Voltage drop on a/c line

[W6HSA]

In testing an a/c powered transistor amplifier, I discovered a 9 volt drop when the amp is keyed. Here are the particulars:

The electrical outlet measures a minimum of 120 volts on a 20amp circuit. The length of power run from the mains to the subpanel is 35'. The length of wire to the outlet is 10'. The wire gauge is 10 throughout. All connections are tight and the breaker is new. The amplifier is capable of the full legal limit of 1500 PEP but develops at most 1350 on my LP-100a. Here is the issue:

When keyed, the voltage at the outlet drops down to 113 to 111 under full modulation. The amp is drawing well over 15 amps, the limit of my test instrument. At 1500 watts PEP, and assuming 70% efficiency, by math that is 17.5amps. I am going to measure at the main breaker feeding the subpanel breaker when the amp is keyed to see if there is a drop there but I thought in the interim I'd post this to get some input from you guys. This is the largest amp I have ever tried to test and I'm in unchartered waters.

It is a home-brew amp that I was given to test. The amp worked for the prior owner who never noticed this or tested it in this way but I have no other data and don't want to run the risk of ruining it. I have no experience with amps this large.

Any input would be greatly appreciated.

 

[GnG8d]

I ran into a situation running my 220v where the meter itself was causing voltage drop. The power company gave me a new one and it fixed my voltage sag.

 

[W6HSA]

I ran into a situation running my 220v where the meter itself was causing voltage drop. The power company gave me a new one and it fixed my voltage sag.

Ah, that may be the issue. I'm going to test the voltage at the mains when the amp is keyed up and check for voltage loss. That meter is 40 years old if it is a day. Thanks for the tip!

 

[-=PEAKABOO=-]

I am an electrical contractor by trade. As electricians we generally allow a 3% voltage drop before upsizing the circuit. I doubt that the branch circuit (10 gauge) is your problem and more than likely the sub panel should have an adequate sized conductor to at least easily carry the load of the amplifier. Even if it were fed with 10 gauge it still be adequate to carry the load of the amplifier with far less voltage drop than you appear to be having. Lets assume you do not have RFI issues with you volt meter.
It is very common to see voltage drop issues with the power company feeders coming into the meter socket.
I would test it at the main feeders coming into the first panel or disconnect located right after the meter socket. Be careful messing with this could give you an instant suntan.
I doubt it is your meter itself, they usually have solid copper buss feeding through them and current is measured with current transformers that are wrapped around these copper busses. The same older meters used in a 100 amp service would also be used in a 320 amp service. The internal buss is easily large enough to carry the current. The connection however may be an issue where the meter plugs into the socket. Generally if this is the case it is either just starting to happen due to weak spring clips that the meter plugs into. If it is an existing problem they generally will rear their ugly head due to other large loads on the meter.
Test it as I mentioned above and through a process of elimination work you way to the amplifier plug and lets see what you find out. Just because an electrician installed something does not mean it was done correctly.
Let me know.

 

[W5LZ]

Wonder why no one mentioned that 70% efficiency for an amplifier was very optimistic? Try 50 - 55% and see how much current you are trying to draw through that 20 amp circuit.
- 'Doc

 

[W6HSA]

Thank you, Peakaboo. I will indeed test at the main meter and work my way back. There is the main panel with the meter, a 220v run to the main subpanel and a 110/20amp run on 10 gauge to the sub panel in the shop/shack. The only thing plugged into the 20amp circuit is that amp. Everything else is plugged into a separate 20 amp circuit.

I'm using a Fluke VOM which has always worked well. I also used an analog meter a colleague brought over that is very substantial and draws a milliamp or two as its load. We could see the voltage drop on it as well.

Thanks for this and I'll report back shortly after I work my way backwards to the shack.

Best,

W6HSA

I am an electrical contractor by trade. As electricians we generally allow a 3% voltage drop before upsizing the circuit. I doubt that the branch circuit (10 gauge) is your problem and more than likely the sub panel should have an adequate sized conductor to at least easily carry the load of the amplifier. Even if it were fed with 10 gauge it still be adequate to carry the load of the amplifier with far less voltage drop than you appear to be having. Lets assume you do not have RFI issues with you volt meter.
It is very common to see voltage drop issues with the power company feeders coming into the meter socket.
I would test it at the main feeders coming into the first panel or disconnect located right after the meter socket. Be careful messing with this could give you an instant suntan.
I doubt it is your meter itself, they usually have solid copper buss feeding through them and current is measured with current transformers that are wrapped around these copper busses. The same older meters used in a 100 amp service would also be used in a 320 amp service. The internal buss is easily large enough to carry the current. The connection however may be an issue where the meter plugs into the socket. Generally if this is the case it is either just starting to happen due to weak spring clips that the meter plugs into. If it is an existing problem they generally will rear their ugly head due to other large loads on the meter.
Test it as I mentioned above and through a process of elimination work you way to the amplifier plug and lets see what you find out. Just because an electrician installed something does not mean it was done correctly.
Let me know.

 

[W6HSA]

Ah, perhaps that is the problem. I have no way of knowing what the efficiency rating is and pulled 70% out of the hat. I'll use a space heater for the test which draws 9 amps to measure the voltage drop as I test.

Wonder why no one mentioned that 70% efficiency for an amplifier was very optimistic? Try 50 - 55% and see how much current you are trying to draw through that 20 amp circuit.
- 'Doc

 

[GnG8d]

The connection however may be an issue where the meter plugs into the socket.

This was my problem, it wasn't actually the meter itself I guess. I've been here 15 years and I've had the meter out many times and it was here when I bought the place. The new meter needed a new socket, but they let me modify the old box to take the new socket.

Anyway, I think you'll need to schedule this with the power company, but it should be free I believe. My FIL retired from the power company here, so I had more latitude. Normally they would make you change the whole box instead of making a new socket fit an old box.

 

[-=PEAKABOO=-]

Wonder why no one mentioned that 70% efficiency for an amplifier was very optimistic? Try 50 - 55% and see how much current you are trying to draw through that 20 amp circuit.
- 'Doc

I assumed he actually measured the current demand of the amplifier.

 

[W6HSA]

I assumed he actually measured the current demand of the amplifier.

What is 'typical' efficiency for an amp of this size. It seems to be well built with a good transformer. I have only measured what it draws up to 15 amps which is as high as I can measure. My colleague has a much more robust device that can measure what it is actually trying to pull.

In doing the math at 55% efficiency, it works out to exactly what I'm seeing for output: 1350 watts but at 20 amps!!!! Seems like I need a 30 amp circuit at 8 gauge wiring?

I just looked at the recommendations for code and wire size for 30 amps. 10 gauge is what's specified. Now I'm confused. That's what I have only with a 20amp breaker. If I was pulling more than 20 amps, it seems to me the breaker would 'go'.

 

[W6HSA]

Peakaboo, I just edited this note to get the numbers correct from my notes.

Here are the results of my test:

Using a space heater that draws 8.5 amps, I went to the main panel. On the 120v leg that the shop is on, the voltage dropped by 2.5 volts with the heater on. The loss continued to the secondary panel and dropped yet again at the shop panel. The total loss was about 4 volts (rounded). Actually, it was 3.2 volts. I just looked at my notes, so that is 3 volts rounded.

I'm not quite sure what to do to correct this as I'm not an electrician. Any advice you can provide would be most appreciated. I would like to be knowledgeable before I call the utility company. I checked the connections at the main 100amp breaker. They were both tight. I had full voltage on one leg and down 2 on the one with the heater. The only thing ahead of it is the meter and the lines to the pole.

I just re-read your post where you indicated 3% was within tolerance. The space heater test was good then, but out of tolerance for the amp. Its the amp? It draws down voltage but stays under 20 amps? I'm stumped. Is it the amp or the utility...

 

[-=PEAKABOO=-]

Well the reason you were only seeing a voltage drop on one leg was because you were only using one "phase" of the service. You probably already knew that but I posted it in case.
It sounds to me like your voltage drop is power company issue. Do you have a larger item you can turn on? AC, Dryer, Stove and oven etc. etc? Turn all of them on and measure you voltage drop.

 

[74IN]

113V with no load seems a bit low. In my experience 117V and higher is the norm around here.

 

[W6HSA]

Well the reason you were only seeing a voltage drop on one leg was because you were only using one "phase" of the service. You probably already knew that but I posted it in case.
It sounds to me like your voltage drop is power company issue. Do you have a larger item you can turn on? AC, Dryer, Stove and oven etc. etc? Turn all of them on and measure you voltage drop.

Yes, I was aware that I was measuring one side of the 220. I was just reporting from my notes. I'll use the 110v washing machine and 110v dryer (gas powered) at the same time and measure at the main. They are both on their dedicated circuit and are run simultaneously all the time. That way, if I have to call the power company and have them check the feed, I'll have that data. Also, my colleague is due over with a 100 amp meter to measure the exact draw of the amp.

Thanks again for your advice.

 

[W6HSA]

113V with no load seems a bit low. In my experience 117V and higher is the norm around here.

Actually, my voltage varies anywhere from 120-125 and averages 123 depending on the time of the day. I was using 120 to demonstrate the voltage drop at the time it was 120. The 113 number was the measure after the amp was drawing current.