# Volts, hp, & Hz to Amps?

Discussion in 'General Electronics Discussion' started by Mountaingoat, Sep 17, 2011.

1. ### Mountaingoat

19
0
Sep 14, 2011
I am trying to figure out how big of a relay I need for a pump I am using sadly it does not say how many Amps it is.

120 V~ / 60 Hz / .5 HP

is all it says so how do I configure how many Amps this is? Thanks

2. ### davelectronic

1,087
12
Dec 13, 2010
Pump power

Hi there. 0.5 hp half horse power pump motor, there must be a power rating on the unit some where i would have thought, a suitable relay can be worked out from that, if not, look for that model on the net try to find its power rating, last option, and dangerous unless your experienced is current testing with a meter pump running under load conditions, this last procedure needs up most care and experience, as your working live mains circuits, try the net first. Dave.

3. ### duke37

5,364
772
Jan 9, 2011
1 BHP = 746W, horses can vary somewhat.
0.5 HP = 373W
Current = Power/ Voltage = 373/120 = 3.1A

Motors take a lot more current to start than to run so I would be inclined to go for a 10A relay.

4. ### davelectronic

1,087
12
Dec 13, 2010
Pump power

This correct, but i did not opt for that theory as there can be load fluctuations, but having said that a 10 amp relay should as Duke says fine, better a higher current rating relay than to low. Dave.

5. ### Resqueline

2,848
2
Jul 31, 2009
1hp = 745.7W so 0.5hp = 372.85W
Theoretical (if ideal) Current = 372.85W / 120V = 3.1A
In real life however, with an efficiency of 62% and a Power Factor of 0.57 - it seems it will draw around 8.8A at 120V.
Relays made for switching mains motors are sometimes also labeled as to what motor hp ratings they can switch.
But as Dave says, when hooking up bare mains connections like that yourself there are many safety related details (& pitfalls) to observe.
Without experience in the field personal guidance would be preferable.

6. ### davelectronic

1,087
12
Dec 13, 2010
Relays power

Hi all.
I thought power factor for transformers, motors power VA ratings was 0.6 , oh well close enough, yes and in practice some of these calculations have some swing variations, still 10 plus amps should be ok, but if this is water pump stuff, i would still go with aeration, less on any battery, load, life etc. Dave. PS Just a good compromise.

7. ### Mountaingoat

19
0
Sep 14, 2011
Yes Dave it is, And thanks that tells me what I need. What I was opting for anyway