Transformer Sizing for 7.5 Horsepower air compressor

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eng1985

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Hi,

I am working on sizing a transformer for a guy who has a 7 horse 1 phase 230v AC air compressor
Any suggestions? his load will be this compressor, welder, lathe and some woodworking equipment. There is what looks to be an existing 10kVA transformer serving a house next to this garage and I know it's not big enough. Does anyone have any simple rule of thumb formulas for sizing kVA transformer for this load on a 200AMP service?

Thanks
Eng1985
 
Re: Transformer Sizing for 7.5 Horsepower air compressor

This is really more than a ?rule of thumb? thing. But it is a relatively simple thing: just find out how much load each item requires, and add them up. There are a few twists, however. One is that you have to add 25% to the largest motor, which I presume would be the compressor.

Looking up a 7HP, 230V ,motor in NEC Table 430.148, I get a value of 40 amps. Multiplying 40 amps times 230 volts, and then adding 25%, gives me 11.5 KVA. So you are right, that 10 KVA transformer cannot handle this load.

Welders are strange animals, at least as far as the NEC is concerned. The amount of load you have to count depends on many things, including the type of welder (e.g., arc welder versus resistance welder) and the duty cycle. It?s all in Article 630.

The rest of the equipment needs to be taken into account, item by item. You need the nameplate currents of the lathe and any permanently connected equipment. If the ?woodworking equipment? is simply a set of tools that will be plugged into wall receptacles, then you need count no more than 180 VA for each receptacle. You also need to account for lighting.

At some point in the process, the guy you are helping will have to hire an electrical contractor to get the installation work done. That contractor will be responsible for the actual sizing of the power supply. I think the best you can offer, until you get more fluent in the use of the ?Service Load Calculation Process? of Article 220, is a round number, based on the few steps I have outlined above.
 
Re: Transformer Sizing for 7.5 Horsepower air compressor

It sounds as if you are sizing the utility transformer. If that is correct why would you be doing it? Thats the job of the utility engineer.
It that you?
 
Re: Transformer Sizing for 7.5 Horsepower air compressor

Thanks charlieb that was what I was looking for - helpful way to outline the load based on what I know. And yes welders do seem like "strange animals" in the NEC. I am pretty sure the air compressor will be his largest load in the garage. I'm not sure how he was doing it but It looked to me like he had an 100amp service to his house, came out of his meterbase and ran to his garage about 100ft away and was running a 5 horse compressor - He said his lights were dipping like crazy when he started it and I said "I bet" lol. OK so As far as wattage wise 1hp = approx 746W is this correct? if so this would be about 5KW? Someone was also telling me that you could look at it like 1 HP = approx 1 kVA would this be accurate?
 
Re: Transformer Sizing for 7.5 Horsepower air compressor

Yes, for AC loads, think & talk in VA not watts.

Rule of thumb is absolute minimum of 1 KVA of XFMR for each HP of 3-phase motor
or 1.25 KVA of XFMR for each HP of 1-phase motor.
 
Re: Transformer Sizing for 7.5 Horsepower air compressor

You need to be talking to the serving electric utility. We would probably charge you some big bucks for fixing the flicker problem your welder is causing. We do not look at welders that are used in residential applications as extra load but as a problem since the use is very intermittent. You are causing a flicker for all of your neighbors as well.

If you insist on keeping and using the welder, we would probably install a larger transformer just for you and charge you the total installed cost for the work. My guess would be $1,500 to $6,000, depending on how much work was involved.

As far as the size of the transformer, that is our job to size it and we do not do it the same way as you size it. :D
 
Re: Transformer Sizing for 7.5 Horsepower air compressor

Originally posted by eng1985: As far as wattage wise 1hp = approx 746W is this correct? . . . Someone was also telling me that you could look at it like 1 HP = approx 1 kVA would this be accurate?
Click to expand...
If you were talking about a unit conversion, which you are not (whether or not you are aware that you are not), then yes, one ?horsepower? is equivalent to 746 watts.

What you are really talking about, however, is how much load, in units of VA (as kiloamp7 pointed out), you must take into account, for a motor of a given rating, in units of horsepower. That is not a matter of unit conversion, but rather a matter of looking it up in the NEC tables. For the example I described earlier, and for the moment disregarding the 25% extra factor, a 7 horsepower, 230 volt motor would be accounted for as 9,200 VA. So the rule of thumb of 1 HP = 1 KVA is not quite accurate, being about 30 % low. But it is correct within ?an order of magnitude,? and can be used to get a quick estimate of the load.

Why is there such a difference between rated HP and the load used in an NEC calculation? Why, for example, can you not take 7 HP, multiply by 746, and use 5,222 VA? Why is the calculated load (9,200 VA) so much higher than you get from a unit conversion? The answer is that the HP rating of a motor does not take into account power factor or the efficiency by which the motor converts incoming electricity to output power. The numbers in the NEC table are empirical, based on years of experience with motor operation. That is why we are required to use the numbers in the table, and are prohibited from using the nameplate ratings.
 
Re: Transformer Sizing for 7.5 Horsepower air compressor

As far as the size of the transformer, that is our job to size it and we do not do it the same way as you size it.
Click to expand...
No disrespect intended. :) No truer words have been stated.

We asked POCO to double the size of the transformers on the poles for a welding shop based on OUR load calculations. This would require a rebuild of the pole, new xfmrs, controlled power outage, and everything that goes with it.
NOPE! POCO said our numbers were wrong!!!
2 weeks after the plant is in operation 2 fuses blow, and 1 transformer blows--on Friday night!
POCO replaced per our specs (on a Sunday), paid for loss time to plant, paid for losses to near-by residences, paid OT to change XFMRs, etc.
Welders are strange animals, at least as far as the NEC is concerned.
Click to expand...
Our calcs were by an EE with many years practical experience! :)
 
Re: Transformer Sizing for 7.5 Horsepower air compressor

Welders have a duty cycle. The 60 Amp buzz box can be wired on on much smaller branch circuit, the welder ampacity is multiplied by the duty cycle to get the branch circuit rating. Small buzz boxes have a 20% duty cycle which means welding 2 minutes out of 10. But if you have a heavy duty commerical welder it will have a much higher duty cycle. Keep in mind a welder is a transformer, basically. Tshea may of had a commerical welder and perhaps the POCO was used to the little buzz boxes....
 
Re: Transformer Sizing for 7.5 Horsepower air compressor

Originally posted by tom baker:
Welders have a duty cycle. The 60 Amp buzz box can be wired on on much smaller branch circuit, the welder ampacity is multiplied by the duty cycle to get the branch circuit rating. Small buzz boxes have a 20% duty cycle which means welding 2 minutes out of 10. But if you have a heavy duty commerical welder it will have a much higher duty cycle. Keep in mind a welder is a transformer, basically. Tshea may of had a commerical welder and perhaps the POCO was used to the little buzz boxes....
Click to expand...
Tom, that doesn't sound right. I have tripped 30A breakers with a small AC welder. Had to up the breakers and wire to 60A.

Rattus
 
Re: Transformer Sizing for 7.5 Horsepower air compressor

Tshea may of had a commerical welder and perhaps the POCO was used to the little buzz boxes....
Click to expand...
Yes industrial welders with 100% duty cycle!

Look at 630.32.A To para-phrase ...over current device rated or set at not more than 300% rated primary current of the welder.

Duty Cycle is the percentage of time when the welder is loaded, ie 2 cycles "on", 2 cycles "off" = 50% duty cycle.
 
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