Amps Per Phase Meaning

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ph196

Member
Location
Michigan
Occupation
EE
Hi,

I have a question regarding how you calculate amps per phase, and what it actually means. I am looking at a spec sheet for a piece of equipment that has a load of 54kW and on the sheet is says 150 amps per phase, the equipment is 3-phase 208V.

My initial impression is that this sounds like 150Ampsx3phases=450 Amps for the entire equipment, but I know that is likely not the case, as that would be extremely high load for a water heater. I am asking so that I can correctly size the breaker, but want to understand how this amps per phase calculation works and how it is used for sizing.

It makes sense to me where the 150 comes from, 54,000W/3 = 18,000 W 18,000W/120V = 150 A but you would not multiply this by 3 right? Why?
 
B

bwat

EE
Location
NC
Occupation
EE
ph196 said:
Is there a reason that the 'amps per phase' actually refers to the total amp draw? I guess that is what I am confused about.
Click to expand...
This is going to sound snarkier than I intend: Why do you think that 'amps per phase' means something different from 'total amp draw'?

Where is the current going from one phase, if it's not showing up on the other(s)?
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Circuits are traditionally described in terms of the amps flowing (or the amp rating) on the individual conductors.

If you have a single phase load, fed by 2 circuit conductors, and that load is drawing 150A on each conductor, that is described as a 150A load.

If you have a three phase load, fed by 3 circuit conductors, and that load is drawing 150A on each conductor, that is described as a 150A load.

People sometimes get confused, somehow thinking that the 150A three phase load is somehow drawing 450 'total amps', and arguably when you have 3 wires carrying 150A each you have more of something then if you only have 2 wires carrying 150A each. But the concept of 'total amps' is not used in the industry and doesn't really make sense without lots of surrounding explanation. So don't use the term 'total amps' unless you are willing to write the entire paragraph describing exactly what you mean.

If you want to talk about the 'total' being delivered to the load, use 'kVA'. The concept of the total power being delivered to the load is well understood in the industry, and makes it clear the difference between a 150A single phase and 150A three phase load.

A 150A 208V single phase load is 31kVA, a 150A 208V three phase load is 54kVA.

-Jon
 
S

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
ph196 said:
Hi,

I have a question regarding how you calculate amps per phase, and what it actually means. I am looking at a spec sheet for a piece of equipment that has a load of 54kW and on the sheet is says 150 amps per phase, the equipment is 3-phase 208V.

My initial impression is that this sounds like 150Ampsx3phases=450 Amps for the entire equipment, but I know that is likely not the case, as that would be extremely high load for a water heater. I am asking so that I can correctly size the breaker, but want to understand how this amps per phase calculation works and how it is used for sizing.

It makes sense to me where the 150 comes from, 54,000W/3 = 18,000 W 18,000W/120V = 150 A but you would not multiply this by 3 right? Why?
Click to expand...
So, this was answered here, but perhaps not clearly. 120 volts doesn't come in to the picture. Your wires are A to B 208V, A to C 208V and B to C 208V. You don't have a neutral, so you don't have anything running at 120 volt. Sot the formula is wattage divided by voltage divided by 1.73. Now if you don't know where or why 1.73 then ask that, because that math comes out to 15 amps.
 
P

ph196

Member
Location
Michigan
Occupation
EE
winnie said:
Circuits are traditionally described in terms of the amps flowing (or the amp rating) on the individual conductors.

If you have a single phase load, fed by 2 circuit conductors, and that load is drawing 150A on each conductor, that is described as a 150A load.

If you have a three phase load, fed by 3 circuit conductors, and that load is drawing 150A on each conductor, that is described as a 150A load.

People sometimes get confused, somehow thinking that the 150A three phase load is somehow drawing 450 'total amps', and arguably when you have 3 wires carrying 150A each you have more of something then if you only have 2 wires carrying 150A each. But the concept of 'total amps' is not used in the industry and doesn't really make sense without lots of surrounding explanation. So don't use the term 'total amps' unless you are willing to write the entire paragraph describing exactly what you mean.

If you want to talk about the 'total' being delivered to the load, use 'kVA'. The concept of the total power being delivered to the load is well understood in the industry, and makes it clear the difference between a 150A single phase and 150A three phase load.

A 150A 208V single phase load is 31kVA, a 150A 208V three phase load is 54kVA.

-Jon
Click to expand...
Thank you, thats a good explanation.

So on a 3-phase circuit breaker, it is measuring the current on each individual 'hot' wire - the amps per phase - and will trip if any given leg exceeds the breaker rating?
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
ph196 said:
So on a 3-phase circuit breaker, it is measuring the current on each individual 'hot' wire - the amps per phase - and will trip if any given leg exceeds the breaker rating?
Click to expand...

Pretty much.

The breaker is separately measuring the current on each phase, and the trips in response.

Note however that the 'trip rating' actually describes a trip curve; the breaker is supposed to permit some amount of overload for some period of time, to allow for things like starting current and inrush current for various load. But the values for this 'trip curve' are all related back to some multiple of the handle rating, and the handle rating is based on the current individually through each phase.

-Jon
 
Joethemechanic

Joethemechanic

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
Maybe they stated "Amps per phase" because not all the heaters they manufacture have balanced phase loading.
 
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