw about 200% of rated current on starting. So I am now feeling much more confident about leaving the switch in place.

Thanks for the input.

 

[grover]

If all you are doing is changing motors then the blower load is the same no matter how large a motor you install.

During normal conditions, yes, but on startup, the 1/2hp motor will potentially draw higher surge current, and could potentially overload undersized components. If the nameplate data indicates the 1/2hp draws less surge than the 1/3hp, though, then there is pretty much no risk.

 

[mudpuppy]

Just wanted to follow up on this. I measured the starting current (under load) of both the old 1/3 hp motor and the new 1/2 hp motor. The old one drew about 18 A on starting and the new one drew about 19.5 A. I was originally ignorant of permanent split capacitor motors and was expecting much higher starting currents, but now I'm not too worried about this slight increase.