208/230 single phase on 277/480 systems

[dkidd]

Since that statement appears to have confused one or more of my readers, let me clarify that if two or more loads have different power factors, then adding just the real power values (i.e., watts) is not going to give you the correct answer for the sum of their loads. You must also add their reactive power values (i.e., VARs), and then use vector addition of the watts and the VARs to get the total apparent power (i.e., VA).

Watts is scalar, not vector.

 

[desert designs]

good morning!

so, can someone explain where/how the 230V works, and comes from for a 208/230V unit on a 120/208 system?



also, upon further thought, regarding the 'wattage' callout vs. VA on a panel schedule:

as it was explained to me, by a PE, 'there's a small difference but for our purposes they're basically the same'. Would y'all not agree that, say you input a value of 1200W from a cut sheet onto a circuit, that it's sufficient for calculation purposes or do some folks really go thru some sort of additional calculations from load to load to ensure they're really calculating what's actually going to be drawn from the physical panel ? I'm thinking, that if the difference is small, you're telling the truth by having that column named wattage, becuase you're simply indicating a watts value specified by the manufacturer? or, sould it really be VA, because that is what it becomes when it's handled in conjunction with other circuits.

 

[roger]

good morning!

so, can someone explain where/how the 230V works, and comes from for a 208/230V unit on a 120/208 system?

The unit is listed for use on either system/voltage. You do understand that 230 and 208 are not from the same source don't you?

also, upon further thought, regarding the 'wattage' callout vs. VA on a panel schedule:

as it was explained to me, by a PE, 'there's a small difference but for our purposes they're basically the same'. Would y'all not agree that, say you input a value of 1200W from a cut sheet onto a circuit, that it's sufficient for calculation purposes or do some folks really go thru some sort of additional calculations from load to load to ensure they're really calculating what's actually going to be drawn from the physical panel ? I'm thinking, that if the difference is small, you're telling the truth by having that column named wattage, becuase you're simply indicating a watts value specified by the manufacturer? or, sould it really be VA, because that is what it becomes when it's handled in conjunction with other circuits.

The difference may be small or maybe large, you could always have a piece of equipment oversized for the application.

Roger

 

[desert designs]

"You do understand that 230 and 208 are not from the same source don't you?"
I do now. Thanks roger. I guess I kind of knew that. I appreciate your, and smart$ hard answers that give me confidence I'm doing the right thing, and also appreciate the theoretical parts as well.

I bounce back and forth between concrete, glass, and gyp. bd. to electrical design and some of the rudiments slip my mind during the process. I mean, here it is saturday morning, and I'm circuiting those units. I have to work harder, but I enjoy curcuiting and totalling everything up, sizing wires etc.

basically I'm asking y'all so I'm less of a 'hack' designer. Which I am, but I get by, and collect the ED fees. but, as a project manager I don't want to ask 'dumb' questions of a contractor, you know, I need to look like I really know what I'm doing.
thanks.

 

[desert designs]

Here's what I have so far....

The fan coils are 7.5KW + 1/3HP, and 5KW + 1/4HP w/ no fuse per carrier's cut sheets. (they have to heater option in addition to the heat pumps, 30A fused disconnects and 60A breakers per carrier).

ab switching maybe overkill in the small change rooms, but I guess I'm giving that option to 'the people' instead of specifying a 1/3 or 1/2 ballast....

and fear not, I understand article 517 patient care grounding.

 

[desert designs]

oh, I'd be interested to read your opinions in potential interference from that transformer....

thanks,
steve

 

[Smart $]

"You do understand that 230 and 208 are not from the same source don't you?"
I do now. ...

FWIW, the 230V reference is for connection to a 240V system. In the US, 230V is not a standard system voltage.

 

[Smart $]

...

also, upon further thought, regarding the 'wattage' callout vs. VA on a panel schedule:

as it was explained to me, by a PE, 'there's a small difference but for our purposes they're basically the same'. Would y'all not agree that, say you input a value of 1200W from a cut sheet onto a circuit, that it's sufficient for calculation purposes or do some folks really go thru some sort of additional calculations from load to load to ensure they're really calculating what's actually going to be drawn from the physical panel ? I'm thinking, that if the difference is small, you're telling the truth by having that column named wattage, becuase you're simply indicating a watts value specified by the manufacturer? or, sould it really be VA, because that is what it becomes when it's handled in conjunction with other circuits.

As Roger noted, it may not be small. What if that 1200W load had a power factor of 0.5? Connected to a 120V system it would draw 1200W?120V?0.5pf=20A, or 2400VA. According to the posted premise, your 1200W load would only draw 10A, or 1200VA. Now couple the preceding with the fact we design distribution around ampacity, overcurrent, overload, etc.?not wattage?and you can see why it must be VA.

 

[desert designs]

would a 0.5 pf be the same as, say a max. momentary draw? I've notice time and again while studying the spec.s that a max. momentary draw can far exceed the recommended overcurrent protection. switch the unit on, and it draws alot of power, then backs down.

on this project for instance, I'm looking at a mammography system with a mfg. rec. OCP of 30A at 208V 1Ph, but it states the thing can momentarily draw 9KW (43A), but has a 'wait state' of only 1.5KW....how does the momentary power draw happen when the OCP device is only 30A ? And, what would you plug in for VA in that instance.....do you calc. it out, or make an educated conservative guess to get a circuit load, then total panel load and then thusly determine the main bus size, feeder etc. ?

 

[desert designs]

also the feeder size for the device. I think I know that the feeder should be sized per the OCP of 30A in this case, so say, #10THHN, #10G, but it doesn't cover the max. momentary demand of 46A, which would require #8...

 

[desert designs]

my completed branch circuiting:







so for the mammography units, it's OCP of 30A at 208V 1Ph, but it states the thing can momentarily draw 9KW (43A), but has a 'wait state' of only 1.5KW. I plugged in 6000VA o/2P. This is really the only assumption I made. the MCA isn't given in the documentation, only OCP and max. momentary.

I should be o.k. eh ? :thumbsup: or :happyno:

thanks,
steve

 

[desert designs]

I think the bus of panel 'a' should be increased, or maybe I'm going too crazy with 6000VA for the mammo. system.

the crux of the issue here is the difference between max. momentary and MCA.:?

 

[desert designs]

here are the spec.s for the mammography scanner in question.

can anybody help with what I should be using for my panel VA on this ?

thanks.

 

[desert designs]

NEC Section 517-73 (a) Item 1:
The current capacity of supply branch circuit conductors and the current rating of overcurrent protective
devices must not be less than 50% of the momentary rating or 100% of the long-time rating,
whichever is greater.



whup! der it is.......

KNOW THE CODE:dunce:

wait though....

"As Roger noted, it may not be small. What if that 1200W load had a power factor of 0.5? Connected to a 120V system it would draw 1200W?120V?0.5pf=20A, or 2400VA."

if the momentary is 45A do I need to divide it by the power factor of 0.6 and show 75A, or 15,600VA ?

 

[Smart $]

NEC Section 517-73 (a) Item 1:
The current capacity of supply branch circuit conductors and the current rating of overcurrent protective
devices must not be less than 50% of the momentary rating or 100% of the long-time rating,
whichever is greater.



whup! der it is.......

KNOW THE CODE:dunce:

wait though....

"As Roger noted, it may not be small. What if that 1200W load had a power factor of 0.5? Connected to a 120V system it would draw 1200W?120V?0.5pf=20A, or 2400VA."

if the momentary is 45A do I need to divide it by the power factor of 0.6 and show 75A, or 15,600VA ?

No. Power factor is only associated with Amperes when determining Real Power, or Watts. Distribution systems are designed to handle amperes of apparent power. Apparent power (VA) = Reactive Power (VAR) + Real Power (Watts=VApf)

 

[kwired]

NEC Section 517-73 (a) Item 1:
The current capacity of supply branch circuit conductors and the current rating of overcurrent protective
devices must not be less than 50% of the momentary rating or 100% of the long-time rating,
whichever is greater.



whup! der it is.......

KNOW THE CODE:dunce:

wait though....

"As Roger noted, it may not be small. What if that 1200W load had a power factor of 0.5? Connected to a 120V system it would draw 1200W?120V?0.5pf=20A, or 2400VA."

if the momentary is 45A do I need to divide it by the power factor of 0.6 and show 75A, or 15,600VA ?

Momentary load is likely only for second at most and probably even in millisecond ranges.

More important with this kind of equipment is the ability to provide that surge of power without a voltage drop that either effects the equipment in supplied or even other sensitive equipment connected to same service, or feeder. That usually equates to larger transformers, conductors, and other equipment than what would otherwise be used for similar capacity types of load that are not as sensitive to voltage fluctuations like maybe HVAC loads.

That X-ray, CT, or mammogram may draw a spike in current similar to a motor starting, but it is also doing other important things at same time that may be effected by voltage drop. The HVAC compressor dropping the voltage when it starts does not have as big of an impact on itself anyway and can take a larger voltage drop without any problems.

 

[desert designs]

thanks.

I think I'm in over my head with the xrays....I'm gonna call a PE pal on monday.

I've been getting by just fine with cut sheets for subway bread ovens, and burger king fryers, and wingstop equipment, hvac and lighting but since I'm specifying the transformer with this big-dog healthcare equipment I better check this for real instead of assuming the usual 45kva xfmr will work, etc.

also, my apologies:ashamed1: for using the word nitpicky when dealing with VA vs. watts. It's quickly become apparent it's important.

thanks again.
steve

 

[the blur]

Not too often you see a piece of equipment listing "statup current". Usually it's a given fact motors will be 8x operating current on start up. Your equipment has a slight increase, nothing to worry about.

However, any job like that will need a PE stamp on the plans.

 

[Smart $]

Not too often you see a piece of equipment listing "statup current". Usually it's a given fact motors will be 8x operating current on start up. Your equipment has a slight increase, nothing to worry about.

...

It's not startup current. It is normal operating current surges after energization, expected and short in duration (~6 sec. at a time). Think of it like a snapshot camera. The surge only occurs as the picture is taken and stored in memory.

 

[the blur]

It's not startup current. It is normal operating current surges after energization, expected and short in duration (~6 sec. at a time). Think of it like a snapshot camera. The surge only occurs as the picture is taken and stored in memory.

That seems to make sense on an xray machine. So then design it for that higher spec, and use the 80% rule, and you should be fine.